1

Dynamic of Vistula River channel deformations

downstream of Włocławek Reservoir

Michal Habel

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Contents

1. Introduction…………………………………………………………….…………..…3

2. Aim of work, scope and research methods…………………………….…………..…5

3. General characteristics of the Vistula valley floor……………………………..……11

3.1. Geomorphology and geological structure……………………..………...…11

3.2. Selected features of the hydrologic system ………………...…….......……16

3.3. The Włocławek dam……………………………………………………….19

4. Morphology of Vistula channel………………………………………..…...…..……22

4.1. Channel type………………………………………………………....…….23

4.2. Channel mesoforms………………………………………………..………30

4.3. Selected factors modifying channel morphology……………...………......37

4.3.1. Natural factors………………………………………………..…....37

4.3.2. Artificial factors……………………………………………..….....42

5. Changes in hydrological regime after damming………………………………..49

5.1. Changes in mean annual water stages……………………….………….….50

5.2. Changes in daily and hourly water stages………………………..……...…52

5.3. Maximum impact range of the dam ………………………………….……55

6. Changes in bed load transport and its lithological characteristics……………...…....62

7. Functioning of Włocławek reservoir and its morphological consequences .……..…74

7.1. Channel deformations………………………………………..………….....74

7.1.1. Changes in the longitudinal and cross-sectional profile……...….74

7.1.2. Dynamics of bed load layer thickness changes …………...……..84

7.1.3. Change in water surface slopes…………………………..…...….87

7.2. Flood plain development …………………………………………….…....92

8. Summary……………………………………………………………………….…….98

9. References……………………………………………………………..…………...102

*This paper was prepared under the project no. N N306 2178 3 financed by Narodowe Centrum Nauki –

Morphodynamics of Vistula valley floor below Włocławek reservoir

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1. Introduction

Rivers are believed to be the most common and significant factor of earth’s

surface formation. At the same time they constitute a very sensitive “organism”, which

quickly reacts to any form of disturbance (Klimaszewski, 1978). Regardless of the size

of the river, its course character and climate zone in which it functions, construction of a

dam is regarded as the strongest possible interference in the fluvial system (fig. 1).

Structures that regulate the course of a river play equally significant role. Such ventures

cause drastic changes in both hydrological phenomena and clastic load transport and, as

a result, lead to the formation of a different channel type.

Fig. 1. Possible impacts due to lack of sediment transport in the downstream reach

below dams (Ksntoush et al., 2010).

Literature related to the influence of hydrotechnical development, including dams,

on the natural environment of river valley floors, as well as on quantitative and

qualitative characteristics of basic phenomena that follow the construction of a dam is

very extensive.

Research papers that are considered most influential, as far as issues related to the

influence of dams on fluvial process in global perspective are concerned, include

publications by Z. Babiński (2002) and K. Berkovich (2011). The work by G. P.

Williams and M.G. Wolman (1984) on the other hand, which presents the results of

research on fluvial processes below 29 dams in USA, constitutes the primary source of

research methods for the subject under discussion.

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Individual issues related to dams operation, including the processes below their

reservoirs, was touched upon in numerous scientific articles. Research on changes in the

morphology of river channels and hydrological conditions were conducted by, among

others: N. I. Makkaveev (1957) – below Rybinski reservoir on the Volga river and

Dnieper reservoir on the Dnieper river; R. S. Chalov et al. (2001) – the Ob river, below

the Novosibirsk Dam; B. V. Belys et al. (2000) – on the Yenisei river, below the

Sayano-Shushenskaya Dam; E. D. Andrews (1986) – below Flaming Gorge reservoir,

on the Green river; M. Kondolf (1997) – below Keshweek reservoir, on the Sacramento

river; N. Zdankus and G. Sabas (2006) – below the hydro power plant in Kaunas, on the

Neman river. X. X. Lu and R. Y. Siew (2006) – below Manwan reservoir, on the

Mekong river. Trends in the development of flood plains adjacent to dammed rivers

were discussed by: Ch. Chiwei (1990), who conducted research on the Huang He river,

below Sanmenxia reservoir; S. N. Ruleva, L. V. Zlotina and K. Berkovich (2002), who

investigated the area of the upper Ob valley, below Novosibirsk reservoir.

Polish literature features numerous scientific articles that include results of

research dealing with the impact of artificial reservoirs on the environment: I.

Dynowska (1984) – on the San river; J. Punzet (1972) – on the San river; M. Kosicki

and J. Krężel (1977) – on the Oder; Z. Babiński and D. Szumińska (2006) – on the Wda

river; A. Bartczak (2007) on the Zgłowiączka river– all of the above-mentioned works

discussed the changes in runoff regime below dams. J. Cyberski (1984), on the other

hand, attempted to outline the problems related to the exploitation of artificial

reservoirs. A. T. Jankowski (1995), as well as A. T. Jankowski and M. Rzętała (1997)

conducted research in the Upper Silesia region. The works of W. Machalewski et al.

(1974), W. Śliwiński (1975), L. Bagiński (2010) and Z. Brenda (1998) focused on the

problems related to the functioning of Włocławek reservoir on the Vistula. Issues

related to erosion below dams, and below the Włocławek dam in particular, are touched

upon in numerous research papers by Z. Babiński (1982, 1992, 1997, 2002). B.

Przedwojski and J. Wicher (1999), on the other hand, studied erosion processes below

Jeziorsko reservoir on the Warta river. R. Głowski and K. Parzonka (2007), as well as

K. Parzonka and R. Kosterba (2010) carried out research on the Oder river, below Brzeg

Dolny reservoir, while R. Malarz (2004) dealt with channel deformations on the Soła

river below minor dams.

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2. Aim of work, scope and research methods

The aims of this work are contained in the answer to the following question:

What was the role of the Włocławek dam in the process of shaping the valley floor

below in the last forty years of its operation. It is a fact that damming a channel greatly

contributes to radical changes in fluvial processes. Most importantly, it disrupts the

continuity of bed load transport and its renewal below dams and, furthermore, it leads to

the formation of a new hydrologic regime. Thus, an attempt was made to answer a

number of component questions: Did the dynamics of channel processes increased or

decreased due to the changes in the operation regime of the hydro power plant? Did the

identified erosion and deposition zones alter their location? Did the channel types

undergo transformation in consequence of dam’s activity? How did the course of

hydrological and morphodynamic phenomena relate to the channel bed, flood plain and

bed load changes in time and space?

The aims were primarily focused on: (a) analysing the course of hydrological

phenomena in the longitudinal profile below the dam (influence range of the hydro

power plant and weirs on water stages fluctuation, i.e. flows), (b) identifying changes in

channel’s cross-sections and the longitudinal profile of the channel bed in reference to

the factor that modifies these characteristics – geological structure, (c) analysing

changeability of conditions for channel sedimentation (bed load).

The research covered the Vistula valley floor fragment (its channel in particular)

stretching from the dam profile in Włocławek (river km 674.85) to the gauging station

profile in Toruń (734.7 km) (fig. 2). The reach under discussion is located entirely

within the boundaries of Kujawsko-Pomorskie voievodeship (central Poland). In terms

of physical and geographical division of Poland proposed by J. Kondracki (2002), the

area under discussion is located in the North European Plain, cuts through Płock Basin

and Toruń Basin (mesoregions of Toruń-Eberswalde ice-marginal valley). According to

the geographical division, the Vistula valley fragment from Płock to Bydgoszcz-Fordon

is referred to as Kuyavia Vistula (Falkowski, 1982). W. Pożaryski (1965) indicates that

the river flows along the hidden anticlinorium of Kujawy and it developed its valley in

the zone of coastal synclinorium, being parallel to the tectonic line which divides the

plate of Eastern Europe from the fold of Western Europe.

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Fig. 2. Location of gauging stations (triangles) in relation to the Vistula reach under

study.

The paper discusses the results of research conducted in the years 2007-2011.

Field works focused mainly on hydrological observations, morphometric measurements

of the channel and geomorphological mapping. The author prepared, among others, 9

depth surveys in the longitudinal profiles of the Vistula and 144 cross-sections. In order

to assess the changes in longitudinal and local slopes, surveys were conducted in regard

to the elevation of water surface under different conditions of water flow in the Vistula

channel at eight gauging stations. Measurements were taken at geodetic accuracy with

the use of GNSS RTK technology (Real Time Kinematics). A total of 36 samples was

gathered, one for each bar in succession within the river channel at the reach from

Włocławek to Toruń. Sampling locations were marked on the enclosed maps.

Bathymetric and geodetic measurements were carried out with the use of a

motor boat equipped with the following devices: LMS-522C GPS sonar (single-beam

echo sounder), geodetic receiver GPS GNSS Trimble 5800. Such configuration ensures

high-precision navigation over a bathymetric measurement route at a constant boat

speed of approximately 7 km∙h-1

and ensures precise readings regarding bottom

depths/elevations in a given water body.

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During the research on the development of the flood plain located 5 km below

the dam, a set of geomorphological maps was prepared for the flood deposition forms

within the new right-bank flood plain. In particular, morphometric measurements were

taken in relation to the natural levees and backswamps. A shallow geological sounding

was carried out to determine the character of sediments that constitute the new flood

level. Also, measurements of thickness were carried out for the overbank deposits found

on top of the flood plain. The natural levees were exposed and their structure was

analysed.

Hydrological observations involved an experiment aimed at defining the influence

range of the Włocławek dam on water stages fluctuation on the lower Vistula.

Observations were carried out every hour at ten gauging stations located at a 230 km

long reach of the Vistula below the Włocławek dam (the area under analysis was

beyond the scope of this paper). Staff gage were temporarily installed at four of the

stations. Apart from the author, a group of 20 observers participated in the experiment.

An aerial reconnaissance over the Vistula channel and the flood plain was

carried out twice: during low water stages and during the passage of a flood wave peak

through the reach under study in May 2010. A total of approximately 1500 high-

resolution photographs were taken, which were later used for typological analysis of the

channel.

By courtesy of numerous institutions, the author obtained archival source

materials, mostly hydrological data, as well as archival cross- and longitudinal sections

of the Vistula channel, which were then used to supplement the data collected during

field works. Hydrological data concerning water inflow to Włocławek reservoir in the

years 1970-2010 and the parameters of 46 flood waves that were allowed to go through

the Włocławek dam were made available by, among others, Hydro Power Plant in

Włocławek (Elektrownia Wodna in Włocławek) of ENERGA SA Capital Group. Data

on hourly water stages from the years 1996-2010 were acquired from the Włocławek

department of the Regional Water Management Authority in Warsaw. The said data

were obtained from a digital limnigraph located at the lower floodgate of the

Włocławek dam. The data related to hourly flows and water stages at Toruń and

Włocławek gauging stations were collected from the IMGW website –

www.pogodynka.pl. Additionally, the work incorporates the information included in the

permit required by Water Law Act for lifting waters in the Vistula river, as well as the

intake and discharge of water by the Włocławek dam and Włocławek hydro power

8

plant. The Regional Water Management Authority in Gdańsk, department in Toruń,

provided archival data concerning the morphology of the Vistula channel from the years

1976-1995. Based on this information, 25 longitudinal profiles of the channel within the

river reach under study were prepared. Hydroprojekt DHV company, department in

Włocławek, granted access to the Vistula channel cross-sections from the years 1969

and 1994.

Own data collected during field works supplemented with archival data allowed

for, among others, determining the dynamics of changes, both in time and space, in the

parameters related to the geometry of valley floor elements, including: channel and

flood plain width, hydraulic mean channel depthand maximum channel depth, wetted

perimeter of the channel. The value of hydraulic mean channel depth and maximum

depth was related to the mean annual water stages on the Vistula in the years 2005-2010

(166 cm at the water gauge in Włocławek and 298 cm in Toruń). Additionally, the

author proposed an index of channel cross-section shape kshp, which constitutes a

quantitative criterion to be used in the analysis of channel’s geometry. The index

represents the ratio of wetted perimeter length to channel width in a cross-section. If

channel bed is flat, devoid of microform and macroforms, the index value tends to be

low, for example close to 1. On the other hand, when channel bed in a cross section

features numerous pools and riffles, the value of the index increases.

The dynamics of changes in the longitudinal profile of the Vistula channel was

investigated by comparing 24 depth soundings on the navigation waterway (in the

thalweg zone) of the Vistula reach under study. 25 of them constitute archival data,

while the remaining 9 were carried out by the author of this work. With the use of

archival cross-sections of the Vistula – from the years 1969 and 1994 – as well as own

measurements taken in 2009, it was possible to compare changes which occurred in the

cross-sections of the channel after 40 years of the Włocławek dam operation. The

morphological analyses were conducted on digital terrain model (DTM) based on the

data collected in field with the use of single-beam sonar and GPS RTK receiver. The

process of preparing DTM in the form of a raster image was discussed by A.

Magnuszewski (1999). In order to compare the morphological changes that occurred in

the longitudinal and cross-sectional profiles of the channel, we related them to multi-

annual mean water stages from the period preceding the construction of the dam in

Włocławek (1954-1970), i.e. 348 cm at Włocławek, 418 cm at Nieszawa, 311 cm at

Silno, 347 cm at Toruń.

9

One of the advantages of surveying channel cross sections with the use of a

single-beam sonar converter LMS-522i is that it provides a digital image of bed

lithology (fig. 3). The converter of the sonar is capable of distinguishing strong and

weak signals, which allows for identification of bed sediments (Smith, Lazar 2003).

Fig. 3. Hydroacoustic image of the Vistula bottom in its cross-sectional profiles of

November 2009. Explanation: A – profile on 677.4 km), bottom with diverse relief,

devoid of alluvia, material in-situ remained (boulders, tills, moraine clay and its

residues); B – profile on 680.05 km, flat bottom devoid of alluvia; C – 706.8 km,

alluvial bottom (sand, gravel).

The analysis of sediment thickness and quartz grain roundness was conducted in

the Laboratory of the Institute of Geography at Kazimierz Wielki University.

Granulometric analysis of sand sediments was conducted on a set of sieves with a

mechanic shaker. The formula proposed by L. Folk and W. C. Ward (1957) was used to

characterize grain-size distribution, as well as lithodynamic features of the deposition

environment. Values for the following basic grain-size distribution indexes were

defined: average grain diameter (Mz), skewness asymmetry (SkG), standard deviation

(δ1) – a measure of sediment sorting. Furthermore, an analysis of quartz grain roundness

10

was conducted with the use of the method proposed by W. C. Krumbein (Mycielska-

Dowgiałło, 2007). Quartz grains of fractions 1.0-0.8 mm and 0.8-0.5 mm were analysed

using the method of visual comparison with the matrix. Upon distinguishing 100 quartz

grains of a given fraction, a stereoscopic microscope OPTA-TECH SL-T equipped with

fibre-optic illuminator was used to visually divide the grains into 10 classes of

roundness according to W. C. Krumbein (Mycielska-Dowgiałło, 2007) (fig. 4).

Consequently, a percentage share was calculated for well and very well rounded grains

(roundness class from 0.7 to 0.9 according to W. C. Krumbein). The author realizes that

visual methods of comparison may be subject to personal errors, however, as E.

Mycielska-Dowgiałło (2007) claims, values obtained by means of, among others,

Krumbein's method tend to be more precise than the ones obtained with the use of

Krygowski Geniformameter – a device for analysing the roundness of grains.

Fig. 4. Matrix for the visual assessment of grain roundness according to W. C.

Krumbein (as cited in Mycielska-Dowgiałło, 2007).

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3. General characteristics of the Vistula valley floor

3.1. Geomorphology and geological structure

Geomorphological research within the lower Vistula valley first began in the

interwar period. The first scientist to engage the problem was R. Galon (1934), who

described the main stages of river development. Later on his research was continued by

W. Mrózek (1958), A. Tomczak (1971) and W. Niewarowski (1987). A summary of

research conducted up to date was offered by E. Wiśniewski (1976, 1987), whose

dissertations discussed geological characteristics and geomorphological evolution of a

valley – from the moment of ice sheet occurrence to the contemporary times. Similarly,

E. Falkowski along with his team (1987), based on detailed geological surveys,

attempted to explain the genesis of the lower Vistula valley floor.

The lower Vistula valley features a complex of river terraces and is

characterized by a sequence of narrowings and widenings (Wisniewski, 1976;

Drozdowski, 1982). Within its entire low-land reach, from the mouth of the Narew river

near Warsaw to Tczew (initial part of the estuary reach), the valley features six

widenings – basins: Warszawa, Płock, Toruń, Unisław, Świecie and Grudziądz,

separated one from another with narrower parts. The alternating character of the

widenings and narrowings (gorges) in the lower Vistula valley was primarily

conditioned by the impact of the last glaciation and the geological structure of the valley

itself. Płock Basin, a 20 km wide basing that stretches over a distance of 60 km, extends

into a 7 km wide gorge and further shifts into Toruń Basin (90 km long and approx. 20

km wide) (fig. 5).

Within the Vistula valley section under discussion, the prevailing

geomorphological units include: Dobrzyń Moraine Plateau and Kujawy Moraine

Plateau along with the gorge that separates them (fig. 5). The Vistula river flows on the

right side of Płock Basin, intensively undercutting the slope of Dobrzyń Moraine

Plateau in Włocławek. The river then runs more to the north and passes through the 25

km long gorge separating Płock Basin and Toruń Basin. Eventually, at Nieszawa town,

before entering Toruń Basin, it draws closer to the slopes of Kujawy Moraine Plateau

(fig. 5).

According to E. Wiśniewski (1976), the Vistula valley fragment under study

emerged from the material left during the transgression of the last glacier. Such

occurrence was a result of meltwater activity during the retreat of the glacier. The

12

process was later continued by the river waters. Varied strength and amount of flowing

waters was recorded in the form of erosion levels and terraces of different origins. The

eleven-stage system of terraces presented by R. Galon (1953) in relation to the outwash

plain (sandur) and the valley of the Brda river, and by W. Niewiarkowski (1968) in

reference to the ice-marginal valley and the Drwęca river valley, was later transferred

onto the Vistula valley by E. Wiśniewski (1976). During the cold and arid periods, on

top of the tall, vast and devoid of vegetation terraces of the Vistula, aeolian processes

tended to develop (Roszko, 1982). In consequence, dunes emerged and forest took hold

of the terrains that had undergone the aeolian transformations, stabilizing the

development of these formations in the process (Mrózek, 1958).

Fig. 5. Fragment of the lower Vistula valley in comparison to the geomorphological

outline (Roszko, 1982). Explanation: A – fragment of Płock Basin; B – gorge section of

the valley; C – Toruń Basin; 1 - ground moraines, 2- end moraines, 3 – dunes, 4 – edge

of valleys and terraces.

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Elevation of the Dobrzyń Moraine Plateau near the Vistula valley ranges on

average from 95 to 100 m a.s.l. The hill located in the Szpetal district of Włocławek

(133 m a.s.l.) constitutes the highest peak of its western part – a pushed moraine

(Głodek et al. 1967) with a glaciotectonically deformed Neogene core (Froehlich,

1970). The Kujawy Moraine Plateau, on the other hand, is lower and its elevation

amounts to 90-95 m a.s.l. The Dobrzyń Moraine Plateau in comparison to the Kujawy

Moraine Plateau located west of the Vistula valley appears to be more diverse in terms

of glacial relief.

The eastern edge of the Kujawy Moraine Plateau located near Nieszawa town, is

particularly well defined in terms of its geological structure. At one of the exposed

fragments of a gravel pit one may observe that the series of sand, gravel and ice-

marginal sediments (series of river sediments) is covered with two layers of boulder

clay. They are separated with a one meter thick layer of fine sands. The series of sand

sediments, identified as river-derived, with its upper part reaching the elevation of 60-66

m a.s.l., locally rests on top of moraine clay and may serve as an evidence of multiple

glaciations. The near-edge zone of the Dobrzyń Moraine Plateau displays similar

geological structure. Deep drillings indicated that the geological structure mainly

involves three layers of moraine clay separated with series of sand formations or

Pliocene clays.

Geological mapping of the channel along with its banks in Włocławek

performed in October 1986 by the team of E. Falkowski (1987) revealed numerous

exposures of soils classified as erosion-resilient – silt and clay, as well as fluvioglacial

formations with interlayers of gravel and pebbles, which form boulder pavement. They

tend to occur in abundance along the left bank and boulevards, and below the mouth of

the Zgłowiączka river. The Vistula valley in Włocławek, due to its geological structure,

features high morphodynamics within the bank zone and the edge zone. Over the

Tertiary formations of the Miocene, and Pliocene in particular, numerous landslides and

weathering wastes creeping can be found (Falkowski, et al., 1987). The landslides on

the right bank, from the wintering harbour in Dolny Szpetal (not operating at present) to

the Zawiśle district, descend in the form of numerous tongues to a very narrow lath-

shaped fragment of the flood plain. The mass movements of the left-bank slope affected

only a minor fragment – near the road bridge, below the mouth of the Zgłowiączka river

(currently a build-up area). As one can conclude from the geological cross-section

14

prepared by B. Fąferek (1960), the Miocene formations in the vicinity of the dam in

Włocławek enter into a direct contact with the Quaternary sediments. The channel itself

is currently established in Pliocene clays and, partially, sand formations. The profile of

the Włocławek dam features a particularly specific distribution of geological strata. For

instance, Miocene sediments (sand with lignite) and Pliocene clays are deposited many

meters above the Pleistocene layers (sand and boulder clay). Grodzka Island in

Włocławek is located on top of a pedestal composed of variegated clay and gravel along

with numerous pebbles, which, in its upper part, form boulder pavement.(photo 1). The

upper layer of these formations lies above the average level of water surface –

approximately 40-41 m a.s.l. (Habel, 2007). E. Falkowski et al. (1987) identified it as a

threshold that took on the form of an islet capable of withstanding erosion processes

that extended over the entire Holocene.

Fig. 6. Geological cross-section in the profile of the Włocławek dam according to B.

Fąferek (1960 – generalized and supplemented with the maximum incision of the

channel bed). Explanation: 1 – sand and gravel, 2 – clay, 3 – Pliocene clay, 4 – fine

sand, 5 – water surface, 6 – line indicating the course of the cross-sectional profile prior

to dam construction, 7 – line indicating the course of the cross-sectional profile 500 m

below the dam in 2009.

15

According to E. Wiśniewski (1976), the surface of the sub-Quaternary

formations is usually composed of sediments of the Pliocene formations (variegated

clay interlayered with silts or fine sands). The upper layer of the Tertiary stratum within

the Vistula valley fragment under study is deposited at the depth of 20-35 m a.s.l. In the

vicinity of Ciechocinek town, on the other hand, geological drillings indicated a lack of

Pliocene formations. Moreover, the Quaternary series is deposited directly on top of

Miocene sediments (fine sands, clayey silts or brown coal), or over the Jurassic-

Cretaceous surface fractured with faults.

Photo 1. Exposed fragments of the bed in the right-bank zone of the Vistula channel in

Włocławek (river km 681.0). In the foreground: boulder pavement remaining from the

erosion of moraine clay – fragment of an out-washed threshold at the right bank of the

channel and also the proximal part of the new flood plain (photography taken in

December 2007).

Another important issue to be considered in relation to the area under study, both

in terms of geomorphology and geology, involves the genesis of the fluvioglacial

sediments that form the terraces – lath-shaped forms along the Vistula channel.

According to E. Falkowski et al. (1987), the development of the valley within the gorge

section of the river is related to the meltdown of dead-ice blocks. In his view, the place

where dead-ice was deposited the longest coincided with the line of the current Vistula

channel. Thus, rainwater and meltwater from the glacier had to flow adjacent to the

dead-ice blocks and the emerging moraine plateaus (present edge of the Dobrzyń and

16

Kujawy Moraine Plateau). This way, oblong covers of fluvioglacial sediments or lath-

shaped glacial alluvia genetically unrelated to the Vistula’s terraces were formed on the

slopes and in the post-glacial areas (lower in terms of hypsometry than the moraine

plateaus). The analysis of the material accumulated in those areas showed that it cannot

be related to the Vistula, as it contains large quantities of coarser gravel and pebbles,

transport of which is rendered impossible due to the gradient of the river, both in its

present and ancient form commonly referred to as “Pra-Wisła” (Falkowski et al., 1987).

Fig. 7. Course of river terraces within the fragment of the lower Vistula plain section

under study between Płock Basin and Toruń Basin (Wiśniewski, 1976).

Explanation: 1 – moraine plateau; 2 – subglacial channels; 3 – glaciofluvial strata; 4 –

erosive meltwater plains; 5 – meltwater valleys; 6 – late-glacial erosive terraces; 7 –

deposition terraces; 8 – dunes; 9 – escarpments of: a – moraine plateaus, b and c –

terrace; 10 – altitude points; 11 – terraces and strata were assigned to numbers from I to

IX.

17

Fig. 8. Fragment of the Vistula valley's gorge section near Bobrowniki, including the

course of terraces and strata according to E. Wiśniewski (1976) – aerial photography

was taken from the altitude of 1000 m during the flood of May 23rd

, 2010, during water

flows of approx. 5750 m3∙s

-1.

Photo 2. Near-bank zone of the Vistula channel exposed during low water stages along

with the channel bank – edge of terrace IV (51-52 m a.s.l.) in Łęg Witoszyn village (km

685). Boulders and pebbles, residues after the erosion of moraine clay, occur both in the

channel bed and at the edge of the terrace (photography taken in June 2007).

18

3.2. Selected features of the hydrologic system.

The basin of the Vistula river covers an area of 194 424 km2. The river stretches

over a distance of 1092 km. The average elevation of the basin amounts to 270 m a.s.l,

and the highest located point of the drainage area in the Tatra mountains reaches the

elevation of 2655 m a.s.l. (Mikulski, 1963). The average slope of the drainage basin

amounts to 1.04‰ and clearly decreases down the river (Soja, Mrózek, 1990).

However, as much as 75% of the river course displays slope lower than 0.3‰ (Starkel,

2001). There are three artificial reservoirs on the Vistula river course. Two are

constructed on upper part on the river in Czerniańsk and Goczałkowice and one on the

lower reach in Włocławek.

While summer floods appear to prevail in the drainage basin of the upper

Vistula, lowlands feature a tendency for meltwater floods. Total annual precipitation

within the river basin in the Carpathian Mountains oscillates between 1000 and 1300

mm. It reaches up to 550 mm in the vicinity of Warsaw and tends to be even lower in

Toruń – as little as 450 mm (Soja, Mrózek, 1990; Sobolewski, 2000). Due to the

prevalence of summer rainfalls, the river is considered to feature a snow/rain-fed system

of supply (Dynowska, 1991).

The mean annual flows on the Vistula tend to increase down the river. If the

flows amount to 6.23 m3∙

s-1

in Skoczewo, then in Zawichost they reach 450 m3∙

s-1

, and

1090 m3∙

s-1

in Tczew. The maximum flows increase from 648 m3∙

s-1

in Skoczewo to

7500 m3∙

s-1

in Zawichost and 7849 m3∙

s-1

in Tczew. Individual parts of the river basin of

the Vistula display a clear difference in the irregularity of flows (Qmax and Qmin), which

indicates varied capacity for retention and water circulation (Soja, Mrózek, 1990). The

lower reach features particular capacity for water to drain into alluvia (Starkel, 2001). In

the river basin of the upper Vistula runoff tends to be slightly higher in the summer

half-year. The lower parts of the drainage basin, on the other hand, feature much higher

runoff in the winter half-year, which for instance in Warsaw amounts to 67% (Starkel,

2001).

The lower Vistula holds about 65% of Vistula's entire water capacity and

represents approx. 30% of Poland's hydro-energetic resources. It displays features of a

transit river with complex hydrologic system. Its water regime is basically defined in

the upper and, albeit to lesser degree, middle part of the river basin. The lower course of

19

the Vistula constitutes nearly 18% of the entire river basin surface. Lowland inflows

ensure moderately stabile discharges (Rusak, 1982).

The surface area of Vistula's subcatchment at the reach under study, between the

profiles of the gauging stations in Włocławek and Toruń, amounts to 8644.2 km2

(approx. 4.4% of the entire river basin). From Włocławek to Toruń there are 29

tributaries, 5 intakes and 11 outfalls, 23 of which constitute minor unnamed streams.

The largest tributaries include: Drwęca (mean annual discharge of 33.15 m3∙

s-1

),

Zgłowiączka (5.05 m3∙

s-1

), Tążyna (1.53 m3∙

s-1

), Mień (1.36 m3∙

s-1

) (Kubiak–Wójcicka,

2006). The volume of water supplied by these streams (on average 43 m3.

s-1

) constitutes

less than 5% of total flows within this reach of the Vistula.

Table 1. Gauging stations on the Vistula within the river reach under study.

Item

no.

Name of station River

kilometre

Gauge datum

(m a.s.l.)

Current technical condition

1 Dam 674.85 - Staff gage and digital limnigraph

2 Włocławek 679.7

41.17 (newly

installed in

2004) (used to

be 42.17)

Staff gage and digital limnigraph

3 Łęg Witoszyn 685.3 42.50 No staff gage since spring 2010

4 Bobrowniki 695.8 40.92 No staff gage since approx. 1995

5 Nieszawa 702.4 38.23 No staff gage since approx. 2000

6 Łęg Osiek 713.5 38.69 No staff gage since approx. 1995

7 Silno 719.8 34.42 No staff gagesince approx. 2008

8 Toruń 734.7 31.96 Staff gageand digital limnigraph

Water stages at the river reach under study are recorded at two gauging stations

belonging to the Institute of Meteorology and Water Management - National Research

Institute, in Włocławek and in Toruń, and additionally at one station owned by the

Regional Water Management Boards of Warsaw, located below the Włocławek dam

(river km 679.85). Until recently it was possible to conduct observation at several more

gauging stations of the Regional Water Management Board (table 1).

High water stages on the lower Vistula occur in March and April and, albeit not

as frequently, in late spring and summer. The former are related to the early-spring

20

meltwater flow, often intensified with the movement of slush and floating ice. The

latter, on the other hand, usually short-term, result from continuous rains. In both cases

the occurring flood waves reach a relative elevation of 3-5 m (Babiński, 1992), the

maximum being 8 m. Lower water stages are most frequent at the turn of autumn and

winter (September-November). While water stages are predominantly shaped by flows,

water level fluctuation may st times result from slush and slush-ice jams (Babiński,

1992). The maximum flow on the lower Vistula was recorded in March 1924. It

amounted to 8620 m3∙

s-1

in Płock and 8305 m3.

s-1

in Włocławek. Such flows have never

again been recorded.

In the years 1970-2005, the river featured mean annual river flows of 895 m3.

s-1

in Włocławek and 1004 m3.

s-1

in Toruń. In humid years (1971, 1974-1975, 1977-1982,

1998-2002) they ranged from 945 to 1342 m3.s

-1 (fig. 9). In arid years (1972, 1984-

1987, 1990-1992, 2003-2004) they oscillated between 580 and 790 m3.s

-1. The

maximum water flow amounted to 5972 m3.

s-1

(on March 30th

, 1979), and the minimum

– 158 m3.

s-1

(in September 1992). Available records related to the maximum flows

during the flood of spring 2010 appear to be contradictory. According to the Regional

Water Management Board in Warsaw, on May 23rd

, there was a temporary discharge at

the Włocławek dam which amounted to 6350 m3.

s-1

(Bagiński, 2010). However, data

provided by the Institute of Meteorology and Water Management - National Research

Institute indicate that the said discharge was no higher than 5765 m3.

s-1

(Walczykiewicz,

2011). Records for the Vistula reach under study mention 46 flood waves in the last 40

years (1970-2010).

The operation of the dam clearly interferes with the natural regime of flows

below Włocławek. Since 1970 it has been lifting and retaining waters of the Vistula in

the adjacent reservoir, total capacity of which amounts to 270 million m3 (Glazik,

Grześ, 1999). While the reservoir does poorly at attenuating flood waves on the Vistula

below the dam, it does, to great extent, limit low water stages. The average time of

water retention in the reservoir in the years 1972-2000 amounted to only 5.2 days

(Gierszewski, 2006).

In the past, floods on the lower Vistula may have also resulted from ice and

slush-ice jams. The average time span of ice phenomena (slush, ice cover) on the lower

Vistula amounts to 60-65 days (Glazik, Grześ, 1999). The most frequently occurring

forms of icing include movement of slush and ice floats released by the dam in

Włocławek. In Toruń, in the years 1970-2000, ice cover occurred once in five years. It

21

is thus less frequent than in the previous years (Pawłowski, 2003). Ice phenomena on

the lower Vistula usually commence in mid November and tend to disperse by the end

of March (Grześ, 1991).

Fig. 9. Flows regime on the Vistula in Włocławek in the hydrological years 1970-2005

(data compilation based on: Elektrownia..., 2010).

Explanation: Q – mean annual flow, Qmax – maximum observed flow, Qmin –

minimum observed flow, F – number of days when flood plain was inundated, Qfb -

discharge above which flood plain becomes inundated (full bank).

3.3. Water barrage in Wloclawek

Most intensive hydrotechnical works at the river reach under study took place

in the 50s and the 60s of the 20th

century and were related to the implementation of

"Programme of complex development of the Vistula river water system", also referred

to as "the Vistula programme". It assumed, among others, that about thirty dams would

be constructed on the Vistula river (Makowski, 1998), about eight/nine of which were

supposed to be erected on the lower Vistula alone. Eventually, the dam in Włocławek

was the only project completed.

Preparations for the construction were commenced in May 1962. Surplus of

channel material was deposited on the left bank, which in result formed an artificial

river bank fragment. The channel was dammed on October 13th

, 1968. The dam was

commissioned on January 17th

, 1970 – upon filling the reservoir.

The Włocławek dam is located at river kilometre 674.85 km (photo 3).

Initially the dam lifted water to the level of 11.3 m. At present it is 14.1 m (Zdulski,

2001). Thus, the largest storage reservoir in Poland was formed, with surface area of 70

22

km2, length of 55 km, average width of 1.2 km and average depth of 5.5 m. It is also the

second largest reservoir in terms of capacity – approx. 270 million m3

of water. The face

dam is 670 m long and its crest is 12 m wide. Its base width at the deepest amounts to

150 m. The dam consists of the following elements:

1. Nine weirs locked with sluice gates, weight of which amounts to 93 tons each.

When normal level of impoundment (57.3 m a.s.l.) is maintained, their discharge

capacity amounts to 7500 m3∙s-

1. Over a stretch of 100 m, the bottom below

weirs is secured with concrete slabs and riprap.

2. A hydro power plant with an installed rating of 160.2 MW. It is equipped with

six Kaplan hydro-units with turbine runners of 8.0 m in diameter. In the years

1971-2000, the power plant annually produced from 550 GWh to 1043 GWh of

electric energy. The maximum flow capacity of the power plant amounts to 2100

m3s

-1 (Zdulski, 2001).

3. A navigation lock with chamber dimensions of 115x12 m.

4. A fish ladder - currently not operating (a new one is designed to be set on the

left bank).

5. A check dam stabilizing waters below the weirs and the power plant (new

element introduced in 1998 – photo 4).

The dam is administrated by the Regional Water Management Authority in Warsaw

- dapartment in Włocławek.

Due to the increasing concern for the safety of the dam, efforts were made to

stabilize the surface of lower water at the level of 44.5 m a.s.l. – the said value is

considered to be boundary for maintaining stability and integrity of the dam and proper

operation of the hydro power plant. For this purpose, in the years 1997-2000, 506 m

below the weir and the power plant, a threshold was constructed (photo 3 and 4), which

temporarily ensures proper conditions for the functioning of the said elements (Polak,

Rosicki, 2007). The threshold constitutes a structure made of riprap with an addition of

gabion on the crest and concrete tetrapods in the body of the threshold (photo 4). The

structure features the following dimensions: total length – 660 m, maximum height –

7.1 m, crest width at the frontal part – 7.0 m, maximum width at the base – 54 m.

23

Photo 3. Dam in Włocławek (photography taken on July 8th

, 2008). Explanation: 1 –

earth dam, 2 – weirs, 3 – power plant, 4 – navigation dock, 5 – fish ladder, 6 – check

dam stabilizing the lower part of the dam.

Photo 4. Renovation of the check dam in Włocławek during a complete cessation of

water flow (photography taken by St. Krzyżelewski on June 6th

, 2007).

24

For most part of the year, the threshold operates in adverse conditions –

particularly when ice goes through at low flows, between 400 and 1000 m3s

-1. When

only a thin layer of water flows over the threshold, the structure is being damaged by

moving blocks of ice. The flow of water over the threshold results in the formation of

higher deep pools at its lower part. The bottom at the lower part of the threshold at the

reach below the weir is relatively stable (higher deep pools range between 1.0 and 1.5 m

in depth). At the hydro power plant, on the other hand, it tends to be unstable (sand

Tertiary formations) and the higher deep pools reach depths of 10-14 m (fig. 6).

Proper maintenance of the dam in the last forty years required great financial

outlays. All repairs carried out so far cost PLN 152 million in total (Bagiński, 2010).

25

4. Morphology of Vistula channel

River channel, by M. Pardẻ (1957) referred to as channel proper, understood as

space contained between banks which holds average waters, arises from a more or less

ideal attempt at natural or artificial adaptation. A river-bed (channel along with flood

plain) may overlap with channel proper if the banks of the latter, at times of highest

floods, are higher than the current water level (e.g. Grand Canyon in USA, Dunayec

River Gorge in Poland). In such a case, the flood plain may be nonexistent. M. Pardẻ

(1957) also distinguishes a low water channel, which often coincides with channel

proper due to river slope being minor within a given reach, or in consequence of near-

bank structures, such as water-lifting weirs. This paper follows the claim made by J. R.

L. Allen (1965), that the range of a channel is limited by the course of bankline at water

surface level no higher than average high water. It tends to occurs at water surface level

corresponding to average water.

Morphology of river channels depends on many factors considered changeable

in time and space. According to D. Montogomery and J. Buffington (1997), river

channel's development predominantly depends on the following factors: debris supply

(its volume, frequency and size), river's transport capacity (frequency and volume of

flows) and – directly and indirectly – vegetation (e.g. it's impact on stability of banks).

W. Jarocki (1957) additionally distinguishes relief of the river basin and climate.

According to F. Falkowski (1978), channel's morphology is a resultant of requirements

imposed by high and low flows.

One might add one more factor to the above-mentioned list, namely the direct

influence of human activity, such as hydrotechnical infrastructure and extraction of

debris. Channel formation is most effective when it is entirely filled with water, at the

so called bankfull stage (Makkaveev, 1957). As water overflows channel banks, during

floods, the rate of water flow decreases and so does its erosive capacity (Klimaszewski,

1978).

The main parameters characterizing channe's morphology involve: its width,

depth, bed slope, size of grains composing the bed, channel formations, type of channel

(Montogmery, Buffington, 1997).

26

4.1. Channel type

The typological analysis of the Vistula river reach under study employed the

classification systems proposed by L.B. Leopold and M.G. Wolman (1957), as well as J.

L. Allen (1965). The authors identified, among others, straight, meandering and braided

rivers. Another criterion of classification involved the view in plan. According to J. C.

Brice (1975), one can distinguish single-thread rivers; straight, sinuous, meandering,

braided and island-braided, and the analysis of channel pattern should be conducted

with the use of topographic maps and aerial photographs. Research of L. B. Leopold

and M. G. Wolman (1957) shows that the occurrence of a meandering or braided

channel type at a given river reach depends mainly on the ratio of channel slope to river

flow value. S. A. Schumm (1981) relates the development of each particular channel

type to the size of bed load transport, texture of the bottom and debris, and the energy of

a river. Moreover, in his view, each channel type represents certain degree of stability.

Russian scientist R. S. Chalov (2001) proposed a classification system of channel types

based on morphodynamic criteria related to rivers that function in different conditions

of geological structure (fig. 10). The typological analysis introduced by D. Rosgen

(1994, 1996), on the other hand, is based on the measurable features of channel

geometry, plan view and channel bed-forming material (fig. 4). Initially, D. Rosgen

(1994) grouped rivers into categories, assigning them letters from A to G in relation to

characteristic parameters: slope value of water surface, shape of cross-section, channel

pattern in plan view, sinuosity, parameter defining the ratio of river valley floor width to

channel width, or width-depth ratio. Consequently, he distinguished another set of

categories, assigning them numbers from 1 to 6, which he related to the prevailing

fraction of transported sediment in a channel – d50 (fig. 4). Employing the classification

proposed by D. Rosgen (1996) allows us to assign an alpha-numeric code to a given

river reach under study. Moreover, since each category represents a strictly defined

range of values referring to particular geometrical parameters, it is possible to

effectively compare reaches selected from different rivers.

27

Fig. 10. Morphodynamic types of channels functioning in different geological and

geomorphological conditions – according to R. S. Chalov (2001). Explanation: A –

incised, confined; B – adapted; C – with wide flood valley; I – relatively straight, non-

branching; II – meandering; III – anabranching; a – Yug river channel; b – Panoy r.

channel; c – Sukhona r. channel; d – Don r. channel; e – Gauja r. channel; f – northern

Dvina r. channel; g – Oka r. channel; h – Żizdra r. channel; i – Pechora r. channel.

Fig. 4. River classification by D. Rosgen (1996), including measurable geometrical

parameters of a channel and type of sediments in which a channel was developed. W –

channel width, D – average depth.

28

Most of the approaches presented above are based exclusively on office works.

There is, however, a number of researchers who attempt to establish a typology of river

channels by supplementing theoretical considerations with field works: M.

Kamykowska et al. (1975), or L. Kaszowski and K. Krzemień (1999), among others,

seek to characterize rivers using code systems and field notes. Thus, channel typology is

defined based on gathered geometrical parameters of a channel, channel forms and

debris, as well as calculated indicators. For the analysis of a given channel system, one

tends to select features which directly or indirectly provide information on channel's

dynamics, in other words, on processes that tend to form and transform it (Krzemień,

2006). This kind of mode of research and proposed channel typology, which focuses on

identifying morphostatic and morphodynamic channel sections, tends to be employed

for mountain channel systems. However, it might be applied more extensively and for

other types of systems. The above-mentioned typological characteristic of channels

served as a basis for the analysis of changes in the Vistula channel below the

Włocławek dam.

A number of different channel formations were distinguished within the river

reach under study. For the purpose of identifying and classifying channel formations

within the Vistula reach under study, the author of this work consulted available

literature on the subject under discussion. M. Klimaszewski (1978) argues that in

consequence of bed erosion, shoals (riffles) and river pools tend to form. The

occurrence of riffles and pools in an interchangeable manner is considered a

characteristic feature of rivers that reach an equilibrium curve. According to J. L. Allen

(1965), under specific hydraulic conditions, bed load transport in a river flowing over a

sand bed results in the emergence of various bed formations. W. Florek et al. (2008)

expresses a similar opinion, claiming that bed formations reflect the hydrodynamic

condition of a channel at a given time. L. Van Rijn (1984) divided bed formations in

alluvial rivers in terms of discharge conditions into: formations of low flows regime

(flat bed, ripple marks or dunes), formations of transition flows regime, formations of

high flows regime (flat bed, antidunes).

Z. Babiński (1982, 1992, 2002), in his research on the lower Vistula, within the

macroformation here understood as the channel along with the flood plain,

distinguished channel meso- and micro-formations. Channel mesoformations include

those, which emerged in consequence of accumulation (islands, bars – positive

formations) and erosion formations (river pools – negative formations). Channel

29

microformations are defined as a dynamic surface layer of channel mesofromations. The

author of this paper took the liberty to extend the group to include thresholds, which

constitute outcrops of erosion-resilient formations. According to Z. Babiński (1992),

each type of a channel corresponds to a different channel bar type and their

classification should not only account for their emerged fragments, but also their

underwater parts. Z. Babiński (1992) also prepared a summary of terminology for all

positive channel sand mesoforms in world literature (gathered from 74 reference items

in English and 23 in Russian). Bearing this in mind, he proposed a schematic system of

channel mesoformations (fig. 12) along with channel types they correspond to.

Fig. 12. Schematic system of channel mesoforms on rivers with a gravelly-sand bottom

(Babiński, 1992).

I – Transversal pattern – straight river: 1 – transversal bars; 2 – transversal bars with

"horns"; a – stage of development; 3 –transversal mouth bars (in regard to inflow).

II – Diagonal pattern – straight-meandering river: 1 – oblique-alternate bars; 2 – oblique

bars.

III – Lateral pattern – meandering river: 1 – lateral bars; 2 – point bars; c – riffles.

IV – Medial-longitudinal pattern – braided river: 1 – Longitudinal bars – longitudinal

bars with lunate bars; 2 – middle-central bars, emerged fragments of middle-central

bars; 3 – middle-transversal bars.

V – Diverse pattern – braided river: 1 –braided river bars, 2 – sand levees (bars) -

linguoid bars, large-scale dunes; 3 – island and near-island bars.

In order to assign the Vistula reach under study to one of the channel types

falling into the classification proposed by R. S. Chalov (2001) or D. Rosgen (1996), the

30

river must be divided into several fragments. For the purpose of typological analysis,

the river reach under study was divided into three parts, taking into consideration

geomorphological, geological and geometric factors. The following attempt to classify

the types of channel is to be considered preliminary and shall be continued in the

summary of this paper.

Reach I. The Vistula river within the first reach under analysis, stretching

between the dam in Włocławek (river km 675) and Bobrowniki village (km 696) (fig.

2), displays features of a young river, restricted by hydrotechnical structures and diverse

in terms of bed's resilience to erosion (Falkowski et al., 1987). It runs between two

moraine plateaus (fig. 5) and takes on the form of a gorge (Wiśniewski, 1976). In plan

view, channel's course is straight, both during low and average water flows, and its

width fluctuate, ranging from 380 to 800 m (fig. 13 - c). The regulated reach of the

river, set out by the irregularly located groynes, is of changeable width, ranging from

280 to 450 m. The zone between the groynes is at present partially filled with sediments

(photo 5). Currently, flood plain’s width at this place ranges from 700 to 1200 m, half of

which is occupied by a channel of diverse width – sediments accumulated here undergo

redeposition (fig. 13 - a).

A number of islands strengthened by vegetation can be found along the entire

length of the overbank zone. Channels located behind the islands (side channels) are

currently sealed off by the regulatory structures. During floods, lateral channels behind

islands are filled with water, and the river may then resemble a multithreaded

anastomosing channel (fig. 8). During average flows, both bed and water surface slope

tend to increase with the distance from the dam in Włocławek towards the Bobrowniki

village. Water surface slope values, which amount to 0.06‰ at average and 0.156‰ at

low water stages, are predominantly shaped by the naturally occurring river thresholds

(fig. 13 – i; photo 1 and 6). The average depth in cross-sectional profiles of the channel

at reach I, at average flows, ranges from 2.8 to 5.9 m – an average of 3.8 m (fig. 13 – f).

The maximum depths in the cross-sectional profiles along the river also tend to vary.

Their values range from 3.3 to 8.9 m (fig. 13 - g). This particular reach of the Vistula

displays one more characteristic feature, as there is a tendency for thresholds (outcrop

for erosion-resilient formations) to appear where the difference between the maximum

and average depth in a cross-section drops to approximately 1.0 m. Both depth-defining

parameters decrease in value with the distance from the dam in Włocławek towards

Bobrowniki (fig. 13 – f, g).

31

Fig. 13. Selected geometrical parameters of Vistula's valley floor in the years 2007-2010 and their values in longitudinal profile at the reach between Włocławek and Toruń.

Explanation: a – width of flood prone area (flood plain, islands and channel); b – width of islands; c – active channel width; d – channel bed slope; e – water surface slope at average flows; f – pentad values

(moving average for five pieces of data) in the cross-sectional profiles of mean channel depths; g –maximum depth; h – shape index of cross-sectional profile (kshp) expressed as a ratio of wetted perimeter to

active channel width; i – location of thresholds at channel's bottom.

32

Photo 5. Intensive channel sedimentation in the basins formed between groynes of the

Vistula at reach I, approximately at river km 688 (right bank). Accelerated process of

channel incision and excessive length of groynes constitute favourable conditions for

sediment deposition. Explanation: 1 – filled groyne field, 2 – flood plain, 3 – groyne

(photo – September 2007).

Photo 6. "Wildered" fragment of the Vistula channel at reach I – km 686.2. Channel bed

restricted by groynes – diverse in terms of resilience to erosion. Explanation: 1 –

threshold, an outcrop of erosion-resilient formations; 2 – inter-groyne bars; 3 – remains

of an island; 4 – course of regulatory route; 5 – groynes; 6 – main directions of water

flows (photography taken on July 8th

, 2008).

33

At the reaches where thresholds occur, thalweg tends to migrate to the side –

river gradually becomes "wild" – which results in lateral erosion that "cuts off" groynes

from the channel banks and opens up channels behind islands to low and average flows

(photo 6). A criterion which facilitates the analysis of channel geometry is the use of

cross-section shape index kshp, which is the ratio of the length of wetted perimeter to

channel's width (fig. 13 - h). The value of the index tends to vary within the entire reach

under discussion – from 1.1 (flat bottom devoid of bed formations) to 3.1 (diversified

bottom with numerous pools and riffles, e.g. bellow the road bridge at river kilometre

680), which certainly contributes to the high changeability of hydrodynamic conditions

in the channel (fig. 3- A). On average the kshp index amounts to 1.8, which allows us to

draw a conclusion that the length of bed's wetted perimeter is by 80% higher than its

width.

The reach I at average water stages, according to the classification proposed by

R. S. Chalov (2001), displays features of a "constrained" incised river, and its channel is

relatively straight and non-branching – type A-1 (fig. 10). The fact arises from

anthropogenic transformations, including channel's location below the dam. However,

during flood flows it bears features of an adjusted river, branching into several channels

– type B-III (fig. 10, fig. 8). On the other hand, according to the classification of D.

Rosgen (1996), relating the reach under study to a specific type may prove to be

difficult due to transformations of river's natural geometrical attributes and the fact that

the parameters defining particular river types are fixed (fig. 4). It might be reasonable to

assume that reach I falls into type D3, D4, D5 or D6, as the width-depth ratio (W/D)

amounts to 128, slope is lower than 0.4‰, and its bed is mostly composed of clay,

boulder pavement and silt (fig. 3 - A and B). Additionally, type B2 may also be taken

into consideration (fig. 4).

Reach II. The second Vistula reach under analysis (river km 696-719.8), from

Bobrowniki to the mouth of the Tążyna river and the town of Silno (fig. 2), resembles a

braided-anastomosed river, with partially preserved islands and lateral channels, as well

as channel sand mesoforms (bars). Especially during floods, the channel in plan view

appears to be multi-branch (fig. 8, photo 7). In terms of geology and geomorphology,

the reach constitutes a transition zone between the gorge fragment of the valley and

wide Torun Basin (fig. 5). The channel has nearly constant width (from 500 to 650 m –

fig. 13 - c), and the regulatory structures (groynes) occur sporadically in the vicinity of

river kilometre 700 and at the reach from km 713 to 719.8. Large islands constitute a

34

major part of the cross-section of river valley floor. The only reach where they do not

occur stretches between river kilometre 696 and 700. Here, the width of the flood plain

is small (fig. 13 - b). The lateral branches behind the existing islands tend to function,

albeit partially, even at mean and low flows, unlike at the higher located reach I.

Channel slopes are diverse – perhaps due to the large quantity and size of accumulated

channel mesoforms (bars). Water surface slopes are nearly equal at average and low

flows, and do not correlate with channel's slope (fig. 13 - d, e). Hoever, they are

influenced by the channel sand mesofroms. Mean depths of the channel in its cross

sections are comparable and on average amount to 2.9 m (fig. 13 – f). The maximum

channel depths in cross sections range from 3.7 to 7.0 m (fig. 13 - g). The cross-

sectional profiles with highest depths are concentrated within a short reach located blow

the dam, at river kilometre 706. Kshp for this particular reach amounts to 1.6 and tends to

decrease down the river (fig. 13 – h).

Photo 7. Fragments of reach II. The channel resembles a braided-anastomosed river

with partially preserved island and sand mesoforms. Explanation: 1 – remaining

fragment of Ptasia Island at river kilometre 697, 2 – newly-formed flood plain, 3 –

Zielona Island at river kilometre 708-711 km (photography was taken during low flows

in June 2008).

35

According to the classification proposed by R.S. Chalov (2001), reach II

functions similarly to rivers that are adapted to their valleys, anabranching – type B-III

(fig. 10). According to D. Rosgen's (1996) classification, on the other hand, we may

tentatively assume that it belongs to type C5. W/D ratio amounts to 200, slope is lower

than 0.4‰, and the bed is mostely composed of sands (fig. 3 - C). Other variants that

may be taken into consideration include type D5 and DA5 (fig. 4).

Reach III. The third reach under analysis, between river kilometre 719.8 and 735, i.e.

between the towns of Silno and Toruń (fig. 2), constitutes a regulated channel with a

number of transverse-riffle sand bars – alternate. At low and average water stages, the

channel is straight, with forced erosion and limited meandering due to groynes (photo

10). There is a large concentration of hydrotechnical structures, due to which the course

of mainstream at times of low and average flows is winding, nearly sinusoidal. During

floods one can notice the consequence of regulatory works being carried out –

straightening of the channel. The Vistula channel in the central part of the valley floor is

regulated and of constant width (photo 8, fig. 13 - c). The adjacent flood plain is up to

approx. 2000 m wide and at present consists of former islands and lateral threads filled

with accumulated debris (fig. 13 – a, fig. 14). Currently, the flood plain, which is higher

than the one in reach I and II, is cultivated. Majority of tree stands and bush patches

were cleared, which accelerates runoff during floods. Channel bed slope is by 5 cm

(0.05‰) steeper than water surface slope (fig. 13 – d and e). Average depths in the

cross-sectional profiles of this river section amount to 3.8 m, meaning they are similar

to reach I. However, the course of depths in the longitudinal profile appears to be more

even, with a tendency to decrease down the river (fig. 13 - f). Numerous groynes and

three road bridges appear to exert strong influence on the course of maximum depths in

cross-sectional profiles, which, at this particular river reach, range from 4.0 to 8.8 m.

While the shape index of channel's cross sections kshp for the entire reach amounts to the

average of 1.9, longitudinal profile displays greater diversity – from 1.1 to 3.0 (fig. 13 -

h). Channel bed relief is particularly diverse in the vicinity of a highway bridge (river

km 725.5), which is considered to be a result of impact exerted by hydrotechnical

structures (groynes and the bridge), as well as numerous sand bars (fig. 13 – h).

36

Photo 8. Flooded Vistula valley floor near Toruń – reach III, in the vicinity of the

highway bridge during the culmination of the flood wave of May 2010; flow of approx.

5750 m3.

s.1

(photography taken on May 25th

, 2010 from an altitude of approx. 1000 m).

According to the classification proposed by R.S. Chalov (2001), reach III

functions similarly to rivers adapted, incised and constrained, in this case, by the

regulatory structures – type B-1 (fig. 10). During floods, water that pours into the flood

plain flows in between former islands. In such cases, the river displays features of an

anabranching stream – type B-III (fig. 11, photo 8). According to D. Rosgen's (1996)

classification, on the other hand, we may assume that it represents type C5. W/D ratio

amounts to 200, slope is lower than 0.4‰, and the bed is mostly composed of sands.

Other variants that may be taken into consideration include type D5 and DA5 (fig. 11).

TORUŃ

37

Fig. 14. Channel type changes in the vicinity of Toruń (Broza Toruńska) resulting from

the regulatory works in the last 130 years. 1876 – braided-anastomosed channel with the

width of up to 2 km; 1888 – channel soon after regulation, with a 350 m wide regulatory

route; 1943 – regulated channel, straight, with forced bed erosion and alternating

transverse-riffle sand bars (according to L. Koc (1972) – supplemented by Z. Babiński,

1992). Explanation: 1 – flood plain and islands; 2 – near-island bars; 3 – transverse-

riffle sand bars; 4 – flood plain’s edge; 5 – steep edges of islands; 6 – transverse and

longitudinal groynes; 7 – course of regulatory route.

4.2. Channel mesoforms

Islands are the largest mesoforms occurring within the reach under study. This

paper follows the definition given by Z. Babiński (1982), who defined them as

formations that vary in shape and are usually covered with willows, sporadically with

older tree stands. Initially they represented various kinds of sand bars or cut-off parts of

flood plain. If a river bar achieves an appropriate relative height, which prevents it from

being flooded and provides favourable conditions for vegetation, it may transform into

an island (Babiński, 1982). Changes concerning islands are often limited to their bank

zones. Within the Vistula reach under study, islands typically do not constitute

individual compact formations, but merge into groups of islands separated with lateral

38

channels. We can distinguish ten formations of this kind between Włocławek and

Toruń, although some of them are currently in residual form.

At present there are two islands in Włocławek (river km 678-682): Włocławska

Island, which in 2008 was once again separated from the flood plain in consequence of

dredging (Śliwiński, 2003), and Grodzka Island, merged with the flood plain. Moreover,

between river kilometre 682 and 700, there is Krzywogórska Island and Korabnicka

Island (currently devoid of lateral branches), complex of Rachocin and Bógpomóż

Islands (fig. 8), separated with partially obstructed channels, and a small remaining

fragment of Ptasia Island approximately at river kilometre 697. Two more islands are

located between river kilometre 700 and 719, Zielona Island and Dzikowska Island,

separated from the flood plain by lateral channels (photo 9). Changes in the shorelines

of islands in the last 40 years can be traced on the maps enclosed.

According to Z. Babiński (1982), islands located at the reach under study can be

divided into central and lateral. The first are genetically linked to large central bars

formed in the Holocene. Morphological analyses clearly indicate that both Zielona

Island (photo 9) and Dzikowska Island near Ciechocinek share such genesis. W.

Juśkiewicz (2006), upon analysis of historical maps and sketches dating to the Middle

Ages, claims that Dzikowska Island reached its current shape after the construction of

flood embankments in 1872, which protect Ciechocińska Lowland. Before that, the

island was dismembered.

Islands surface slope at the reach stretching from Włocławek to Toruń ranges

from 1.0 to 2.4‰ and is considerably higher than water surface slope in the main

channel. According to Z. Babiński (1982), islands with higher surface slope are less

dismembered, for instance Zielona Island and Dzikowska Island. On the other hand,

islands with lower slopes are highly dismembered and divided into small rhomboidal

islets. This feature is displayed by Grodzka Island, Rachocin Island, Bógpomóż Island.

Relief of these formations is diverse, which is a result of frequent inundation of their

surfaces. Height differences across these formations reach several metres, for instance

on Dzikowska Island (up to 7 m).

Within the Vistula reach under study one may distinguish positive, accumulation

channel sand mesoforms, sizes of which tend to be proportional to channel's width.

Their emerged surface usually attains the level of annual mean water stages. They tend

to be highly stable and inert. They may remain in an unchanged form and at the same

39

river reach even for many seasons (Babiński, 1992). In 2008, within the reach under

study we distinguished 36 river bars – either temporal or permanent.

Photo 9. Zielona Island at river kilometre 708-712 during a flood at water flow of

approx. 6350 m3.

s.1

. Explanation: 1 – lower part of floodplain forest 2 – elevated

fragment of the island, 3 – lateral channel artificially sealed with a groyne (photography

taken on May 23rd

, 2010 from an altitude of approx. 1000 m).

Natural thresholds – outcrops for erosion-resilient formations detected in the

channel zone – constitute a separate group of mesoforms. Hydrotechnicians sometimes

refer to them as "reefs" (Polak, 1996). This appears evident from the presence of

thresholds within the river reach. They tend to have considerable sizes, comparable to

river sand bars, and their boundaries can be distinguished with the use of bathymetric

maps or aerial photographs. Their highest fragments tend to emerge during low water

stages (photo 6 and 10). E. Falkowski (1990) and T. Falkowski (2004) described similar

formations on the middle Vistula, referring to them as "culminations of alluvial

basement". The author distinguished five mesoforms of this type within the river reach

under study (fig. 13 – i). They attained their present form through "uncovering" the

upper part of an erosive (fossil) valley. The said formations consist mainly of

Pleistocene sediments, such as boulder clay and compressed coarse grains of

40

fluvioglacial sediments, and their upper parts are often covered with boulders (photo 1,

5, 19). According to E. Falkowski et al. (1987), irregular distribution of the outcrops for

the alluvial bed sediments exerts strong influence on the pattern of the mainstream in

the river channel reach, as well as the course of its erosion and deposition processes.

The authors argue that random distribution of erosion-resilient sediment outcrops

underneath alluvia is conditioned by glaciotectonic deformations of Miocene, Pliocene

and older Pleistocene layers. M. Klimaszewski (1978) describes the occurrence of

"sulay" formations in the rivers of tropical and subtropical regions. The said formations

represent rocky bends and ridges, similar in terms of morphology and development of

fluvial processes to the thresholds on the lower Vistula. Sulay formations can be found

in a wide valley floor of an anastomosing river, or one that rapidly transforms its

channel. They tend to emerge where weathering of rocks proceeds slower than within

the adjacent reaches (selective erosion). Rivers incising into a weathered bed display

low slope and even profile. Similar in the form are "parohy" – stone outcrops

(Słownik..., 1881) found on the Dnieper and Dniester rivers.

The main determinants conditioning the configuration of channel mesoforms

within the river reach under study include: geometrical parameters of river valley,

hydrotechnical structures and diverse lithology of channel bed. The analysis of channel

mesoforms was conducted in relation to three Vistula reaches distinctively different in

terms of dynamics.

41

Photo 10. The emerged fragment of the threshold located at the right bank of the Vistula

channel at river kilometre 685.5 km (approx. 10 km below the dam). The upper layer of

the formation composed of moraine clay is covered with boulders, maximum diameters

of which range between 80 and 90 cm. The photography was taken on June 25th

, 2007.

It shows a groyne field located 150 m off the right bank during temporarily lowered

flows (to 350 m3.

s-1

) at the Włocławek dam.

Reach I – (between river kilometre 675 and 696) currently devoid of channel

sand mesoforms (bars). Lithological structure of the bed indicates large diversity of

sediments in terms of resilience to erosion. However, one can distinguish five erosion

formations within the reach – thresholds. M. Klimaszewski (1978) argues that

thresholds occurring at the bottom of river channels can be one-sided, two-sided,

transverse or diagonal in relation to the river course. In this particular case, four of them

are one-sided and all of them are located closer to the right bank (photo 1, 2 and 10).

One of them displays a well developed pattern in plan view and additionally diagonally

dams 90% of channel's width (fig. 15). Two thresholds took on the form of islets

capable of withstanding erosion in the channel. One of them is located at river kilometre

686 (photo 6), and the second at river kilometre 690 (fig. 16).

42

Fig. 15. Bathymetric plan of the Vistula channel near Włocławek (river kilometre

682.5-684) of November 2009 illustrating the arrangement of bed formations and depths

during low water flows. Explanation: 1 – channel's reach before 1970; 2 – islands before

1970; 3 – river kilometres; 4 –threshold's range; 5 – groynes; 6 – direction of water

flow.

Fig. 16. Bathymetric plan of November 2009, made on a considerably narrowed reach

of the Vistula channel near Gąbinek (river km 690), showing the morphology of a

positive channel mesoform, a threshold, composed of erosion-resilient formations. The

43

arrangement of bed formations and depths was established during low water flows.

Explanation: 1 – channel's reach before 1970; 2 – islands before 1970; 3 – river

kilometres; 4 – threshold's range; 5 – groynes; 6 – direction of water flow.

Thresholds, similarly to river bars, tend to limit the area of channel's cross-

section, yet seem more stable than bars. According to R. Sołtysik (2000), the occurrence

of thresholds may locally lead to the development of a multi-channel valley floor. These

formations often act as erosive bases, which stabilize the vertical arrangement of the

channel (Falkowski et al. 1987; Habel, 2010b).

As demonstrated in the study by K. Polak (1996), thresholds tend to undergo

gradual erosion. In consequence, some of the formations currently classified as one-

sided used to function as transversal thresholds which dammed the Vistula channel – an

example here being the threshold near Grodzka Island (photo 1). The author, during his

research on the river reach under discussion conducted 15 years earlier, did not identify

as many thresholds as there are at present. This may indicate intensively progressing

erosion of the alluvia within the river reach, which results in revealing new fragments of

fossil valley’s bottom

Reach II – between river kilometre 696 and 719.8. The first channel sand

mesoforms below the dam can be found at Bobrowniki and are typical of a braided river

(photo 7, fig. 17). The genesis of the middle-central and longitudinal bars (a group of

central bars – fig. 12, set IV and V), which are considered to be the most numerous

formations at this reach, is related to river's excessive bed load and its accumulation in

the axis of the channel (Leopold, Wolman, 1957). It should be noted that central bars

tend to form during floods. Water flowing around them at low water stages tends to

underwash channel banks, which in turn leads to lengthening the bars and widening the

two surrounding channels (Leopold et al., 1964).

44

Fig. 17. Middle-central bar (1) on a braided Vistula reach between Bobrowniki and

Nieszawa during low water stages (type IV-2 – fig. 12). The emerged formation is

accompanied by submerged linguoid bars (2) (photomap of September 2005).

In September 2009, during low water stages (195 cm at the gauging station in

Toruń), a detailed survey of an exemplary formation was conducted – an emerged

fragment of one of the central bars located at river kilometre 713.5. The analysis

enabled us to characterize the selected morphological parameters of the formation.

Moreover, the emerged fragment served as an object for conducting elevation

measurements and compiling a digital terrain model. Additionally, an emerged fragment

of a linguoid horn-shaped bar was found below the front of the main formation (fig.

18). The front of the mesoform was positioned transversally to the river course and the

surface area of its emerged part at the time amounted to 1.89 ha. The difference between

relative heights amounted to 0.95 m, with a culmination in the frontal part at an

elevation of 38.44 m a.s.l. The maximum depth amounted to 131 m, and the length was

estimated to144 m. The inclination angle of the central bar was contrary to the direction

of the river course (fig. 18 – profile A-B) and its longitudinal slope amounted to 20‰.

The fragment of the linguoid bar, on the other hand, was 96 m long and 30 m wide. Its

slope amounted to 4.54‰ and was in line with the direction of the river course. (fig. 18

– profile A-B). Cubic capacity of the emerged formation, calculated from its base equal

to water level in the channel, was estimated to approximately 4 924 m3.

45

Fig. 18. Digital terrain model for the emerged fragment of the central bar at river

kilometre 713.5 along with its longitudinal (A-B) and cross-sectional (C-D) profile.

Arrows indicate the direction of water flow in the channel. Grid dimensions – 20 x 20

m,

At the wider sections of the braided channel as well as in the lower parts of the

bends, due to favourable conditions for bed load accumulation during low water stages,

the mainstream was observed to meander and new formations emerged – lateral bars.

The largest formation of this type was observed in the vicinity of Ciechocinek, near the

left bank (fig. 19, fig. 12, set III). The highest fragments of this mesoform, for most of

the year, rose above the water surface – as evident from the fact that vegetation was able

to take hold on top of it. After floods of May and June 2010, the surface of the bar

transformed. In the years 1951-1972 Z. Babiński (1982) observed lateral bars to appear

on the Vistula near Włocławek. He claims that the said formations emerged behind

obstacles such as river groynes, in perifluvial zones. At the end part of reach II, in the

vicinity of Silno, due to the drop of water surface in a cross-sectional profile caused by

a rapid decrease in regulated channel's width by approx. 30% (fig. 13 - c), intensive

deposition of bed load took place and numerous longitudinal bars along with linguoid

bars were formed (fig. 20).

46

Fig. 19. Vistula reach at Ciechocinek (river km 707-710) during low water flows,

featuring numerous bars. 1 – middle-central bars; 2 – lateral bar, 3 – Zielona Island, 4 –

lateral channel behind the island. Arrows indicate the direction of water flow (photomap

of September 2005).

Fig. 20. Boundary in terms of hydrotechnical development Vistula reach near the mouth

of the Tążyna river (km 717-720). Channel features the following formations: 1 –

middle-central bar with numerous surrounding linguoid bars; 2 – boundary line dividing

reaches of different channel types (former boundary between Russia and Prusia back in

the 19th

century). Arrows indicate the direction of water flow (photomap of September

2005).

47

Sand mesoforms occurring within this reach of the Vistula correspond in terms

of their arrangement to a braided channel and constitute central bars with numerous

individual linguoid bars. Field surveys showed that the groynes within this particular

river reach do not operate properly. They are cut off from the banks, thus hampering the

development of lateral bars and sedimentation in the basins between groynes.

Reach III – stretches between river kilometre 719.8 and 735. Its channel is regulated to

nearly constant width (fig. 13- c). There are numerous groynes at both banks of the

channel (fig. 21). At the initial fragment of the Vistula reach under discussion, between

river kilometre 791.8 and 725, no bars were observed to emerge in the year 2007-2010,

which may be a result of progressive narrowing of the channel, as well as the decrease

in the amount of bed load within the reaches located above. From river kilometre

725 km (from the highway bridge A1 in Lubicz), transverse-riffle sand bars are formed

(fig. 12 - set II 1). According to Z. Babiński (1992), the shape of the bars resemble

oblong tongues, yet they do not constitute typical linguoid bars. They tend to occupy

over 50% of channel's width (fig. 21). They do not connect with channel's banks, as

they are separated by lateral channels ranging in width from 30 to 150 m. The fronts of

the bars, which at the same time constitute their highest fragments, tend to reach the

level of average low water stages and may, albeit seldom, reach average water stages.

During low-water periods, the surfaces of the bars emerge and tend to be washed away

by flowing water. Consequently, the eroded material may form linguoid bars (Babiński,

1992). The transverse-riffle bars observed at the river reaches under study do not

occupy central part of the channel, but tend to shift towards its left or right bank (fig.

21). Additionally, if low-water periods hold for extended time, transversal bars with

"horns" may occur. Also, a point bar tends to form at the meandering river section in

Toruń (km 729-730 km), which Z. Babiński (1992) refers to as a pseudo point bar, It is

considered different, as it tends to transform during low water stages, shifting its course

from nearly central to longitudinal, with a lateral thread at the convex part.

48

Fig. 21. Regulated Vistula reach at Toruń (river km approx. 746-748). Formations found

in the channel: (1) transverse-riffle bars, (2) linguoid bars, (3) river pools. Arrows

indicate the direction of water flow (photomap of September 2005).

Transverse-riffle bars tend to accompany river pools, average depth of which

ranges from 4 to 6 m in relation to average water stages (Babiński, 1992). Within the

said erosion formations, by hydrotechnicians referred to as potholes, one may find pools

reaching depths of up to 12 m. The said pools arise due to the influence of local

obstacles, such as groynes' heads. River pools display lengths that are comparable to

transverse-riffle bars, however, their widths are lower, ranging from 100 to 150 m.

These formations are interconnected with 1-3 m deep passages (in relation to average

water stages), due to which the river displays a sinuous course during low-water periods

(Babiński, 1982, 1992). River pools at the Toruń section are generally deeper than those

at reach II.

49

4.3. Selected factors modifying channel's morphology

4.3.1. Natural factors

1) Ice phenomena

Ice phenomena on rivers cause various changes in the morphology of river

channels and flood plains (Grześ, 1991; Pawłowski, 2008). The said transformations

include, among others, pattern of thalweg in a river, shape of cross section, impact of

ice on channel's banks. As ice cover begins to form, hydraulic conditions related to

flows tend to change in consequence of a decrease in the active surface of channel's

cross section. When a channel is filled with a considerable amount of ice, the course of

thalweg tends to shift and, if possible, water runs off through a flood relief channel (or a

lateral channel, if there are island – Pawłowski, 2003). The change in the course of

thalweg also tends to occur as the ice cover decomposes. For instance, in January 1974,

on the Vistula reach at Polska Island, an ice jam occurred and the dammed-up waters

flowed across the cultivated flood plain, damaging a flood embankment in the process

(Śliwiński, 1975). P. Gierszewski (1991) described a similar situation. In 1924 an ice

jam occurred near Starzewo and Ciechocinek. His morphometric analysis of Vistula's

left-bank food plain showed that a vast erosion formation occurred – a 900 m long, 20-

50 m wide and up to 2.5 m deep two-branch through.

Instances were recorded when ice filled up to 88% of channel's total capacity

(river profile in Płock, winter 2006). At certain points ice reached all the way to the

bottom of the channel (Pawłowski, 2008). In such cases, the concentration of water flow

in a cross section of the channel leads to the occurrence of local pools (Grześ, 1999; Sui

et al., 2006). According to M. Grześ (1999), bed erosion in a 150 m wide pool may

reach up to 2 m. At the same time, eroded material accumulates at the banks.

Marked daily water stages fluctuation along with short-term changes in water

flow velocity below dams significantly hamper the process of ice cover formation

within these reaches (Hayse et al., 2000). At the Vistula reach under study, the process

of ice cover formation appears to be spatially diverse due to the Włocławek dam

operation (daily water stages fluctuation) and the influence of warmer waters from

Włocławek reservoir. In fact, up to the distance of 25 kilometres below the dam, the ice

cover does virtually not form at all. Thus, the dominant form of icing in the vicinity of

Włocławek is stranded ice on top of the banks. It is only from Ciechocinek (40 km

below the dam) onwards that morphological conditions within the channel and

50

physiochemical conditions of water become stable enough for slush-ice jams to form,

which may then gradually transform into a solid ice cover that triggers bed deformations

(Grześ, 1991). Slush-ice jams at Ciechocinek reach proved to be particularly threatening

at the time when an accumulation zone continued to exist, that is in the 80s and 90s of

the last century. Currently such threats do not occur.

b) Natural flood waves

In the last 40 years (1970-2010), a total of 46 flood waves was recorded at the

Vistula reach under study. Water flows exceeding 2400 m3.

s-1

(referred to as acceptable

flows corresponding to bankfull stages) were agreed upon as a limit value for the river

to burst its banks and flood the plains stretching between Włocławek and Toruń

(Instrukcja..., 2006). The value of bankfull discharge for the Vistula reach below the

dam in Włocławek corresponds to the results of many-year observations conducted by

S. Siebauer (1947), whose estimation amounted to 2320 m3.

s-1

.

The analysed flood waves occurred up to four times per year and lasted from 2

to 36 days – 8 days on average (fig. 22). The flood waves featured volumes ranging

from 0.42 to 13.7 million m3 (on average 2 million m

3, which is five times the capacity

of Włocławek reservoir). The maximum discharge of flood waves ranged from 2420

m3.

s-1

to 5972 m3.s

-1 (on average 2999 m

3.s

-1).

The largest flood wave in terms of duration, volume and culmination discharge

occurred in spring 1979 (fig. 22). It came with intensive thawing of snow and ice, as

winter at the end of 1978 and the beginning of 1979 was frosty and snowy. The second

largest wave in terms of duration (27 days) was the one of autumn 1974. It was caused

by intensive precipitation in the south of Poland. The third longest wave (26 days), and

the second in terms of volume (9.4 million m3) and mean discharge (4203 m

3.s

-1) was

the one of June 2010 (fig. 22). The floods in the hydrological years of 1979 and 1980

displayed the highest percentage share in annual runoff – 35 and 18% respectively –

with an average of 7,8% (Elektrownia..., 2010).

In the period under analysis, the highest frequency of floods was observed in

spring (53.3%). It tended to be lower in summer (24.5%), considerably lower in winter

(15.6%), and the lowest in autumn (6.7% – fig. 22). The waves of winter half-year were

higher in terms of volume (on average 2.2 million m3; 1.8 million m

3 in summer) and

duration (on average 7.9 days; 6.5 days in summer). The waves of summer half-year, on

51

the other hand, displayed higher discharge culminations (one third of the waves with a

peak discharge exceeding 4 000 m3.

s-1

).

Fig. 22. Flood waves on the Vistula at Włocławek-Toruń reach in the years 1970-2010

(compilation based on the data: Elektrownia..., 2010).

A – duration of floods in days, B – parameters of flood waves: V – wave volume in

million m3, Qmax – highest observed discharge during a flood in m

3.s

-1, Q2 – average

discharge during a flood in m3.

s-1

,

c) Geological structure of the valley floor

The development of channel processes depends on, among others, bed's

resilience to erosion (Klimaszewski, 1978). If the lithological structure is diverse,

sediments may be eroded selectively. According to W. Jarocki (1957), durability of a

channel depends on the compactness of bed formations. The impact of geological

structure on the morphology of alluvial river channels was thoroughly discussed in the

works by E. Falkowski (1978, 1980), and further explored by T. Falkowski (2004,

2005). River valleys lined with alluvia display a double bed: the first one being erosive,

incised in rock, and the second being alluvial, made up of river sediments, which

covered the former valley formation (Klimaszewski, 1978).

52

Research by E. Falkowski (1978, 1980) and T. Falkowski (2004) demonstrated

that thickness diversity of alluvia, as well as the distribution of culminations of alluvial

basement stand out as the decisive morphogenetic factors to shape the contemporary

Vistula channel and other lowland rivers in Poland.

Detailed study of the Vistula reach between the Włocławek dam and Toruń

resulted in a visualisation of channel bed lithology in a longitudinal profile (fig. 23),

which was compiled based on, among others, data gathered from 144 cross sections

supplemented with hydroacoustic images of the bed (fig. 3). It was thus possible to

distinguish three consecutive channel reaches that exhibit characteristic morphological

and lithological features.

Reach I – river kilometre 675-696 (from Włocławek to the Bobrowniki village). This

section displays diverse bed relief and is mostly devoid of alluvia. Only material in-situ

remained (boulders, moraine clay and its residues, silts) (fig. 3 – A and B, fig. 23).

Underneath the erosion-resilient formations (Quaternary and Pliocene in genesis) of

varied thickness, one can find Miocene formations (fine sands, silt sands and brown

coal) (Fąferek, 1960). According to Z. Babiński (1997), in 1995 the range of the river

reach amounted to 12 km. At present it is approximately 20 km.

Reach II – between river kilometre 696 and 710 (from the Bobroniki village to the city

of Ciechocinek). It features a "shifting" bottom composed of sandy sediments deposited

on top of an erosion-resilient bed, locally exposed in the form of thresholds – similarly

to the reach between river kilometre 705 and 707 (fig. 23, fig. 3 – C).

Reach III – river kilometre 710-735 (from the city of Ciechocinek to Toruń), featuring

alluvial bed composed of sand and gravel of considerable thickness (fig. 23).

As far as the reach between the dam and Ciechocinek is concerned, river

thresholds are attributed the dominant morphogenetic role. The morphometric analyses

based on figure 15, 16 and 23 indicate that pools of considerable sizes tend to form

below these formations, undercutting the thresholds and causing headward erosion,

which in turn may eventually level out the river profile at this particular reach. Where

the channel bed is less resilient, deep longitudinal incisions tend to form. Apart from the

vertical changes in the channel, one may also observe gradual deterioration of

hydrotechnical structures – their continuity is disrupted and the groynes are being "cut

off" from the banks (photo 6). The fact that the near-bank zone of the channel is highly

"constrained" by the hydrotechnical structures bears no impact on the occurrence of the

phenomenon. It is believed to be a consequence of river's drive to migrate laterally.

53

Fig. 23. Lithology sketch of Vistula channel's surface formations in relation to the

longitudinal profile of the bed. Explanation: 1 – boulders left after clay erosion; 2 – clay

and silt; 3 – sand-gravel formations; blue line marks the course of the longitudinal slope

of water surface, established at low flows between Włocławek and Toruń (compilation

based on: Detailed Geologic Map of Poland (SMGP) – sheets for Włocławek,

Bobrowniki, Ciechocinek and Toruń – as well as own hydroacoustic surveys, shallow

drills and geological exposures).

The presence of these positive erosion formations may not only influence the

changes within the channel, but also within the entire flood plain. According to M.

Grześ (1991), thresholds on the lower Vistula cause slush-ice jams to occur, which in

turn may result in jam-floods that reshape the surface of the valley floor. Another

evidence of the morphogenetic role of these formations arises from the attempt to

correlate the development of multi-branching reaches (with islands) with the occurrence

of thresholds. E. Falkowski (1990) indicates that Włocławska Island and Grodzka Island

are situated on top of an erosion-resilient pedestal. Perhaps in the past, as a result of

progressive development of the channel (incising), the said pedestals constituted

thresholds. In consequence of lateral migration, a change in the course of thalweg took

place and the threshold began to emerge. The material, which at present constitutes the

island, accumulated on its surface and in its shadow.

The locally occurring residua of fluvioglacial formations deposited on top of

variegated clay and boulder clay proved to be particularly resilient. Such formations can

be found, for instance, at Grodzka Island. On the other hand, the least resilient, apart

from the contemporary alluvia, are marginal formations, here developed into fine sands,

54

silty sands and interlayered dusts, for example at river kilometre 696 (Bobrowniki

village).

The channel along with the Vistula flood plain commonly interact with the non-

alluvial terraces (fig. 7, photo 2), which tends to influence the morphology of the near-

bank zones in the channel, as well as its capability for horizontal changes. The

typological analysis of the dynamics of lower Vistula's banks compiled by M. Banach

(1998) shows that, within the river reach under study (60 km), the banks, at a 9.5 km

long left-side section and 9 km long right-side section, are composed of Quaternary

sediments (clay, sand, silt), Pliocene deposits (dust and clay), as well as anthropogenic

elements (embankments, concrete boulevards, groynes). The remaining parts of the

banks are mostly composed of formations resulting from channel and flood deposition.

4.3.2. Artificial factors

a) Operation of the dam

In consequence of damming a channel, a reservoir is formed, in the backwater of

which the flow of water decreases to a value that is critical for bed load transport.

Consequently, the material is discharged in the form resembling a delta (fig. 24 - Ab). In

the lower basin of the reservoir, the conditions tend to be favourable for at least partial

decantation of the transported suspension (Babiński, 2002). The shortage in clastic load

transport is consequently replenished below the dam in the course of bed erosion and,

frequently, lateral erosion (i.a.: Z. Babiński 1982, 1992, 2002; A. B. Veksler and V. M.

Donenberg, 1983; G. P. Williams and M. G. Wolman, 1984; E. D. Andrews, 1986; B.

Belyj et al., 2000; K. Juracek, 2002; M. Kondolf, 2004; Z. Wang, Ch. Hu, 2004; N.

Zdankus, G. Sabas, 2006; B. Przedwojski, M. Wierzbicki, 2007; W. Parzonka, R.

Kosierb, 2010; K. Bierkovich, 2011) (fig. 24 - E). Just below the front of the erosion

zone, an accumulation reach tends to emerge (fig. 24 – Ab), which displays features of a

braided channel (Babiński, 1992; Babiński, Habel, 2009).

Intensive morphological changes in the channel below a dam tend to occur

already at the stage of its construction. According to Z. Babiński (1982, 2002), due to

the shift of thalweg in the line of dams, in other words, redirecting the energy of water

towards the banks in the initial phase of dam operation, intensive lateral erosion takes

place. In the case of the Vistula, prior to the construction of the dam in Włocławek,

55

thalweg ran near the right bank. However, after the dam was commissioned, it shifted to

the left and, in consequence, the bank of Włocławska Island retracted.

Fig. 24. Model of channel processes in an alluvial/lowland river under the influence of a

dam (Babiński, 2002): Ab – bed load deposition zone, As – suspended load deposition

zone, E – erosion zone; vectors indicate the directions of channel processes

development.

Bed erosion below dams is most notable in the direct vicinity of the structure.

For instance, bed erosion below the Hoover dam in a cross-sectional profile amounts to

an average of 7.5 m (Williams, Wolman, 1984), while below Kuybyshev Reservoir on

the Volga it may reach up to 31 m (Raynov et al., 1986). The zone of intensive bed

erosion below dams occurs at a certain reach and moves at various rates as time goes

on. The movement rate of the front of this zone is closely connected with the dynamics

of waters flowing out of the reservoir, topography and the geological structure of a

given channel bed (Kondolf, 1997). As far as the movement rate of the front of an

erosion zone is concerned, the process of erosion is limited before reaching a proper

erosion base (sea level, lake level etc., tributary of a larger river) or an erosion-resilient

bed.

According to the data gathered by Z. Babiński (2002), movement rate of the

fronts of erosion zones on alluvial rivers tends to oscillate between 0.4 and 8 km per

year. However, in the initial period of dam operation, it may even exceed 42 km a year.

The shift in channel type that accompanies the erosion reach is related to the

two-direction character of channel processes development, namely, incision of the

thalweg zone and, at the same time, covering the areas outside thalweg, lateral channels

and groyne fields in particular (Babiński, 2002). Thus, accumulation forms tend to be

left out of channel process on increasingly longer reaches, transforming into a new flood

level (fig. 25). Forms such as mid-channel bars and central bars merge into a new level

56

and are consequently replaced by point bars and lateral bars (Babiński, 2002). The

following stage in the process of stabilizing the formations allows vegetation to take

hold on their surfaces.

Certain reaches of English rivers constitute interesting examples of typological

changes in channels below dams. As a result of the attenuation of the flood wave peak

and, most importantly, reduction of discharge, which in 40-60% of cases was related to

water intake for the purpose of agriculture, the parameters of the channels, such as

width, depth and surface area of cross sections, decreased by half (Babiński, 2002).

Channel erosion below a dam is virtually impossible to stop. Hence, a number of

adverse consequences, both natural and economic, are to be expected. These may

include damaging hydrotechnical infrastructure over long channel reaches or hindrance

to navigation. Solutions are implemented to prevent and/or limit erosion processes. One

of the most promising concepts involves replenishing bed load below dams with

material transported from the upper part of a reservoir where accumulation occurs. Such

ventures were undertaken, among others, on the Danube river, below the Freudenau

Dam near Vienna (Wedam et al., 2004) and on the Sacramento river, below the

Keswick Dam (Kondolf, 1997). Another possible solution, frequently applied on

lowland rivers, involves constructing check dams transversely to the river course.

Examples of such structures include four check dams made of fascine and stone on the

Warta river, below Jeziorsko reservoir (Przedwojski, Wierzbicki, 2007), and a single

check dam made of stone and concrete below the dam in Włocławek (Polak, Rosicki,

2007).

The process of bed erosion below dams contributes to the replenishment of bed

load (and part of suspension) accumulated in the upper basin of the reservoir (fig. 24).

Frequently – albeit not always, as one can conclude from the available literature

(Babiński, 2002) – a shallower aggradation reach may emerge. The appearance of an

aggradation reach tends to coincide with a partial discharge of debris below a very

dynamic and intensively developing erosion zone. The fact that the maximum riffle of a

channel tends to occur directly at the front of an erosion zone appears to support the

claim. The said phenomenon follows the rule that transport capacity of a river tends to

break at formations of different elevation (e.g. thalweg – flood plain), or in a zone

where the river became "overloaded" with clastic load. Furthermore, American

researchers argue that the occurrence of an aggradation reach frequently results from

57

widening the channel and the appearance of new formations that are considered typical

of braided rivers, for instance mid-channel bars, central bars etc. (Babiński, 1992).

Fig. 25. Horizontal channel deformations on alluvial rivers at the reaches below dams –

from braided-anastomosing to straight. Yellow colour indicates areas of a new, lower

flood plain. According to: A – R. C. Chalov et al. (2001); B – Ch. Chiwei (1990); C – Z.

Babiński (2002).

In the case of the river reach below the Włocławek dam, upon reaching the

average annual value of bed load transport, that is approximately 0.7 million m3, which

is considered to be a threshold value for the transport capacity on the lower Vistula

(Babiński, 1992), the river "drops" it, forming a channel reach of forced accumulation.

In consequence, the Vistula river in the years 1980-1990, at the Nieszawa-Ciechocinek

reach (20-40 km below the dam), displayed features typical of a braided river –

including central and lateral bars. As indicated by Z. Babiński (1992), the surfaces of

these bars emerged to the level of up to 0.2 m above the average water stages, or – albeit

sporadically – even up to 0.4 m upon the transit of a high flood wave. Thus, during

average – and even more so at low – water stages, they may serve as a visual indicator

of the appearance of an accumulation zone below the dam.

58

b ) Channel-regulating structures

Regulating a channel of low, average and high water results in changes in the

hydromorphological features of a river valley floor. In consequence of constructing

groynes – lengths of which range from several to several hundred metres and which are

perpendicular to the banks – hydrodynamical conditions in the channel undergo

considerable changes. The said changes include straightening, shortening and narrowing

the channel (Korpak et al., 2009). The changes in the course of thalweg and river

currents cause the channel process to shift towards erosion and accumulation (Babiński,

1985). An increase in the slope and water discharge energy occurring in this type of a

'canal' causes the channel to deepen at the reach along and above the structures. The

eroded material is then usually deposited below the regulated reach (Korpak et al.,

2009), within groyne fields, or at the wider sections of the channel. According to A. K.

Teiseeyre (1991), fragmentation of thalweg, particularly during low water stages, and

consequent braidening of the river, constitute a forced mechanism of river adaptation to

the new flow conditions and increased bed load transport. Thus, the initial channel

pattern transforms into a transitional one, such as a meandering-braided channel, often

referred to as pseudo-braided (Teiseeyre, 1991) or a forced-meandering channel

(Babiński, 1985; Falkowski, 1975).

Research on the regulated channel of the upper (Łajczak, 1995; Czajka, 2005),

and middle Vistula (Warowna, 2003) appear to support the claim that the highest rate of

sediment accumulation occurs within groyne fields. Rapid sedimentation tends to

proceed until it reaches the level indicated by the peak surfaces of groynes, which

causes individual basins to merge and sets out a technical route that runs adjacent to the

main channel (similar to the situation in photo 5). Moreover, as argued by L. Starkel

(2001), Z. Babiński (1985) and B. Wyżga (1999), eventually a new level is formed,

which, given time, becomes a new flood plain. According to Z. Babiński (1992), the

reason for sediments to accumulate in between consecutive groynes arises from the

tendency of river currents to break on the heads of groynes, and of bed load to be

deposited in their "shadow". In between groynes (in groyne fields), a rotary current

tends to form, which is of vertical axis and markedly lower velocity than in the main

channel (fig. 26, photo 5). The tangential velocities of a water stream are lowest in its

centre (Babiński, 1992).

The local erosion zones which emerge in the vicinity of groynes are particularly

noticeable near the head of a groyne, as it is subjected to the strongest "attacks" from

59

the inflow side. Thus, piled up water is forced to bypass it in order to make its way into

the main channel. Moreover, at times of increased water stages, groynes take on the role

of dams or thresholds, forcing water to flow over and deepen their lower station. In

consequence, river pools with depths reaching up to 12 m tend to form on the lower

Vistula, near the head of groynes (below their bodies) (Babiński, 1992).

E > A → T1 < T2; E < A → T2 < T1

Fig. 26. Model of the course of erosion and accumulation processes in the regulated

reach of Vistula river – initial phase, according by Z. Babiński (1992). Explanation: 1 –

accumulative zone, 2 – water currents, 3 – groynes; E – erosion, A – accumulation, T -

bed load transport.

There are 291 groynes in the channel of the Vistula reach under study. Their

total length amounts to 21.16 km – 16.5 km of which is found on the unregulated route

between river kilometre 675 and 719.8, and 4.66 km on the regulated channel fragment,

i.e. between river kilometre 718.9 and 735. There are on average 3.3 groynes per one

kilometre of unregulated river. On the regulated reach, on the other hand, there are as

many as 8.7 groynes per kilometre. The average length of these structures amounts to

112 m on the unregulated reach, and 27 m at the regulated section. The groynes located

at the reach between Włocławek and Silno are highly varied in terms of length (from 20

to 280 m). However, they become more regular at the section between river kilometre

719.8 and 735 (lengths of 10 to 60 m).

The first and foremost consequence of constructing flood embankments, that is,

regulating the channel of high water, is a change in the conditions of water flows over

the river valley during considerable floods. To some extent, embankments protect

60

adjacent terrains but, on the other hand, as a result of the decrease in valley retention,

flood waves tend to be higher and their course is accelerated ( Łapuszek, Witkowska,

2006). According to K. Klimek (2008), In consequence of erecting embankments to

withstand high waters, a zone is set out at the bottom of the valley where

geomorphological changes tend to be most dynamic. There are approximately 32 km of

embankments along the river reach under study. The lines of embankments are marked

on the maps enclosed.

As indicated in the research by A. Tomczak (1987) and W. Juśkiewicz (2006),

which involved analysing historical maps, construction of flood embankments at the end

of the 19th

century on Ciechocińska Lowland caused changes in the morphology of

islands within the Vistula channel. In particular, it contributed to the vertical

deformations of the river valley – an increase in bed erosion and thickness of sediments

being accumulated during floods within the flood valley and on the surface of islands.

This vertical direction of fluvial processes allowed for, among others, stabilization of

sand islands in the channel, which at times are flooded and undergo transformations

exclusively during high floods.

There is a total of 22 km of flood embankments at the Vistula valley floor

fragment under discussion – 17 km on the left side of the flood valley (river km 683.4-

689.8, 708-718), and 5 km on the right side (river km 679-680, 711-713).

5. Changes in hydrologic phenomena on the Vistula river after

damming

Appearance of a threshold (dam) in the course of a channel, which prevents

uninterrupted gravitational flow of water, results in changes of hydrological regime both

above and below the obstacle. In the case of an artificial form of obstruction, a dam, a

number of hydrological phenomena undergo changes. The most frequently occurring

ones involve:

– a decrease in annual amplitude of water stages fluctuation both within the reservoir

and in the river channel (Wiliams, Wolmman, 1984; Dynowska, 1984; Lu, Siew, 2006,

Bierkovich, 2011),

– an occurrence of daily water stages fluctuation below the dam related to the hydro

power plant operation (Makkaveev, 1957; Williams and Wolman, 1984; Andrews,

61

1986; Babiński, 1992, 2002; Chalov et al., 2001; Juracek, 2002; Wang and Hu, 2004;

Lu and Siew, 2006; Zdankus and Sabas, 2006; Babiński, Szumińska, 2006; Bartczak,

2007).

The most anticipated consequence of dam construction is attenuation of flood

waves. Investigation on 29 American dams located on rivers with alluvial channels

shows that the average annual peak of floods decreased by 3 to 91%, 39% on average

(Wiliams, Wolnman, 1984). In the case of certain Russian dams (for instance below

Rybinsk and Sayano–Shushenskaya reservoirs), hydrological regime was reversed. In

those locations flood waves were observed to transform into low-water periods (Belyj et

al., 2000). A classic example of dam operation influence on the course of flood regime

on the river Nile is the Aswan Low Dam. Before the dam was erected, there had been a

single life-giving flood per year. However, after the dam was commissioned, the floods

attenuated to the level below the bankfull stage. The lack of floods on the Nile adversely

affected the shape and form of the cultivated flood plain. The Peace River may serve as

a similar example. The waters of this stream, upon eliminating flood waves through the

construction of the W.A.C. Bennett Dam, no longer reached the Anthabasca lake

located below, which in consequence had its water stages permanently lowered by 0.6

m (Babiński, 2002).

The natural hydrologic regime of the lower Vistula is disturbed by the dam in

Włocławek, which operates since September 1968. It is most noticeable in the direct

vicinity of the dam, however, its influence appears to extent over the entire lower reach

of the river. The following part of the chapter discusses changes in hydrological

phenomena on the Vistula which occurred after the construction of the Włocławek dam.

5.1. Changes of mean annual water stages

The interference caused by the Włocławek dam is most apparent in view of the

changes of water surface level in the channel below the dam, which are one of the

indicators for the channel incision rate. Access to a full record of hydrological data from

the years 1956-2010 gathered at two gauging stations located at the extreme points of

the river reach under study allowed for the comparison analysis of the course of mean

annual water stages in the profiles of Włocławek and Toruń (Fig. 27-A).

62

Fig. 27. Changes in the course of water stages on the Vistula river before and after

commissioning the dam: A – course of mean annual water stages in Włocławek and

Toruń profiles; B – differences between Włocławek and Toruń: 1 – including the

change in 'zero' on the water gauge in Włocławek; 2 – excluding the change in 'staff

gage zero' on the water gauge in Włocławek; 3 – water stages in Toruń ((Babiński, 1997

– supplemented with the data from the Institute of Meteorology and Water Management

National Research Institute – Internet 1).

Prior to the construction of the dam in Włocławek (1961-1965), water stages

measured at both sites had had similar values – higher by 12 cm in Włocławek (fig. 27 -

B). From 1968 to 1990 water stages in Włocławek were continuously decreasing in

comparison to the readings at the gauging station in Toruń (fig. 27 - B). After 12 years

since the construction of the dam, the difference between these two points amounted to

1.0 m, reached 1.71 m in 1995 and 1.74 m in 1999 (fig. 27 - B). The effects of changes

in water stages were most apparent in Włocławek, where certain hydrotechnical

structures ceased to fulfil their functions, including the boulevard, water intakes for

industrial establishments, port, sluice gate on the dam (photo 11).

Detailed analysis of differences in the mean annual water stages in Włocławek

and Toruń showed, apart from steady increase, certain deviations in the following years:

1969, 1972, 1992, 1993, 1994 and 2004 (fig. 27 - B). Z. Babiński (1997) claims that in

1969 (the first year of dam operation) the 34 cm increase in the mean annual water

stages in Włocławek was related to the preliminary phase of erosion in the direct

vicinity of the dam (incision of the channel). At the time, 4.6 km down the river,

material deposition and shallowing of the channel occurred in the gauging station

63

profile. In 1972 bed erosion in this section of the alluvial channel reached the maximum

depth. The later decrease in water stages was related to the movement of the erosion

zone down the river (incision the channel below Włocławek). The situation changes in

the years 1992-1994, when waters in the channel were lifted due to the emergence of

two natural thresholds. The occurrence of these forms may be interpreted as the possible

outcome of the Vistula channel alluvia erosion. K. Polak (1996) identified two

thresholds located below the dam at river kilometre 680 and 686, axes of which at that

time ran crosswise to the channel (fig. 13). After 1994 both these thresholds were

partially washed out. This further increased the difference in the mean annual water

stages between Włocławek and Toruń (fig. 27 - B). Over time, the bed erosion zone

propagating down the river revealed more thresholds, among others, at river kilometre

683 (fig. 28). Since the year 2000, this particular form has exerted marked influence on

mean annual water stages in Włocławek. The said difference between Włocławek and

Toruń decreased from -1.74 m in 1999 to -0.94 in 2004 (fig. 27 - B). The analysis of the

archival orthophotomaps and aerial photographs show that the threshold at river

kilometre 683 became active after the year 2000. The investigation of water surface

slopes appear to confirm that the threshold stabilizes water surface above the water

gauge, located approximately 3 km up the river. Changes in the bed and banks of the

channel in the vicinity of this erosion mesoform caused, among others, further decrease

in mean annual water stages in Włocławek, from -1.18 m 2005 to -1.40 m in 2010 (fig.

27 - B).

Due to incision of the Vistula channel in Włocławek and, in consequence,

lowering the mean level of water surface, in November 2004 a new reference elevation

was agreed upon for reading water stages off the water gauge in Włocławek. it was

assumed that the so called "water gauge zero" would from that point on be located at

41.12 m.a.s.l

64

Photo 11. Vistula waters "shift back" from the boulevard in Włocławek – result of

progressing deep erosion and lowering of mean water surface elevation in the channel

(photography taken by Z. Babiński – May 1995).

Fig. 28. One of several thresholds which diagonally dam the Vistula channel at river

kilometre 683 (bathymetric map – fig. 15). A-B line indicates the course of channel

depth in the crosswise profile of May 2008. At the right bank one may observe emerged

fragments of the bed, which constitute forms that are highly resilient to washing out

(Habel, 2010b).

65

5.2. Changes in daily and hourly water stages

The operation regime of the dam in Włocławek in the last 40 years can be

divided into three characteristic systems of operation:

Period I - from January 1970 to February 2002 the power plant operated at

peak-capacity - intervention mode. Such a mode of operation contributed to drastic

hourly changes in flow rate (daily water stages fluctuation) below the dam (fig. 29 - A).

Peak demand for electric energy usually occurred twice a day, between 7 a.m and 1 p.m,

as well as in the evening, between 6 p.m and 9 p.m. There were typically five cycles of

power plant operation every 24 hours. Two of them involved peak-capacity operation

(temporary max. flow of approx. 1600 m3.

s-1

), while the remaining three corresponded

to the so called baseline, which relates to the ecological flow, that is approx. 450 m3.s

-1

(fig. 29 - A). In consequence of changeable flows, water stages fluctuation occurred

below the dam, daily amplitude of which ranged from 2.0 to 3.0 m (Babiński, 1982).

Observations by Z. Babiński (1992) indicate that the maximum daily amplitudes of

water stages fluctuation occurred in the zone of increased flows, approximately 710

m3.

s-1

(water stages/mean flows zone). According to Z. Brenda (1998), flow rates in

such a range occurred at that time approximately 70% of the year. Daily fluctuation of

water stages in that period was noticeable within the entire reach down to the town of

Chełmno (125 km away from the dam). In the Fordon profile (100 km away from the

dam) the amplitudes of water stages fluctuation reached up to 50 cm (Machalewski et

al., 1974). There were additional water discharges during low-water periods for the

purpose of navigation (Babiński, 2002).

Period II - it was assumed that from February 2002 the power plant would work

exclusively in constant-flow mode, i.e. the supply of water to Włocławek reservoir was

meant to be equal to the discharge from the dam (fig. 29 - B) and the minimum

acceptable flow was to be maintained at 350 m3.s-1 (Decyzja..., 2001). However, the

provisions stipulated in the new decision were in effect only for half a year.

Period III - from September 2002 intervention-flow system had to be implemented.

For approximately 6 hours a day water discharges from the reservoir ceased entirely, i.e.

maintenance of biological flow stipulated in the permit required by Water Law Act from

2001 was breached (fig. 29 - C). The said actions were taken on workdays, usually

66

between 8 a.m and 1 p.m, excluding periods when Włocławek reservoir was fed with

large amounts of water (Komunikat..., 2007; Komunikat..., 2010).

Fig. 29. Hydrograms of the course of hourly water stages below the dam in Włocławek,

illustrating three different operation regimes of the dam:

A – peak-capacity–intervention mode; B – constant flow mode; C – repair-intervention

mode; C1 – e.g. of water supply to the lower reach of the Vistula channel by the

alimentation wave for the purpose of navigation (compilation based on the data obtained

from a digital limnigraph of Regional Water Management Authority in Warsaw -

Inspectorate in Włocławek); Q – water level corresponding to the average mean water

flows; Qbiol. - water level corresponding to the minimum acceptable flow of 350 m3.s

-1

(Decyzja..., 2001).

Such mode of operation was implemented in order to carry out maintenance works,

which included repairs of spillway, sheathing of the check dam that stabilizes the

surface of water below the weirs and the power plant, as well as filling the 12-17 m-

67

deep pools (spot incisions) that occur after flood wave flows through, directly below the

stabilizing check dam (photo 3 and 4).

The analysis of hydrotechnical data from the digital limnigraph belonging

toRegional Water Management Authority in Warsaw , which is set to constantly collect

data at the lower outer port of the Włocławek dam navigation sluice, allowed for the

estimation of daily amplitudes of water stages fluctuation on the Vistula (fig. 30). The

study involved data gathered below the dam on hourly basis in the (hydrological) years

1997-2009. The results showed that, in consequence of the peak-capacity - intervention

mode, daily amplitudes of water surface fluctuation ranged from 0.5 m to 2.0 m for 70%

of a year (in the hydrological years of 1997-2001), and from 0 to 0.5 m for 18.1% of a

year. On the remaining days the amplitudes amounted to more than 2.0 m, to the

maximum of 3.4 m. Such system of dam operation was most apparent in close vicinity

of the dam. The highest rate of amplitude in the range from 0.5 to 2.0 m occurred in the

channel up to 30 km away from the dam. Farther away fluctuation gradually minimized.

Furthermore, due to short breaks between water discharges, waves tended to overlap

and made the impact of the dam appear more perceptible at shorter distances.

From February to September 2002, as a result of constant flows, the daily

amplitudes of water stages fluctuation amounted to, on average, 0.2 m and did not

exceed 0.5 m (fig. 29 - B, fig. 30 - b). Such operation regime of the power plant was

close to the natural one. From September to October 2009, due to the implementation of

intervention-flow system of work, the daily amplitudes of fluctuation exceeded 3.0 m

(fig. 29 -C, fig. 30 - c). Over 80% of days in a year displayed amplitudes ranging from 0

m to 1.0 m, and approximately 3% - over 2.0 m. Observations showed that operation of

the dam during the repairs conducted at the lower station of the dam, which involved

limiting water discharge from Włocławek reservoir for approximately 6 hours, resulted

in occurrence of water surface fluctuation at the station in Fordon (100 km below) with

an amplitude reaching approximately 0.5 m. In such situations, at the gauging station in

Nieszawa (approx. 25 km below the dam), a several-hour long decrease in water surface

level occurred, ranging from 0.4 to 0.6 m, with a delay of approximately 5 hours in

reference to Włocławek. In Toruń, on the other hand, decreased flows were noticeable

after approximately 10 hours and caused amplitudes ranging from 0.6 to 1.2 m. The

intervention mode of dam operation during low flows exerted a particularly adverse

impact on the water environment of the Vistula, since water flow tended to be lower

than natural over a long reach of the river. At the time, considerable fragments of river

68

bed started to emerge within the bank zone (photo 1 and 2) and at the entire width of

channels located behind islands

Fig. 30. Distribution of daily water stages fluctuation on the Vistula in Włocławek,

reflecting three types of dam operation regimes in the hydrological years 1997-2009.

Explanation: a – peak-capacity – intervention mode; b – constant flow mode; c –

intervention-flow mode (compiled with the use of data obtained from the digital

limnigraph of Regional Water Management Authority in Warsaw - department in

Włocławek).

From September 2002 to the beginning of 2010 over 30 intervention discharges

of water from the reservoir were performed to increase the depth in the navigation route

for the large-size load transport on the Vistula (fig. 29 – C and C1). At the reach

between Włocławek and Silno (distance of approximately 45 km) navigation with large

vessels at the discharges lower than 800 m3.

s-1

is rendered impossible (due to bed

thresholds uncovered by erosion – fig. 15, 28, photo 6). "Water management instruction

for the dam in Włocławek" from 2006 stipulates that such large discharges may be

performed only when water flow through the hydro power plant is maintained at the rate

of 1170 m3.

s-1

, and the discharge may last no longer than 12 hours. The volume of water

discharged (an artificial small flood wave) in such situations shall amount to

approximately 30 million m3 (7.4% of total capacity or 56.6% of reservoir's useful

capacity). Before the scheduled discharge, water in the reservoir is meant to be retained

for the period of 2 to 7 days.

69

5.3. Maximum impact range of the dam

Previous researches on the Vistula related to the impact range of the dam in

Włocławek on water stages fluctuation are, in view of the author, insufficient, as they

tend to focus exclusively on a short river reach below the dam (Glazik 1978; Babiński

1982, 2002; Brenda 1998). So far a claim has been maintained that at the beginning of

the dam operation, the daily fluctuation of water stages caused by the operation of

Włocławek power plant occurred on a 200 km-long reach down the river (Machalewski

i in., 1974). Such a statement has been made in all publications dealing with the subject

of hydrology of the lower Vistula thus far.

In June 2007, during an intervention discharge of water performed to allow for

the transport of a tanker from the river shipyard in Płock to Gdańsk, an experiment was

conducted to identify the range of hourly water stages fluctuation occurrence in the

channel. Detailed research results concerning this subject were included in author's prior

publication (Habel, 2010a). This paper will discuss only the most significant issues

presented more thoroughly in the publication, as the scope of the observations extended

beyond the river reach under study and covered 12 stations on the lower Vistula – from

the dam to Tczew (fig. 32 and 33) – a 234 km-long reach of the river.

The influence range of the dam on hydrological conditions in the channel was

discussed in relation to the analysis of stage discharge curve of annual water stages. Due

to the fact that an individual supply wave may resemble a flood wave, a method

proposed by A. Ciepielowski (1987) was employed, which involves graphic analysis of

stage discharge curve of a single flood. Based on this, the following parameters of the

wave under study were characterized: wave elevation, understood as the difference

between the crest and the base of the curve; mean velocity of wave movement (in w

km∙h-1

) and supply wave crest, i.e. maximum discharge (Qmax); duration of the entire

supply (T) (in hours), defined as total time in the phase of increase (Ti) and decrease

(Td) of the wave; time ratio of wave decrease phase (Td) to wave increase phase (Ti):

maximum and mean values of hourly water stages fluctuation in the phase of wave

increase and decrease (in cm∙h-1

); total wave volume (V) (in million m3), understood as

a sum of volumes of the increase phase (Vi) and decrease phase (Vd) (fig. 31); extreme

values of average water surface slopes (Imax, Imin) (in ‰), as well as of irregularity

degree of average slopes W = Imax/Imin; mean velocity of wave movement, as well as

70

flood crest velocity (w km∙h-1

) – with the use of the simplified Kuskov formula (1) for

shallow rivers (Arkuszewski et al., 1971).

Fig. 31. Model of flood wave (Lambor, 1962 – altered). Explanation: 1 – increase curve,

2 – decrease curve, Qmax – wave crest/maximum discharge, Ti – duration of increase

phase, Td – duration of decrease phase, Vi – wave volume during the increase phase, Vd

– wave volume during the decrease phase.

v = α √g Hśr [1]

where:

Hśr – mean height of a wave in cross-section (cm),

g – gravitational acceleration, 9.81 (m/s),

α – Kuskov coefficient which takes into account channel morphology (assumed value of 0,53

for

channel roughness coefficient n ≈ 0.0325);

Values of peak flows were obtained from current discharge rate curves for the

period under study. Consumption curves were prepared based on the data of IMGW

published on www.pogodynka.pl. Wave volume was determined by means of a graphic

method that involves measurements of areas between the curve of increase, decrease

and the base of the flood wave (Lambor, 1962). Data related to total volume of water

discharge and temporary flows in the dam profile were obtained from the Regional

Water Management Authority in Warsaw - department in Włocławek.

Data analysis showed that elevation of the supply wave (the height of the wave)

did not exceed bankfull stage at any of the stations and ranged from the maximum of

183 cm in Włocławek to the minimum of 77 cm in Grudziądz (160.1 km below the

dam). In Tczew, 234 km below the dam, the culmination amounted to 91 cm (fig. 33 -

71

A). Local modifying factors clearly influenced the course and shape of the wave. The

said factors include (among others): capacity of the channel and valley to retain water,

hydrotechnical structures (in particularly groynes), channel sand mesoforms. For that

reason elevation of the wave, which should decrease downstream, increased in the

measurement profile of Silno, Korzeniewo and Tczew in comparison to the higher

located reaches (fig. 33 - A). It is result of reducing hydraulic capacity of the channel,

in particularly at the reach near Silno. The wave movement rate within the Vistula reach

under study amounted to, on average, 5.03 km∙h-1

. The highest values were observed in

the profile of Silno – 5.8 km∙h-1

, then directly below the dam – 5.7 km∙h-1

, and in

Włocławek – 5.6 km∙h-1

. At the reach between Chełmno and Tczew, wave velocity

stabilized at the level of 4.2-4.4 km∙h-1

. Results bearing great significance for defining

changes in the hydrological phenomena on the Vistula were obtained from the analysis

of movement rate of wave front along the river course. It decrease with the distance

away from the source of water discharge. The peak of the wave reached the maximum

speed (8.8-11.25 km∙h-1

) in the reach between the dam and the profile of Nieszawa.

From the profile of Silno, the movement rate of wave front halved, reaching the value of

4,9 km∙h- 1

.

As the waved lowered, the culmination flow decreased from 1170 m3.

s- 1

in Włocławek to 720 m3.s

- 1 in Tczew (fig. 32). Total time of wave run between the

profiles of Włocławek and Tczew amounted to 44 hours. The entire channel-supplying

wave proved to be shortest in Włocławek – 26 hours, and longest in Tczew – nearly 56

hours (fig. 32).

Time of concentration (growth) of a wave (counted from the moment of flow

increase to flow culmination in a stream) in the dam profile, Włocławek and Łęg

Witoszyn amounted to 10 hours, which corresponds to the duration of water discharge

from the reservoir. From water gauge profile in Silno, growth time started to lengthen:

Silno – 11 hours, Toruń – 12 hours, Solec Kujawski and Fordon-Bydgoszcz – 15 hours,

Grudziądz – 17 hours, Tczew – 34 hours. Additionally, the wave decrease time curve

tends to elongate with the river course.

72

Fig. 32. Propagation of the alimentation wave caused by the intervention water

discharge from Włocławek reservoir in June 2007 (prepared with the use of own field

observations as well as data from the limnigraph of Regional Water Management

Authority in Warsaw at the dam in Włocławek and at the port in Korzeniewo).

73

Fig. 33. Course of the selected parameters of a channel-supplying wave in the

longitudinal profile of the lower Vistula (June 25th

- 28th

, 2007). A - elevation height of

the wave in cm, B - maximum value of water level fluctuation in cm∙h-1

.

The maximum and mean values of hourly water stages fluctuation during wave

concentration (growth) were higher than at the time of wave decrease. The average

increase of water stages during the growth phase in the dam profile amounted to 16.6

cm cm∙h-1

. It was lower in Toruń – 12.0 cm∙h-1

. Similar value to the one in the dam

profile was recorded at the station in Silno – 16.1 cm∙h-1

, despite considerable distance

between these two stations. The growth rate in the Fordon-Bydgoszcz profile amounted

to 8.1 cm∙h-1

, and 3.5 cm∙h-1

in Tczew.

The maximum recorded hourly fluctuation of water level in the phase of increase

amounted to 49 cm∙h-1

in the dam profile and 48 cm∙h-1

in Włocławek. It decreased to 21

cm∙h-1

in Niszawa. Then further increased to 30 cm∙h- 1

in Silno and dropped to 20 cm∙h-

1 in Toruń. Farther downstream the value did not exceed 20 cm∙h

- 1 and amounted to

only 5 cm∙h-1

in Tczew (fig. 33-B).

Changes of hourly water stages occurred at considerably lower rate in the phase of

decrease and amounted to an average of 11.6 11,6 cm∙h-1

: at the dam – 7.2 cm∙h-1

, in

Silno - 6.6 cm∙h-1

, in Toruń - 2.8 cm∙h-1

. The highest values of the maximum drop rates

of water stages in the dam profile and at the water gauge in Włocławek amounted to,

74

respectively: 30 and 29 cm∙h-1

; in Nieszawa – 14 cm∙h-1

; Silno – 12 cm∙h-1

; Toruń – 9;

in Tczew – 4 cm∙h-1

.

This part of the analysis indicates that the supply wave in the measuring profile in

Silno, at the distance of approximately 45 km away from the dam, while constituting the

beginning of an unregulated reach, exemplifies similar dynamics to the dam profile (fig.

32, 33 - A). In the Nieszawa profile, fluctuation of water stages attenuated and the wave

was flattened. Such course of a wave at this reach is related to better conditions for

water retention in the channel (larger width) and the river valley. In the vicinity of

Nieszawa and Ciechocinek, the Vistula river flows into Toruń Basin, where the channel

is composed exclusively of sand formations (fig. 23).

Observation of the consecutive three supply waves, which occurred in 2007 on

July 22nd

, September 23rd

and October 14

, showed that propagation of waves, and the

value of hourly water stages fluctuation in particular was considerably influenced by the

initial filling ratio of the Vistula channel (immediately before the water discharge from

Włocławek Reservoir). (fig. 34).

Figure 34 shows that the higher were the water stages in the channel prior to the

discharge, the lower was the wave and its duration was shorter. On the other hand,

movement rate of a wave front increased. The said relationship becomes apparent when

comparing the course of waves of June and July 2007, which were preceded by low

water flow (approx. 450 m3.s

-1), and the wave that occurred in November, the same

year, with initial water flow that did not exceed the average values (approx. 950 m3.

s-1

).

Observation of four different waves allowed to formulate a conclusion that the impact

range of the power plant operation on the hydrologic conditions depends on the extent

to which the channel is filled with water. The higher are the water stages, the shorter is

the distance at which fluctuation occurs (fig. 34).

While assessing the influence of the dam in Włocławek on the water stages

regime of the Vistula river one may assume that during low discharges the largest

hourly fluctuation of water stages occur within the reach between the dam and Toruń

(distance of 60 km) and range from 49 to 20 cm∙h-1

. Taking into consideration that

water stages fluctuation in a large river displaying a natural course does not exceed 10

cm∙h-1

(Zdankus, Sabas, 2006), it can be assumed that fluctuation lower than the said

value is observable only further down the river, in Korzeniewo and Tczew (i.e. over 160

km below the dam). The maximum hourly fluctuation at these stations amounted to

respectively: 7 and 5 cm∙h-1

. However, at the farthest located station (Tczew), daily

75

fluctuation of water stages reached over 80 cm, while elevation of the alimentation

wave amounted to 91 cm Thus one may assume that the impact of dam operation in

Włocławek on the course of hydrologic conditions, such as hourly fluctuation of water

stages, extends to over 160 km reach downstream, while its range of influence on daily

changes reaches over 230 km downstream. So far a claim has been maintained that the

distance does not exceed 200 km (Machalewskim et al., 1974).

The research conducted on the Vistula reach near Płock by A. Magnuszewski

(2002) show that, in relation to this particular river, the maximum range of flood waves

influence on groundwater stages may reach up to 1000 m. Thus, it appears that water

stages fluctuation below the Włocławek dam also influences ground waters in the

Vistula valley over a 230 km-long reach.

Fig. 34. Hydrographs of hourly water stages at the selected gauging stations during the

passage of four channel-supplying waves, at various channel filling ratios, preceding

water discharge from Włocławek reservoir. June 1st-24

th, 2007; July 2

nd-22

nd, 2007;

September 3rd

-23rd

, 2007; November 4th

-14th

, 2007 (Bydgoszcz-Fordon – own

observations; Włocławek – data obtained from the Regional Water Management

Authority in Warsaw; Tczew – data obtained from the Institute of Meteorology and

Water Management National Research Institute in Warsaw).

76

In May 2007 the new repair-intervention regime of operation of the Włocławek

dam caused ecological catastrophe on the lower Vistula (photo 12). After supplying the

channel for the purpose of navigation it took an hour to close the deficiency of useful

capacity in Włocławek reservoir. In order to achieve it, water level below the dam was

lowered to the value of biological flow. Additionally, discharge was entirely ceased for

approximately six hours (due to the planned maintenance works at the lower station of

the dam). Overlapping of these two factors caused considerable lowering of water

stages at the reach between Włocławek and Grudziądz, which lasted approximately 10

hours. All aspects combined resulted in great fish and mollusca mortality (Chełmiak

2007).

Photo 12. Employee of a company extracting gravel from the river in Fordon -

Bydgoszcz helping molluscs which remained on shoals return to water – result of

lowering water flows at the Włocławek dam to the value below the biological flow.

May 2007 (photo taken from the archive of Nowości Toruńskie).

77

6. Changes in bed load transport and its lithological characteristics

River's capacity for bed load transport plays a significant role in shaping their

morphodynamics and channel morphology (Allen, 1965; Dębski, 1970). The lower

Vistula currently does not comprise a homogeneous fluvial system. Due to human

activity, its channel lacks typological continuity. Study showed that different channel

types display different conditions for bed load transport in terms of its size, type of

material, as well as mode of transport.

Changes in the relationship between the components of fluvial transport are best

illustrated by the process that typically takes place in flow-through dammed reservoirs.

Such reservoirs display a tendency for retaining clastic load, including entire bed load

and a considerable amount of suspension. Damming the Vistula in Włocławek caused

complete cesation of bed load transport. It was assumed that the reservoir intercepts

approximately 42% of suspension (Babiński, 2005). However, in the years 1971-1995 it

was found to intercept as much as 88% of Vistula's clastic load (Babiński, 2002).

Disruption of river load movement continuity due to the existence of an artificial

reservoir results in increased bottom erosion below the dam, and thus, river tends to

replenish the material transported (fig. 24).

The process of redeposition of sediments from the channel bottom and banks

does not fully meet the transport capacity of the fluvial system. According to the

available literature, no dammed river has ever reached the same value of bed load

transport to its mouth, as it had had before the dam was erected (Babiński, 2002). As

mentioned before, the waters of the lower Vistula may, in a humid year, transport the

maximum of 4 million tons of bed load within the reach above Włocławek reservoir

(braided-anastomosing channel - in the profile of Kępa Polska) and over 1.5 million

tons within the regulated reach in the profile of Toruń. The minima in an arid year

amounted to, respectively, nearly 1.0 and 0.5 tons. The above-mentioned data indicate a

disproportion in the amount of bed load transported and show that the smallest

differences in bed load transport between the reaches take place in arid years (2 times),

while the most considerable ones occur during humid periods (2.7 times). The reason

for this disproportion is limiting river's capacity for bed load redeposition below the

dam as a result of bed erosion (Babiński, 1994).

In order to characterise bed load, which typically consists of grains of various

sizes (Skibiński, 1976), certain indexes must be introduced, for instance: average size of

78

grain (so called graphic arithmetic average) – Mz, median value d50, as well as indexes

of sorting, kurtosis and quartz grain roundness. Acording to M. Ludwikowska-Kędzia

and E. Smolska (2007), the analysis of relationship between the basic indexes of grain

size constitutes a source of information related to the environment of deposition and its

dynamics. J. Szmańda (2010), on the other hand, claims that based on grain size of the

alluvia one may conduct indirect assessment of the rate of rank flows: shear stress

(erosion) and deposition velocity. Bottom material and the ways it reacts constitute an

integral part of river mechanism, while mechanical composition of bed sediments

transforms in time in relation to the conditions of water flow in a channel (Kaniecki,

1976).

Detailed analysis of bed load grain-size distribution in the Vistula reach under

study was conducted with the use of 36 samples collected from the fronts of sandbars

found in the channel.

Prior studies regarding grain size of bed-load material in the lower Vistula reach

were subject of PIHM analysis under the supervisory of K. Dębski (Materiały..., 1954)

and were later continued by A. Born (1958). Research on the material obtained from the

fronts of sandbars was initiated by Z. Babiński (1992). As a result of his considerations

in this field, methodology of gathering representative samples for the bed load of the

Vistula was established, which was later employed by D. Giriat (2003), who analysed

selected textural features of bed load samples obtained on the lower Vistula in order to

define the extent of influence of the Włocławek dam on sediments. However, due to the

fact that samples were collected at random only from 8 out of total 30 sandbars

occurring at the time in the river reach under study, D Giriat (2003) did not obtain

satisfactory results. A. Kaniecki (1976) and Z. Babiński (1992) emphasize the necessity

of relating the places of sampling to channel geometry and flow conditions.

River sediments collected from 36 sandbar fronts were subject of detailed

analysis focused on textural features of grain size (average diameter of a grain,

skewness, sorting). Due to the fact that the river reach under study was diverse in terms

of channel and mesoforms typology, the sediment material obtained was divided into

two groups:

I – 28 samples collected in a braided reach of the river, unregulated, highly transformed,

where sand mesoforms prevail, such as central and lateral bars, stretching between

Bobrowniki (river km 694.25), where first sand bars occur below the dam and

Ciechocinek (river km 718.5);

79

II – 10 samples collected at the regulated reach, where channel is straightened and

narrowed to a constant width, featuring numerous alternate transverse-riffle bars, which

stretches from Silno (river km 721) to Toruń (river km 735).

Both in the first (unregulated – braided) and the second reach (regulated –

straightened), sediments consists mainly of sand fraction. Its average percentage share

in the samples under study amounts to, respectively, 99.3% and 99.6%. Gravels

constitute the remaining fraction, 0.6% and 0.3% respectively. Large homogeneity of

sediments under study appears to find confirmation in the indexes of grain size.

The values of grain mean diameter (Mz) indicate differences in the dynamics

of bed load transport. For the first group of samples (reach I) Mz indexes range from

0.335 to 0.556 mm (from 1.58 to 0.84 phi), 0.401 mm (1.32 phi) on average, while in

the case of the samples collected in reach II the indexes are higher and range from 0.404

to 0.622 mm (from 1.31 to 0.68 phi), 0.477 mm on average (1.07 phi). The above

analyses indicate that in the case of both groups an average grain represents middle-

grained sands with the exception of sample 18 in reach I and three samples in reach II

(no. 28, 30-31), where Mz was of coarse sands (fig. 35 A, appx. no. 2). Grain mean

diameter in reach I amounts to 0.513 mm (0.96 phi), thus, it is 1.25 times bigger than in

the case of those found in 2008. It can be assumed that the decrease in bed load

diameters is related to the changes in water environment's energy, arising from different

geometric parameters of the channel. As far as the regulated reach (II) is concerned, the

Mz index in 1988 amounted to 0.478 mm (1.06 phi), which means it was nearly

identical to the value calculated for the samples collected in 2008. Z. Babiński (1992)

points out that at the time of his research, one particular factor tended to exert marked

influence on the development of sedimentation process. Namely, the operation of the

dam in Włocławek, which conditioned the presence of erosion and deposition zones

below the dam and, consequently, triggered changes in the average diameters of a grain

in the longitudinal profile.

J. Skibiński (1976) also indicates that in the case of the lower Vistula river, the

genetic conditions for the formation of material in which the river forms its channel (i.e.

glaciofluvial sediments) may impact the differences between the values of bed load

diameter in the longitudinal profiles.

Detailed analysis of the Mz index in the longitudinal profile of the Vistula

indicates a clear pattern – a given type of bars arises under different environment's

80

energy. And so, one may divide all 36 bars into 4 types of identified forms: braided,

lateral, central and transverse-riffle bars.

Fig. 35. Line diagram of grain-size distribution (A-C) and roundness (D) of sediments

collected from the fronts of bars in the longitudinal profile of the Vistula in June 2008.

Explanation: Mz – distribution of mean diameter of sediment grains (a) in 2008 and (b)

1988 (Babiński, 1992); SkG – asymmetry of grain-size distribution (skewness

distribution); δ1 – distribution of sorting (standard deviation); Ro – distribution of

percentage content of rounded and well round grains in a sample, according to W.C.

Krumbein's model (Mycielska-Dowgiałło, 2007).

The first two are characterised by lower mean diameter of grains that form their fronts

(in the range 0.33-0.45 mm – sample 1-5, 8-17, 21-26, 36) in comparison to central and

transverse-riffle bars (Mz in the range of 0.42-0.62 mm – samples 6 and 7, 18-20, 27-

35) (fig. 35-A). This is why the course of Mz value in 1988 and 2008 could be so

diverse at reach I. The Mz values at reach II appear to be comparable with those from

the 80s' of the last century since the channel type remained unchanged at the time of the

dam being operational.

81

The course of the Mz index in the longitudinal profile of the river in 1998 and

2008 did not reflect the typical tendency for grains to become smaller with the distance

from a dam (Williams, Wolman, 1984; Kondolf, 1997; Juracek, 2002). It is assumed

that as the river bed becomes lower below a dam, an increase in the diameter of bed

forms occurs, and that there is a nearly direct relationship between them. This means

that reduction of deep erosion in time and space is related to the formation of a bottom

that is more resilient to the process and vice versa – an increase in bed material diameter

impedes the rate of river bed lowering.

At the reach under study, in the initial phase of the dam operation, alluvia were

redeposited selectively (replenishing missing bed load) to the point when their resources

within the channel were completely depleted (fig. 3 and 23). Alluvial material

redeposited at the reach below the dam (from 0 to 20 km below) participated in the

transport. Its features are known to reflect environment’s energetic It is often argued

that the bigger the energy of water flow, the thicker on average are the elements in the

sediments. Finer subfractions tend to be more common in the environments that display

lower dynamics, while in the enviroments of greater dinamics, the characteristic

fractions usually feature larger share of coarser subfractions (Racinowski et al., 2001).

Fine sands in reach I constituted on average 4.9% of the sample. In the regulated

reach (II) – merely 1.9%. The laegest share of such sands in reach I (between 8% and

15.5%) was found in the samples that had been collected directly below the reaches of

intensive lateral erosion (samples no. 5, 12) and from the fronts of lateral and braid bars

at the terminal part of the unregulated reach (samples 22, 25). The largest share of fine

sands in reach II (3.3% and 5.1%) occurred in samples no. 35 and 36, which were

collected in Toruń, within the impact zone of bridges.

The sediment samples were also analysed for sedimentation environment

dynamics diversity index (SkG) – skewness asymmetry of grain size distribution (fig.

35-B). Negative values of SkG index indicate material enrichment in thinner fractions

and elimination of finer ones, while positive values of the index point at the enrichment

of the material in finer fractions and reduction of coarser ones.

Sediments of bar fronts at reach I display mainly negative values in the

symmetric intervals and fine skewness of the distribution (fig. 35 - B), which means

that sediment under analysis is enriched in fractions coarser than the average. This may

suggest that debris is currently in the phase of erosion or there are tendencies for bed

material to be redeposited (Racinowski et al., 2001). Positive values of the SkG index,

82

exclusively in the intervals of symmetrical or slightly coarse skewness, were found in

several samples (no. 5, 8, 9, 12, 14, 15, 16, 23 – fig. 35 - B, appx 2), which may

suggest an occurrence of conditions that are favourable for debris deposition

(prevalence of periods featuring lower dynamics of deposition environment) within a

short reach of the river, between Bobrowniki and Nieszawa (river km 698.5), as well as

in the vicinity of Siarzewo and Ciechocinek (Kozia and Zielona Islands – river km 705-

709). Slightly positive (coarse) skewness of grain size distribution and negative (fine)

skewness of the remaining samples constitute a background which allows us to

conclude that river debris is currently in the phase of massive transit (Racinowski et al.,

2001).

Sediments in reach II (regulated) show negative values of the SkG index,

particularly in the interval of symmetrical grain size distribution. Exception here being

three samples displaying positive values and belonging to the positive interval of the

distribution. First of the samples (no. 28) was collected approximately 500 m above the

highway bridge near Toruń (river km 725.5), which may indicate a favourable influence

of a hydrotechnical structure – bridge – on the deposition of debris. The remaining two

samples (no. 30 and 31 – fig. 35 - B), which show enrichment of material in finer

fractions, were collected at the reach of marked influence of a point bar in Toruń (by Z.

Babiński (1992) referred to as a pseudo point bar). According to L.B Leopold (1982), in

the upper part of the meander, conditions tend to be favourable for material deposition

due to, among others, sudden drop in water discharge rate, which triggers grains that

move in saltation or their precipitation from the suspension.

Another factor that characterizes transport and deposition dynamics is

standard deviation (δ1), which is to be understood as a measure of sediment sorting

(Mycielska-Dowgiałło, 2007), that is spread of elements in a given grain size

distribution, which shows whether sediment, in terms of grain size, is highly or poorly

concentrated in relation to the mean value (Racinowski et al., 2001). According to the

theory of A. Shields (Barnik, 1998), the level of sediment sorting affect the conditions

under which grain movement may commence. It is assumed that the better is sediment

sorted, the lower is the energetic diversity of flow regime where sediment is formed. In

other words, the lower is the rate of sedimentation, hence higher selective velocity of

currents, the better sorted is the population.

Great majority of sediment samples is moderately and well sorted (fig. 35 - C).

Sediment sorting is more diverse in reach I than in reach II, which may indicate more

83

variable morphodynamic conditions (erosion, transport and accumulation) within the

reach. The unregulated reach (I) is also highly diverse in terms of geometry and

lithology of the channel (fig. 45 and 23), which further strengthens the effects of the

dam operation, in particular the dynamics of hourly fluctuation of water stages (fig. 32,

33). One of the morphological effects of frequent surges and drops of water stages in a

channel is a progressive erosion of the emerged surfaces of bars. In time they become

lower and, consequently, bed load transport forms tend to appear on their surfaces,

representing different degrees of bed load transport intensity (photo 13).

Photo 13. Large ripplemarks on the surface of one of the bars in the vicinity of

Ciechocinek (river km 712) after a several-hour long continuous flow of water over its

surface – the result of the Włocławek dam operation. The photograph was taken in

September 2001 during low flows. Arrows indicate the direction of water flow

(photograph by M. Hojan).

Sediments collected from the bars located in the vicinity of Kozia Island and

Ciechocinek were found to be best sorted, as the spread of elements in the population of

samples in grain size distribution oscillated around the boundary value for very well and

moderately sorted sediments (fig. 35 - C). According to R. Racinowski et al. (2001),

good and very good sorting may be related to the fact that bed load in this particular

place is either in transit or in the phase of deposition. As hydrological research showed,

84

the river reach under discussion is predisposed to reduce water stages fluctuation rate

caused by the dam operation, and thus, sediment here tends to be best sorted.

Moderately sorted sediments may occur within this reach due to intensive redeposition,

hence, considerable supply of bed load as a result of lateral erosion of Kozia and Ziolna

Islands (process of bed load differentiation does not keep up with the supply), as well as

the influence of a threshold on variable dynamics of flow rates within the reach.

Most poorly sorted (moderately at most) is the population of samples collected

at the terminal part of the unregulated reach in the vicinity of Otłoczyn (fig. 35 - C),

which may by a consequence of considerable changeability of environment energy

within this section of the river. The said fact is considered a result of water lifting

caused by the narrowed reach of the regulated channel located below (fig. 13). Due to

variable flow rate dynamics, central and lateral bars are being fragmented into smaller,

oblong, emerged forms and numerous submerged fragments - linguoid bars (photo 13).

It is possible to reconstruct transport dynamics and the deposition process in the

longitudinal profile of the Vistula reach under study with the use of a method suggested

by E. Mycielska-Dowgiałło (2007), the analysis of sedimentation trends in fluvial

environment. It provides means to interpret the distribution of population in samples

under analysis by referring them to a diagram of interrelationship between mean

diameter of a grain (Mz) and sorting ((δ1), covering three tendencies of the system (fig.

36):

System 1 – sorting diminishes with the increase in grain mean diameter. The

configuration registers the occurrences of temporary increases in the transporting energy

of the.

System 2 – trend opposite to system 1. Sorting diminishes with the decrease in

grain mean diameter. It accompanies the decrease in environment’s energy.

System 3 – constant sorting, regardless of changes in grain mean diameter,

typical of environments poor dynamics and low changeability of transporting energy.

The diagram depicting relationships between Mz and δ1 in subsequent sediment

samples (fig. 36) shows that the sedimentation environment in reach I (unregulated) is

dominated by the conditions of frequent disruption of sediment sorting caused by the

increase in flow competence and the presence of deposition of mainly coarse-grained

bed load (deterioration of sorting with the increase in grain mean diameter). This

appears evident form the increase in the share of coarse-grained sand subfraction and

fine-grained gravel in the following subsequent samples under analysis: no. 1-2, 4-5, 8-

85

9, 14-15, 17-18, 19-20, 26-27). Variability of current environment dynamics may in this

case be caused by the Włocławek dam operation and, in consequence, the occurrence of

hourly water stages fluctuation within a long reach below the dam (up to 30 cm∙h-1

in

Silno and 20 cm∙h-1

in Toruń). In such conditions sediments undergo resuspension,

which often leads to deterioration in sorting of sediment being redeposited. Conditions

in which a drop in environment's energy was observed, that is deterioration in sediment

sorting with the increase of finer-grained content, were indicated in five sediment

samples (no. 6-7, 9-10, 11-12, 15-16, 23-24), in which deposition affects mainly fine-

grained debris (increase in fine-grained sand subfraction). This may point at a relatively

short transport of material triggered from the bottom at the time of greater environment

dynamics (Gierszewski, Habel, 2011).

Fig. 36. Analysis of sedimentation trends in the Vistula at the reach between

Bobrowniki and Toruń according to the model suggested by E. Mycielska-Dowgiałło

(2007) in comparison to the diagram of interrelationship between grain mean diameter

(Mz) and sorting (δ1). The analysis involved 36 sediment samples collected from the bar

fronts located on the unregulated (a) and regulated (b) river reaches. Explanation: 1 –

trend towards the occurrence of increases in transporting energy in sedimentation

environment; 2 – trend towards frequent drops in transporting energy; 3 – trend towards

stabilization of conditions – typical of an environment featuring poor dynamics.

The second trend appears to prevail in reach II, hence, there is a decrease in

competence of deposition environment and, in consequence, deposition of fine-grained

debris. This tendency is reflected in three groups of samples: 28-29, 30-31, 33-34 (fig.

36). However, sediment samples collected from the bars at a short reach of the channel

86

indicate clear record of impact exerted by the bridges located in Toruń – increases in

transport energy in sedimentation environment (trend 1) for the groups of samples no.

34-35, 35-36 (fig. 36).

Another index under analysis, which allows for the reconstruction of

sedimentation environments, dynamics and length of sediment transport at the

investigated river reach is quartz grain roundness in collected samples. The shape of

grains in sediment, regardless of their diameter and surface features, depends on many

factors, namely: initial shape of a grain, its physical and chemical features, time of

processing, character and transport energy in sedimentation environment (Mycielska-

Dowgiałło, 2007). In a river, quartz grains are not subject to further processing

(Mycielska-Dowgiałło, Woronko, 1998) but, depending on the dynamics of flows, may

undergo segregation based on their roundness (Młynarczyk, 1985).

The author visually divided grains into 9 classes of roundness according to W.C.

Krumbein (Mycielska-Dowgiałło, 2007; fig. 4). The analysis focused exclusively on the

percentage share of rounded and well-rounded grains, i.e. of roundness class ranging

from 0.9 to 0.7 (fig. 4). The results were presented in a line diagram juxtaposing 36

samples in the longitudinal profile of the river for two selected fraction ranges: 0.5-0.8

mm and 0.8-1.0 mm (fig. 35 - D). A complete disruption of bed load movement

continuity occurred, as well as replenishment of its deficiency at the reach below the

dam by means of intensive redeposition of older alluvial sediments. The material that at

present participates in fluvial transport, sediment samples with the lowest numbers in

particular (samples 1-7), can be assumed to be source sediment.

The percentage share of rounded and well-rounded grains ranges from 3% to

43% and appears to be locally diverse, especially in the unregulated reach (reach II). In

reach I, rounded and well-rounded grains constitute 18% of sediment fraction in the

range of 0.5-0.8 mm and 13% in the range 0.8-1.0m, while in reach II – 20% and 23%

respectively. This means that finer grains are likely to be more rounded in reach I, while

in reach II it is coarser grains that tend to be of greater roundness. Which brings us to a

conclusion that sediments may have been transported in different ways. This

observation allowed us to reconstruct the prevailing hydrodynamic conditions for the

deposition of sediments on the bar fronts within the river reach under discussion. It

appears that the unregulated reach provides better conditions for the selection of finer

grains (fraction of 0.5-0.8 mm). The regulated reach, on the other hand, shows tendency

for triggering and transporting coarser fractions (0.8-1.0 mm) – greater environment

87

energy. In the upper part of reach I (samples no. 1-8), there is a clear difference between

the fractions of 0.5-0.8 mm and 0.8-1.0 mm (fig. 35-D). Finer fractions are clearly better

rounded (fig. 35 - D), which may arise from the fact that they are poorly sorted – impact

of the dam operation (water stages fluctuation – water flow rates). On the other hand,

the fact that finer fractions tend to be more rounded in the terminal part of reach I may

be related to the decrease in river's transport capacity caused by water lifting (fig. 35 -

D). Considerable diversity in term of quartz grain roundness in the river reach near

Ciechocinek may result from stabilization of environment's energetics and increased

sediment deposition (slow grain selection, great degree of roundness and sorting –

samples no. 11, 15), as well as from the fact that river transport there tends to be

supplied with the material from channel banks (deterioration in roundness and sorting of

sediment – samples no. 10, 12, 16). In reach II, on the other hand, roundness of quartz

grains tend to improve linearly along the river course – due to the fact that sediments

remain in transit (long-term sediment transport) over a presumably long distance

(samples no. 26-30), until they reach the meander section, where sediment accumulation

tends to be intensified (samples 31-33; fig. 35 - D). Thus, it can be assumed that, as far

as the reach under discussion is concerned, roundness degree of quartz grain tends to

increase with the distance from the Włocławek dam (moving away from the source of

sediment material being transported).

88

7. Functioning of Włocławek reservoir and its morphological

consequences

7.1. Channel deformations

7.1.1. Changes in the longitudinal and cross-sectional profile

The basic aspects of bed erosion development below dams include: rate of

channel bed incision and movement of erosion zone front downriver (Williams,

Wolman, 1984). Most frequently, the phenomena of erosion and deposition are studied

in relation to the observations of changes in the longitudinal and cross sections of a river

channel. Comparison analysis of their morphometric parameters recorded at various

periods allows for determining changes in the morphology of a channel. As K. Klimek

(1983) emphasizes, study of changes in cross sections can provide reliable results.

However, they need to be based on several decade-long periods of observation. River

cross-sections in gauging profiles are most frequently monitored (fig. 37). K. Krzemień

(2008) claims that monitoring changes in longitudinal profiles allows for distinguishing

morphostatic and morphodynamic sections of a river. Cross-sectional profiles, on the

other hand, help identify morphodynamic zones.

Fig. 37. Changes in cross section of the Vistula channel in the gauging profile in

Włocławek in the year 1966-2009 (river-km 679.7 – #9). Source: 1966, 1969 – cross

sections obtained from Hydroprojekt, department in Włocławek; 1994 – Śliwiński,

Polak, 1995; 2009 – own measurements.

Comparison analysis of changes in hydraulic mean depth of the Vistula channel

in the cross-sections prepared prior to the construction of the dam (1969), as well as 25

89

and 40 year later (1994 and 2009), shows a steady trend – incision of the channel (fig.

37 and 38). The differences in depths tend to decrease downstream from the dam, which

seems to confirm a general tendency in bed erosion processes below dams (Wiliams,

Wolman, 1984; Chalov et al., 2001; Juracek, 2002; Wang, Hu, 2004; Berkovich, 2011).

Comparison of mean depths, calculated for the cross sections of 1969 and 2009, indicate

that in the direct vicinity of the dam (up to approx. 10 km below) the mean depth of the

channel increased on average by 3.5 m (fig. 38), At the lowest located reach, 10-20 km

away from the dam, the difference in mean depths increased on average by 2.1 m (fig.

38). Comparison of changes in relation to the reach further down the river, below

Bobrowniki, is possible only for the period of 1994-2009, as no data is available for the

preceding years. At the reach from 20 to 30 km below the dam, the channel incision on

average by approximately 0.6 m (fig. 38).

Fig. 38. Changes in time of hydraulic mean channel depths (points), measured in cross

sectional profiles of the Vistula reach under study in relation to mean water stages in

the years 1956-1970 (continuous lines denote moving average). Source: 1969 – cross

sections of Hydroprojekt, department in Włocławek; 1994 – Śliwiński, Polak, 1995;

2009 – prepared by the author.

Detailed analysis of mean depths marked on cross sections allows for the

assessment of vertical erosion below Włocławek reservoir. With the use of data from

three different periods: 1969-1994, 1994-2009 and 1969-2009 (tab. 2), it was possible to

estimate the annual rate of mean channel depth increase (later referred to as channel

incision rate).

During the 40 years of dam operation (1969-2009), the mean rate of channel

incision in its direct vicinity (reach from 0 to 5 km below) was estimated to 8.6 cm∙year-

90

1 (tab. 2). To give a sense of scale, based on the analysis of data gathered on the Oder

river below Brzeg Dolny reservoir over the period of 31 year, the said value was

estimated to 6.2 cm∙year-1

(Głowski, Parzonka,2007). On the other hand, at the initial

stage of the Włocławek dam operation, between 1969 and 1994, the incision rate

amounted to 9.2 cm∙year-1

, and in the years 1994-2009 it dropped slightly to

7.5 cm∙year-1

. At the reach further down the river, approximately between river

kilometre 680 and 685 (Włocławek – Łęg Witoszyn), in the years 1994-2009, the value

amounted to 11.1 cm∙year-1

, thus, process of erosion intensified in comparison to the

initial stage of water dam operation (tab. 2). It is often assumed that bed erosion below

dams tends to diminish in time (Wiliams, Wolmman, 1984). In this case, however, at

the reach between Włocławek and the gauging station in Łęg Witoszyn, channel

incision clearly intensified in the last 15 years (tab. 2, fig. 38). Such occurrence may

arise from the geological structure of the channel, which favourable conditions for

selective erosion (numerous one-side thresholds).

Table 2. Mean annual rate of channel incision on the Vistula at the 45 km-long reach

below the Włocławek dam in the years 1969-2009. Source: 1969 – cross sections of

Hydroprojekt, department in Włocławek; 1994 – Śliwiński, Polak, 1995; 2009 – own

measurements.

River kilometre

River reach

Distance

from the dam

in km

Increase rate of mean depths observed in given

periods (in cm∙year-1

)

1969-1994 1994-2009 1969-2009

675-680

Dam - Włocławek city

0 to 5 9.2 7.5 8.6

680–685

Włocławek - Łęg Witoszyn

5 to 10 8.2 11.1 8.6

685-696

Łęg Witoszyn - Bobrowniki

10 to 21 4.4 5.7 5.3

696-702

Bobrowniki - Nieszawa

21 to 27 - 4.0 -

702-713

Nieszawa - Łęg Osiek

27 to 38 - 1.5 -

713-720

Łęg Osiek - Silno

38 to 45 - 2.3 -

91

fig. 23). However, in the opinion of the author, it is the consequence of channel

becoming narrower due to increased incision of the midstream zone and bed load

deposition between the channel regulating structures. The said phenomenon was

particularly intensive in the 80s of the last century. As illustrated with figure 37, within

the 5 km-long reach under discussion, channel lost over 40% of its active width (fig.

38). The reach located below, for instance, between river kilometre 685 and 696 (Łęg

Witoszyn–Bobrowniki reach), features lower rate of channel incision in spite of large

number of groynes (tab. 2, fig. 38). This is possibly due to two island of considerable

size, lateral channels of which relieve the main water flow zone during floods (fig. 8).

Thus, horizontal changes within this reach of the channel are lower (fig. 39,).

Fig. 39. Changes in channel width, measured in cross-sectional profiles of medium

water channel within the Vistula reach under study (continuous line indicates moving

average). Sources: 1968 – topographic map in the scale of 1:10 000; 1995 –

orthophotomap of the Polish Geodetic and Cartographic Documentation Centre

(CODGiK); 2009 – photomap of ODGK in Lublin 2005 – supplemented with mapping

conducted by the author in 2009.

The above mentioned analyses of mean depths in cross-sectional profiles show

that in 1994 the erosion zone below the Włocławek dam extended over a 20 km-long

river reach below the dam. This seems to confirm the results of research conducted by

Z. Babiński (1997, 2002), who indicated that after 25 years, the erosion zone covered a

reach of 26 km. Z. Babiński (2002) also calculated the mean rate of erosion zone

movement to 1.1 km∙year-1

(based on: 1 – channel changes in cross sections, 2 – channel

changes in longitudinal profiles). According to the author, erosion zone in 2009 already

crossed the boundary between the regulated and unregulated reach, meaning, it

92

extended over a distance of 40 km downstream from the dam (fig. 38, tab. 2). The

results obtained appear to be convergent with the results of research conducted at the

same reach by Z. Babiński (1997, 2002). One may also draw a conclusion that the

annual rate of channel incision is locally diverse, and is highest at the reaches where

considerable horizontal changes occurred in the channel (narrowing its active width).

Due to limited data (cross sections) from the year 1969 and 1994, it is impossible to

precisely indicate when the incision of the regulated channel at the reach from Silno to

Toruń commenced.

As vertical changes progress in a channel below a dam, the river adapts to new

hydrodynamic conditions. Observation conducted below the Włocławek dam showed

that the erosion and deposition zones movement was followed by horizontal changes in

the shape of channel cross sections, which resulted from the processes of both sediment

erosion and deposition.

Figure 39 presents the dynamics of channel width changes in the longitudinal

profile of the Vistula reach under study in the following three periods: 1969, 1994,

2009. During the initial stage of the Włocławek dam operation (1968-1995), the

horizontal changes mainly involved a decrease in channel's active width (fig. 39, tab. 3).

In the periods of increased water stages, intensive deposition of sediments occurred

between the groynes and in the channels located behind the islands. At that time, a

decrease in channel width was observed at the river reach under study by approximately

16%. Most significant changes affected the reach between Włocławek and Łęg

Witoszyn (fig. 39, tab. 3), where channel width decreased on average by 212 m. Thus,

the rate of channel narrowing amounted to mean 8.5 m∙year-1

.

In the later years (1995-2009) changes in the Vistula channel width were not as

rapid. However, at the time, process of channel widening commenced in consequence of

lateral erosion. As Z. Babiński (2002) suggests, such phenomenon tends to occur only

later on during water dam operation. While the banks at the river reach under study tend

to erode over short channel sections, the mean rate of width changes calculated for the

entire river reach under study showed that the active flow zone in the channel actually

continued to narrow (by 0.2 m∙year-1

) (tab. 3). Most intensive broadening of the channel

in that period occurred at the reach between Bobrowniki and Nieszawa, particularly

from river kilometre 694 to 699, where both the old and the new level of the flood plain

were subject to lateral erosion. As Z. Babiński (1982, 1992) indicates, transformation of

channel morphology below the Włocławek dam triggers changes in the course of

93

thalweg, which in turn results in lateral erosion of the channel. The process tends to be

most intensive within the erosion section of the channel, upon the formation of erosion-

resilient bed and uncovering river thresholds. In the region where mesoforms occur, the

banks of the flood plain and islands are exposed to intensive undercutting (photo 14).

The process proved to be particularly strong at the following reaches: between river

kilometre 683 and 684 (fig. 28), at river kilometre 686 km (photo 6), between river

kilometre 690 and 696, between river kilometre 705 and 706.

Table 3. Mean annual rate of width changes in the Vistula channel active cross section

at the reach from the dam in Włocławek to Toruń in the years 1968-2009. Sources: 1968

– topographic map in the scale of 1:10 000; 1995 – CODGiK orthophotomap; 2009 –

photomap of ODGK in Lublin 2005 – supplemented with mapping conducted by the

author in 2009.

The analysis of banks retreat rate shows that degradation, in the case of most

reaches at least, is related to undercutting and washing out caused by flowing water,

which is a consequence of forced relocation of the midstream of the river (fig. 28).

According to the research conducted by Z. Babiński (1982), the process of erosion in

the years 1973-1976 greatly affected the banks of Włocławska Island (within the reach

River kilometre

River reach

Distance

from the dam

in km

Rate of channel width changes in given periods (in

m∙year-1

)

1968-1995 1995-2009 1968-2009

675-680

dam

0 to 5 -3.7 -0.5 -4.2

680-685

Włocławek - Łęg Witoszyn

5 to 10 -8.5 -0.3 -8.8

685-696

Łęg Witoszyn - Bobrowniki

10 to 21 -2.7 -0.4 -3.1

696-702

Bobrowniki - Nieszawa

21to 27 -2.2 +0.1 -2.1

702-713

Nieszawa - Łęg Osiek

27 to 38 -1.5 -0.3 -1.8

713-720

Łęg Osiek - Silno

38 to 45 -0.1 -0.8 -0.9

94

from river kilometre 677.5 to 678), and progressed at the rate of 1.75 m ∙year-1

.

Research carried out by the author in the years 1995-2010 indicate that the banks

erosion rate of Kozia Island amounted to 2.0∙year-1

. Furthermore, recent observation of

changes in river banks between Bobrowniki and Nieszawashowed that, within the reach

between river kilometre 697 and 699, from September 2005 to November 2009, the left

bank of the channel retreated by up to 125 m (annual rate of 25 m∙year-1

). Moreover,

one of the islands, Ptasia Island, lost considerable part of its surface. The right bank is

also exposed to erosion – a fragment of the new low flood plain at river kilometre 698

in particular.

Photo 14. Left erosion bank of the Vistula channel near the threshold at river kilometre

683. Undercutting of the new low flood level, which emerged in the years 1980-1995

(photography taken in September 2007)

Another aspect that needs to be taken into account in the study of the Vistula

valley floor morphodynamics below Włocławek reservoir is the course of channel’s

longitudinal profile. Analysis of its changes in time allows for the assessment of erosion

zone and debris deposition zone movement rate below the dam (Babiński, 2002). A

series of 28 measurements of bed elevation in channel's longitudinal profiles was

95

conducted in the years 1978-2010, serving as the starting point and basic material for

the analysis.

Incision of the channel in the thalweg zone illustrates a general tendency in

fluvial processes within the Vistula reach under study. The said trend can be traced in

figure 40, which presents river longitudinal profiles and indicates extreme dates of

surveying: 1978 and 2009. The scale of the phenomenon appears to decrease

progressively with the distance from the dam (fig. 40). River bed in the thalweg was

incision the most (on average by 1.62 m) at the reach between Łęg Witoszyn, located 10

km away from the dam, and Bobrowniki (21 km from the dam). The least incision

section (on average by 0.52 m) was found at the terminal part of the river reach under

study, between Silno and Toruń (45-60 km below the dam). Changes in five separate

reaches can be analysed in the last column of table 4.

Fig. 40. Changes in river bed elevation in the longitudinal profile of Vistula's thalweg

zone between Włocławek and Toruń against the background of the moving average of

water surface profiles between the gauging stations in Włocławek, Silno and Toruń.

Propagation dynamics of the erosion and deposition zones below the dam in

Włocławek can be traced by analysing changes in the mutual position of lines in the

96

longitudinal profiles of river bed in the thalweg zone in selected years. Figure 40 shows

an erosion zone appears where the longitudinal profile lines representing data from later

years run below the profile lines from the year before. An accumulation zone, on the

other hand, tends to occur within a river reach where the lines of later years run above

the lines representing an earlier period. Figure 41 illustrates the stages of fluvial

processes below the Włocławek dam. Quantitative data of channel deformations divided

into five characteristic river reaches are presented in table 4.

Table 4. Vertical changes in the Vistula channel bed in the thalwegline of the selected

reaches below Włocławek reservoir.

Tendency of bed changes in comparison to the previous period: ↓ - erosion, ↑ -

deposition, ↔ - stabilization (transportation).

Stage I. Years 1978-1980. The course of the lines in the longitudinal profile of the

thalweg zone indicate that after 10 years since the dam commenced operation, at the

reach from Włocławek (kilometre 680) to river kilometre 700 (near Nieszawa), a bed

erosion zone was developing (approx. 25 km below the dam). Between river kilometre

10 and 20 km below the dam, bottom incision in short time on average by 22 cm, and

by 4 cm 6 km further downstream (tab. 4). Directly below the erosion reach, an

accumulation zone emerged with its front located at the 709th

km (vicinity of Łęg Osiek

– approx. 35 km below the dam – fig. 41 - A), and the bottom was covered with an

River kilometre

River reach

Distance

from the

dam

in km

Vertical changes in thalweg zone in given periods of

time (in cm).

1978-1980 1978-1990 1978-1995 1978-2009

685-696

Łęg Witoszyn -

Bobrowniki

10 to 20 -22 -33 ↓ -34 ↔ -162 ↓

696-702

Bobrowniki - Nieszawa

20 to 26 -4 -18 ↓ -48 ↓ -131 ↓

702-713

Nieszawa - Łęg Osiek

26 to 37 +18 +7 ↓ -49 ↓ -47 ↔

713-720

Łęg Osiek - Silno

37 to 45 -4 +31 ↑ +57 ↑ -55 ↓

720–735

Silno-Toruń

45 to 60 -2 +62 ↑ +51 ↓ -52 ↓

97

approximately 20 centimetre-thick layer of sediment (tab. 4). Farther reach, from Łęg

Osiek to Toruń, exemplifies vertical changes of the bottom that are typical of or close to

natural course of fluvial processes. In other words, alternating sections of minor erosion

and deposition occurred (fig. 41 - A).

Stage II. Years 1980-1990. After 20 years since Włocławek reservoir was filled, the

difference between bed elevations clearly indicate the progressive development of an

erosion zone, front of which moved 7 km downstream, near Łęg Osiek (approx. 32 km

below the dam – fig. 41 - B). The erosion zone movement rate was estimated to an

average of 0.7 km∙year-1

. From river kilometre 710 downstream, over a 25 km-long

reach, a rapidly developing deposition zone emerged, almost reaching Toruń (fig. 41 -

B), and the bottom at the reach between Silno and Toruń was covered with a 60

centimetre-thick layer of sediment (tab. 4). The zone movement rate was estimated to an

average of 2.5 km∙year-1

. Within the erosion reach, in the vicinity of Bobrowniki, a

short deposition fragment occurred (fig. 41 - B). However, the amount of deposition in

this part of the channel did not lift the bottom above the level from 1978.

Stage III. Years 1990-1995. Further development of channel bed erosion was observed.

At the reach from Silno to Toruń, sediment of the deposition zone from the years 1980-

1990 was subject to erosion (fig. 41 - C). This may serve as a premise to claim that the

erosion zone front entered the regulated reach, where five years before the front of

deposition zone occurred (approx. 58 km below the dam) (fig. 41 - C). Data in table 4

indicate that at the reach between Silno and Toruń, within thalweg zone, channel

incision by an average of 10 cm. At the reach between Łęg Osiek and Silno, a zone of

increased bed load deposition was found (fig. 41 - C), which led to the occurrence of

numerous bars that are typical of a braided river. Deposition at the terminal section of

the unregulated channel reach resulted from the fact that bed load supplied by the

erosion zone located upstream exceed river’s transport capacity (fig. 20). Assuming that

the erosion zone reached river kilometre 733 (vicinity of Toruń), its movement rate

amounted to 5.2 km∙year-1

, and, upon entering the regulated reach, increased several-

fold in comparison to the unregulated reach.

Stage IV. Years 1995-2009. The data presented clearly indicate that the erosion zone

below the dam covered the entire Vistula reach under study (fig. 41 - D). If that the

movement rate of this zone at the regulated reach amounted to 5.2 km∙year-1

, in the

period of 14 years under discussion, the front of the erosion zone would have reached as

98

far as 72 km downstream. However, it is currently located in the vicinity of Fordon (100

km below the dam) – or perhaps even further down the river.

Fig. 41. Dynamics of longitudinal profile changes of the Vistula channel below

Włocławek reservoir in the following periods: A – 1978-1980; B – 1980-1990; C –

1990-1995; D – 1995-2009. Numbers next to vectors indicate the maximum value of

vertical channel deformation in a given period.

Based on the analysis of dynamics changes in the longitudinal profiles of the

channel over a period of 40 years of the Włocławek dam operation, one can assume that

the most intensive development of the deposition zone, both in its length and the

thickness of the bed load layer covering the bottom in thalweg, occurred in the period

99

between 1980-1990 (tab. 4). The erosion zone, on the other hand, covered the entire

Vistula reach under study. The diversity of erosion and deposition zones movement

rates was not only related to the geometric features of the channel and the geologic

structure of the bottom, but also to the water flow conditions prevailing in the period

under discussion (arid years – humid years, size and frequency of floods).

The analysis of changes in channel morphology within a short, two kilometre-

long reach under observation for multiple years, allows for determining erosion-

deposition tendencies for the entire river. Comparison of changes in mean depths,

calculated from the bathygraphic curves for the selected reaches of the Vistula, allows

for, among others, tracing individual stages of the deposition zone entrance, which

continuously developed below the reach of intensive bed erosion (Babiński, Habel,

2009). The analysis includes the results of research for two Vistula channel fragments –

below Nieszawa and at Toruń.

The first investigated channel reach is located at Nieszawa, between river

kilometre 700 and 702. The first bathymetric measurements were taken by Z. Babiński

in the 80s of the last century. The author continued to survey the channel in the years

2007-2011. The publication by Z. Babiński (1992) shows that the average depth of the

channel at the reach between Włocławek and Silno, prior to the construction of the dam,

amounted to 2.2 m. The analysis of a sequence of eight depth surveys and bathymetric

plans allowed for preparing a graphic illustration of differences in bottom morphology

within the river reach under discussion (fig. 42). The results of morphometric analyses

were presented in the form of bathygraphic curves and tabulation (fig. 43).

Measurements taken in the years 1984-1985 indicate that mean depths of the

channel within this reach oscillated between 2.34 and 2.24 m. In 1987 the channel

became shallower – its average depth amounted to 1.93 m (fig. 43). This indicated

aggradation caused by the river. Figure 41 - B shows that, from 1978, a deposition zone

emerged in the vicinity of Nieszawa, below the erosion reach. Thus, it can be assumed

that the shallowing of the channel, which was observed in May 1987, was related to the

deposition of bed load directly below the front of the erosion zone.

100

Fig. 42. Bathymetric maps of the Vistula channel in the vicinity of Nieszawa. Depth of

the channel is presented in reference to water level – 418 cm at the gauging station in

Nieszawa (it is 24.41 m a.s.l.). Data from the years 1987 and 1995 – Z. Babiński, 1992;

2008 – survey prepared by the author.

Since May 1988 to July 2008, channel was progressively incision (fig. 42),

which indicates the presence and development of an erosion zone within this river reach

(fig. 41 B, C, D). Data concerning the differences between mean depths show that the

channel in Nieszawa, in the years 1988-2008, incision by an average of 1.1 m (fig. 43 -

B). Thus, the mean rate of channel incision amounted to 7 cm∙year-1

.

This particular example clearly indicates that the process of channel incision

may not always take on characteristics of a linear function. Surveys conducted in May

2008 and June 2011 showed that there was a slight decrease in mean depths at the

Vistula reach under study – by 20 cm (fig. 43 - B). This phenomenon is interpreted as

101

an effect of sediment build-up at the channel bottom during three floods that occurred in

May and June 2010 (fig. 22). Field observations allowed us to draw a conclusion that,

after 2010, the Vistula channel below Nieszawa undergone widening due to lateral

erosion. The material eroded in the process transformed into sand mesofroms that

occurred within the discussed reach in Nieszawa.

Fig. 43. The course of changes in mean depths of the Vistula channel near Nieszawa

(river km 700 to 702) presented on bathygraphic curves (A) and in the form of

tabulation for the selected morphometric parameters (B) (Babiński, Habel, 2009 –

supplemented with the data from 2011).

The second reach under analysis is located near Toruń, between river kilometre

730.5 and 732.5, and includes the regulated section of the Vistula. Three surveys were

conducted in the 80s of the last century (Babiński, 1992) and one in 2011. The

comparison of the course of bathygraphic curves and calculated mean depths of the

channel allows us to assume that mean depths of the channel were similar in the years

1985-1988, ranging from 3.02 to 3.11 m (fig. 44 - B). On the other hand, bathymetric

measurements taken in July 2011 gave the result of 3.54 m. Thus, in comparison to the

depth in 1988, the channel incision by approx. 0.5 m. (fig. 44). The course of the

bathygraphic curve of 2011 clearly indicates that depths are increasing within the range

102

of 4.5 - 9.0 metres (fig. 44 - A). The capacity of the channel, in comparison to the data

from 1988, increased by over 6%.

Fig. 44. The course of changes in relation to the bathygraphic curves (A) and the

selected morphometric parameters (B) of the Vistula channel in the vicinity of Toruń

(km 730.5-732.5). Data from the years 1985-1988 – Z. Babiński (1992); 2011 – study

by the author.

Taking into consideration the typical feature of regulated rivers, namely their

tendency to continuously lower their bottom (Babiński, 1985; Wyżga, 1993; Łajczak,

1995, Korpak et al., 2008), the average incision value of the Vistula in Toruń by appro.

0.5 m (assuming that erosion process at this reach commenced after 1995) exceeds the

effect of regulation by approx 60% (fig. 41 - D, tab. 4). Such stipulation arises from the

research conducted by Z. Babiński (1985), who argues that the development rate of

erosion processes at the regulated reach near Toruń amounts to an average of 1.05

cm∙year-1

(according to the data from the years 1888-1985).

103

7.1.2. Dynamics of bed load layer thickness changes

The channel reach under study is characterized by high dynamics of sediment

movement within the bottom zone, which was demonstrated in the analysis of bed load

layer thickness changes in longitudinal and cross-sectional profile. The said

phenomenon results from the operation of the water dam in Włocławek and arises from

the need for replenishing bed load below dams (Williams, Wolman, 1984). Erosion of

bed material ceases upon reaching silt-loam sediments, which are resilient to washing

out. Where such resilient forms do not occur, the process proceeds until the bottom of

the channel reaches appropriate gradient (Babiński, 2002).

The dynamics of vertical deformation of the alluvial channel bed are presented

in figure 45. It includes results of morphometric measurements of the channel

conducted in three consecutive periods: 1976-1980 (average value calculated from 14

measurements at various dates), 1985-1990 (average value from 12 measurements),

2007–2010 (average value from 9 measurements) (fig. 45 – B1 and B3). Each period is

presented in the form of a band chart of specific width. The above mentioned data is

demonstrated against the background of general thickness of alluvia found at the Vistula

reach under study (fig. 45 - A), interpreted as a zone between the extreme minimum and

maximum bottom elevations, measured at a given kilometre of the river in the years

1976-2010.

Alluvial rivers' capability to process sediments is highest during floods

(Rotnicki, Młynarczyk, 1989). According to E. Falkowski (1978), channels gain their

prevailing morphologic contour in the course of high, short floods, during which alluvia

are deeply processed and numerous moving pools occur. However, shallow erosion-

resilient forms, which constitute the upper parts of the fossil valley, may hinder their

free vertical movement. Thus, the highest depths in the Vistula channel are lined with

the lowest layers of alluvia, while the lowest depths coincide with the maximum riffles

(fig. 45 - A). Research by E. Wiśniewski (1976), E. Falkowski et al. (1987), as well as

the analyses of geological cross-sections of the Detailed Geological Map of Poland in

the scale of 1:50 000, show that the alluvia of the Vistula valley floor, in the

longitudinal profile of the area under study, may constitute covers varying in thickness.

In Włocławek, which is located at the initial section of the gorge fragment of the Vistula

valley (fig. 5), the channel is incised into Pliocene forms, and the fossil valley of the

Vistula is filled with sand-gravel sediment, thickness which can reach up to 20 m (fig.

104

6). A. Tomczak (1987) estimated the depth of alluvia in Toruń Basin to an average of

10-12 m, while E. Wiśniewski (1976) claims that the bottom layer of alluvia in the

vicinity of Ciechocinek can be 17 metres deep.

In view of author's research, thickness of the contemporary alluvia in the

longitudinal profile of the Vistula reach under study vary greatly, ranging from 4.8 to

10.7 m in the gorge section of the river, and from 6.0 to 9.8 m in Toruń Basin (fig. 45 -

A). The average thickness of alluvia at Toruń Basin is higher by 1.5 m in comparison to

the gorge section.

The longitudinal profile of the studied reach, for the period of 34 years under

discussion, displays a tendency of the river to process deeper alluvia both at the gorge

section of the valley and within Toruń Basin (fig. 45 – B1 and B3). Such occurrence

indicates that the channel below the dam in Włocławek is progressively shifting its

pattern and is becoming straightened. At the same time, sediments are being processed

within layers of decreasing thickness – difference between the minimum and maximum

values in this period (fig. 45 e.g. B3). Situation such as this may arise from the bed load

deficit (fig. 23). In the years 1976-1980, at the reach between Włocławek and

Ciechocinek (gorge section of the valley), the alluvia were being processed within the

layer of thickness ranging from 2.4 to 2.7 m (fig. 45 - B). In the years 2007-2010,

however, the process occurred only in the range of 1.7 to 2.3 m (fig. 45 - B3). Thus,

within 30 years, the capacity of the river to vertically transform alluvia decreased by

22%. On the other hand, at the reach between Ciechocinek and Toruń (Toruń Basin), in

the first period under discussion, the layer where alluvia were being processed displayed

thickness ranging from 2.5 to 3.9 m, and in the recent years – from 2.2 to 4.1 m, with a

general tendency to narrow the range of sediment processing by 5%. Sediment thickness

up to which alluvia were being processed in Toruń increased slightly in the years 2007-

2010 in comparison to the previous period (fig. 45 - B3).

The conducted analysis appears to support the claim that erosion and deposition

zones below the water dam in Włocławek exist and tend to change their location. The

evidence is to be found in the changes in the location of the layer where alluvia were

processed in the years 1985-1990 (B2) against the background of data from the years

1976-1980 (B1) and 2007-2010 (B3) (fig. 45). Comparison with the earlier period (B2

and B1) shows slight incision and decrease in thickness of the layer where sediments are

processed (entrance of erosion zone) at the reach from Włocławek to Nieszawa, while at

the reach from Łęg Osiek to Toruń one may observe an increase in level, within which

105

106

the river transformed its bottom (deposition zone) (fig. 45). Here, the erosion zone

appears to be convergent with the situations presented in figure 40 and 41, as well as

with the data in table 4.

The above-mentioned analysis appears to support the claim that the Vistula

channel at its george section (Włocławek-Ciechocinek) reached the erosion floor of the

Vistula valley. In other words, the alluvial cover once formed by the river waters was

removed (fig. 23). Comparison between the course of bottom elevation lines marked out

during depth surveying in November 2009 and the course of bottom layers of alluvia

shows that channel bed is covered with material displaying thickness ranging from 0.1

to 2.5 m (fig. 45). Alluvial resources within the reach between Ciechocinek and Toruń,

on the other hand, show even greater thickness, hence alluvia are processed at greater

depths.

Moreover, the analysis of thickness diversity of alluvia allows for determining a

precise borderline between the distinguished geomorphological units, i.e. between the

gorge section of the valley and Toruń Basin (fig. 5). On the other hand, identifying

lithological structure of the Vistula channel (fig. 23), along with the analysis of the

dynamics of vertical changes in processing its alluvial sediments (fig. 45), allow for

making prognosis in relation to the course of fluvial process for a few years to come.

7.1.3. Change in water surface slope

Water surface slope tends to transform when changes in flows (water stages)

occur in a river. Channel bed morphology is believed to be a decisive factor determining

its shape (Dunne, Leopold, 1978). When water stages are low, it draws close to the

channel bed slope and is more diverse. At high water stages it corresponds to an average

river gradient. During floods it may even resemble valley slope. Changes in gradients

tend to occur when there is lack of balance between the energy of flowing water and the

resistance of debris particles that form channel bed and river banks (Pasławski, 1973).

In the last 40 years, the diversity of longitudinal gradients of water surface in the

longitudinal profile of the Vistula reach under study was related to the course of erosion

and deposition processes. After only four years of Włocławek reservoir filling, within a

five metre long erosion reach, water surface gradient during average flows decreased

from 0.196‰ to 0.109‰. After over 25 years, surveys indicated a decrease to 0.077‰

107

(Babiński, 1997). Measurements taken by the author in November 2009 returned an

even lower value – 0.06‰ (table 5). As the channel depth increased in the thalweg zone

in the vicinity of Włocławek by approximately 1.62 m (after 40 years), the average

water stages decreased by a similar value (fig. 27). In consequence, the value of

longitudinal water surface gradient decreased within the entire reach under study (fig.

40). The said tendency for water surface gradient to decrease within the reach located

directly below dams is typical and proceeds in a similar way below other hydrotechnical

objects of this type in the world (Williams, Wolman, 1984, Berkovich, 2011).

The diversity of water surface gradient in the longitudinal profile of the river

reach under study is to be considered in two separate analyses. The former aims to

discuss the changes of values in longitudinal gradients during steady flow within

characteristic river segments (fig. 46 - A), the latter seeks to compare changes in the

longitudinal profile of water surface slope, measured in the thalweg of the river, in two

situations displaying different water flow values (fig. 46 - B). The following results of

the research were presented in author's earlier publication (Habel, 2010b)

Analysis I. Focuses on the values of longitudinal slopes within seven characteristic

river reaches, located between gauging stations listed in table 5. The slopes are defined

by values obtained during low water stages (560 m3.s

-1) – survey of May 19

th, 2009;

during average water stages (850 m3.

s-1

) – survey of November 3rd

, 2009; and during

flows corresponding to the bankfull water stage – survey of March 31st, 2010 (2300

m3.

s-1

). For the purpose of comparison, the chart includes data obtained from

measurements carried out by Z. Babiński (1997) in September 1995.

Data from the years 2009 and 2010 show that the lowest gradients occurred

within the reach located directly below the dam (km 674.9-679.4), and their values,

regardless of changes in the rate of flow, were close to 0.06‰. As the distance from the

dam increases downstream, the values of longitudinal slopes gradually increases,

reaching the maximum of 0.221‰ during low water stages at the reach between Łęg

Osiek and Silno (fig. 46 - A, table 5). The highest gradient value was recorded during

average water stages (950 m3.

s-1

) at the reach between Nieszawa and Łęg Osiek

(0.192‰). During bankfull stage (2300 m3.

s-1

), the river reaches the highest gradient at

the regulated reach (between Silno and Toruń) – 0.173‰, and between Nieszawa and

Łęg Osiek – 0.170‰ (fig. 46 - A, table 5).

108

On the Vistula between Silno and Toruń, regardless of water stages, longitudinal

gradients tend to be very stable and reach similar value – from 0.165‰ to 0.176‰ (fig.

25 - A). The said values are close to the ones, which occur within the entire regulated

Vistula reach (e.g. from Fordon to Grudziądz – 100 to 160 kilometres below the dam).

As the discharge of water changes in the channel, at the reach from Łęg Osiek to

Silno, a considerable shift in gradient occurs. While at low water stages it amounts to

0.221‰, during bankfull stage it decreases to 0.128‰ (fig. 46 - A). This indicates

diverse morphology of this particular river reach and, most importantly, constitutes an

evidence of accumulation of a large quantity of channel forms (fig. 20).

Table 5. Differences of water surface slope in space and time, and when changes in

flows (water stages). The values of flow relates to Toruń gauging station.

Characteristic

segments of the

river and river

kilometers

Distance

between

stations in

km

The values of the measured water surface slopes in ‰

21.09.1995

Q830 m3∙s-1

(according

Babiński, 1997)

12.08.2009

Q=520 m3∙s-1

19.05.2009

Q=560 m3∙s-1

03.11.2009

Q=950 m3∙s-1

27.11.20

09

Q=1400 m3∙s-1

31.03.201

0

Q=2300 m3∙s-1

23.05.2010

Q=6350

m3∙s-1

Zapora –

Włocławek

674,9 - 679,4

4,5

0,077

0,071

0,061

0,06

0,069

-

0,155

Włocławek – Łęg

Witoszyn

679,4 - 685,3

5,9

0,108

0,102

0,093

0,107

0,098

0,127

0,157

Łęg Witoszyn –

Bobrowniki

685,3 – 695,8

10,5

0,168

0,141

0,145

0,156

0, 158

0,159

0,142

Bobrowniki –

Nieszawa

695,8 – 702,4

6,6

0,093

0,168

0,154

0,155

0,154

0,152

0,135

Nieszawa - Łęg

Osiek

702,4 - 713,5

11,1

0,194

0,181

0,199

0,192

0, 166

0,170

0,137

Łęg Osiek – Silno

713,5 – 719,8

6,3

0,129

0,222

0,221

0,164

0,192

0,128

0,130

Silno – Toruń

719,8 – 734,7

14,9

0,165

0,164

0,165

0,176

0,164

0,173

1,91

109

110

The comparison of gradients in the years 1995 (orange dashed line in figure 46 -

A) and 2009 (red line in figure 46 - A) shows that in most cases the changes were

inconsiderable. However, two fragments represent a marked gradient increase. Within

the reach from Bobrowniki to Nieszawa it increased from 0.093‰ to 0.155‰, which

was mostly caused by marked loss of alluvia in this section. The average channel depth

increased there in the last 15 years by approx. 60 cm (tab. 2, fig. 38). Furthermore,

lateral erosion of the channel intensified (tab. 3). On the other hand, at the reach from

Łęg Osiek to Silno, the slope decreased from 0.129‰ to 0.164‰, and mean depth

increased by nearly 35 cm (tab. 2, fig. 38). At the reach from the dam to Bobrowniki,

the slope either remained unchanged or decreased slightly, which is a direct result of

stabilizing water surface by numerous exposed thresholds located at the bottom (fig. 23

and fig. 15, 16).

Analysis II. Geodetic surveys of water surface elevation (station pole every 20 m) in

the thalweg of the river, allowed for preparing a visual model of shape diversity

displayed by the longitudinal profile of water surface in the Vistula reach under study,

including the assessment of local slopes. Figure 46 - B demonstrates data from two

surveys. The former was conducted on May 19th

, 2009 at low water stages (560 m3.

s-1

),

the latter on November 3rd

, 2009 during average water stages (850 m3.

s-1

). The largest

channel mesoforms are partially emerged above water surface during low water stages

(flows), and usually become entirely submerged at average water stages. Thus, the

shape of the profile tends to be diverse during low water stages as the presence of

channel mesoforms is revealed. The profile appears more levelled out during average

water stages, although the influence of river bed on its shape is still apparent. The data

allowed for indicating five types of longitudinal profiles in relation to the reach under

study (Houghtale et al., 1996). Measurements taken during low water flows show that

the profile directly below the dam is mild, and at times even entirely flat (river km 683,

690, 696). For instance, the threshold at river kilometre 683 stabilizes water level in low

water channel within a nearly five kilometre-long reach (fig. 46 - B). While the distal

part of the largest channel mesoforms tend to feature steep profiles, short reaches

display profiles are critical, with slopes ranging from 0.54‰ to 0.60‰. In such cases

water movement tends to be rapid (fig. 46 - B, river km 684, 691, 701, 705, 709). Large

inclination of water surface at low channel depth causes triggering material in its bed

(Leopold, 1982). In consequence, the thalweg of the river cuts off channel mesoforms..

On the other hand, profiles that display shape reverse to water surface slope occur

111

below the biggest channel mesoforms (thresholds and bars). Thus, below the threshold

at river kilometre 683, over approx. 200 metre long reach, water surface is temporarily

inclined at 0.064‰ in the direction opposite to the course of the river. Below a bar of

considerable size, at river kilometre 698, the reverse-shape profile extends over a nearly

500 metre-long reach. (fig. 46 - B).

With a sudden increase and decrease of water stages in the Vistula below the dam

in Włocławek, considerable temporal surges in water surface gradients occur. It

influences, among the others, the intensity of erosion-deposition processes in a river

channel (Babiński, 1992). According to the results of research conducted by J. Skibiński

(1976) on the Vistula in Warsaw, observing water surface slopes in a river channel

simultaneously in its various profiles at times of increases or decreases in the values of

flows, allows for establishing channel morphology between subsequent measuring

profiles.

Similar experiment was carried out on June 27th

, 2007 at the Vistula reach under

study. Observation of changing water stages was carried out for 30-35 hours at six

gauging stations – dam, Włocławek, Łęg Witoszyn, Nieszawa, Silno, Toruń – during a

single channel-supplying wave discharged for the purpose of navigation (for further

discussion see subchapters 5.2 and 5.3). The results of calculations for longitudinal

slopes between these gauging stations, as well as relationships between gradients and

water stages are presented in the form of a scatter diagram (fig. 47). Each measurement

taken at hour intervals is marked in the diagram as a dot.

Analysis of the data indicates (fig. 47) that, within the reaches under study, the

amplitude of changes in water surface slope during a water rise amounted to 0.09‰ at

the dam-Włocławek and Nieszawa-Silno reaches, and was lower by nearly half at the

remaining sections. The highest gradients, among all the reaches, were found during the

initial phase of water rise (fig. 47). On the other hand, in the terminal phase of water

stages increase, during the culmination phase of a supply wave, the slopes tended to

reach their initial value. With the exception of the reaches between the dam and

Włocławek (values higher than initial), and between Nieszawa and Silno (lower than the

initial). As the supply wave waters began to fall, gradient decreased and reached a value

lower than the initial. The exception being the reach between the dam and Włocławek,

where the relationship between the slopes and water stages was affected by the sudden

cessation of water discharge from Włocławek reservoir (fig. 47). The most rapid surges

in water surface slopes, observable even when water level rose by merely 20 cm,

112

occurred at the reaches between Łęg Witoszyn and Nieszawa, and between Nieszawa

and Silno. Thus, a conclusion can be drawn that in consequence of releasing even minor

amounts of water from Włocławek reservoir, short-term yet intensive bed erosion of

alluvia is triggered at the discussed reaches of the Vistula. Z. Babiński (2008) points out

that the moment of maximum bed load traction rate during floods coincides with the

occurrence of maximum water surface slopes. Increase in the size of erosion, on the

other hand, depends on flow rate, which in turn is related to the mass of water and

channel slope (Klimaszewski, 1978). Assuming that the higher is the slope and water

mass, the larger is the intensity of erosion, one may conclude that within the 60

kilometre-long reach of the Vistula, the fragment most exposed to erosion would be the

one located in the direct vicinity of the dam (dam-Włocławek) (fig. 47). The said

relationship is less apparent at the Vistula reaches between Łęg Witoszyn and Nieszawa

(impact of thresholds), and between Nieszawa and Silno (influence of bars and good

conditions for increased channel retention). Between Nieszawa and Silno, despite the

increase in water mass, a drop in gradient occurred. This results in an increase of flows

caused by the occurrence of numerous sand bars, irregular hydrotechnical development

and existence of lateral channels behind Zielona Island and Dzikowska Island. In

consequence of decreased water flow rate, accumulation of transported river load

occurs. Moreover, earlier morphometric analyses indicated that Ciechocinek reach

displays best conditions for sediment deposition (fig. 13, 40).

113

Fig. 47. Dependency of water surface slopes (i) (in ‰) from water stages (H) (in

metres) in the Vistula channel within five characteristic river reaches. Explanation: A –

water rise stage, B – fall stage. Data of June 24th

, 2007 – obtained during intervention

discharge for the purpose of navigation.

114

8.2. Flood plain development

The flood plain of the lower Vistula reflects a tree-step development of a valley

floor in the Holocene: a braided river transforms into a meandering one, just to go back

to its braided pattern during the last centuries (Falkowski, 1978, Andrzejewski, 1994).

According to the research by E. Wiśniewski (1976), the accumulation phase in the

Vistula valley, during which the fossil terrace was filled and the older level of the flood

plain was formed, took place either during the Subboreal period (6000-2500 BP), or in

the first half of the Subatlantic period (2500-1250 BP).

The flood plain constituted the lowest level in the system of river terraces

proposed by E. Wiśniewski (1976). During the last 200 years, due to increased

anthropopression, its morphology and area coverage changed considerably. Within the

reach under study, it is discontinuous, on one or both sides of the gorge section

(Włocławek-Ciechocinek reach), and becomes continuous within Toruń Basin (fig. 13 -

a, photo 8).

At present, within the reach from Włocławek to Bobrowniki, the Holocene flood

plain constitutes the older, higher level. Its right-bank fragment in Włocławek is 250

metres wide and runs approximately 7 km downstream from Włocławek (fig. 7 – level

I). Its structure is composed of overbank deposits, as well as peat and fen soil. Structural

drilling conducted up to the depth of 4 m revealed their presence at the depth of 2-3 m

below the surface (Wiśniewski, 1976). Level I can also be found below Włocławek, in

the vicinity of Korabniki and Gąbinek villages, where it reaches 4-5 m above the

average level of the Vistula (approx. between river km 685 and 686) and is located at

the elevation 47 m a.s.l. (fig. 7). The youngest flood sediments formations can be found

at the depth of 2-4 m. The diameter of grains increases with depth. Moraine clay is

deposited at the depth of 21 m (Wiśniewski, 1976). The next fragment of the flood plain

is found in the vicinity of Bógpomóż, Bobrowniki and the mouth of the Mienia river.

From the place where the Vistula flows into Toruń Basin (Otłoczyn-Silno), the older

flood plain, which accompanies the river continuously and which can be up to 2000 m

wide (photo 7 and 8). The largest number of drills within the flood plain was performed

in the vicinity of Ciechocinek. Flood formations occur, namely fen soils, silts and fine

sands, occur up to the depth of 4-5 m. Geological structure frequently includes

organogenic sediments (Wiśniewski, 1976). The said formations are either deposited on

the surface, or covered with a layer of flood sediment. According to E. Wiśniewski

115

(1976), deposition of fossil organogenic and marginal formations indicates the presence

of Vistula fossil valley terraces. Most probably, these sediments were formed within the

old cut-off river channels. The flood plain in Toruń (approx. 735 km) is composed of

5.0-7.0 metre-thick complex of fine and silty sands. Underneath, within the entire width

of the plain, a layer of cobblestones is deposited, and further below one can fine

Pliocene clays and clayey formations (Grobelska, 2002).

Flood plains, unlike river channels, are subjected to the influence of flood waters

only for short time (Babiński, 1990). However, it is enough to change their surface. Due

to the operation of the dam in Włocławek, a new anthropogenic flood plain level

emerged (fig. 48), referred to by Z. Babiński (1992) as the new, lower flood plain.

L. Starkel (2001) and B. Wyżga (1999), among others, studied the low flood plain of

Carpathian tributaries of the Vistula river, which emerged in consequence of sediments

deposition in between groynes and was further stabilized by vegetation. Nevertheless,

the rate of development of the new Vistula flood plain directly below the dam in

Włocławek appears to be higher than in the case of the regulated Carpathian rivers

discussed by those authors.

Already after a year since the dam had been commissioned, material eroded

from the channel bed within the thalweg zone was found to accumulate in lateral

channels, behind islands and in between regulatory structures. As the erosion zone

moved down the river, sediment was found to accumulate in the near-channel zone.

According to Z. Babiński (1992), after 20 years of dam being operational, a new level of

flood plain was formed at the Vistula reach in Włocławek. (fig. 25). Currently, this

process occurs up to river kilometre 698, in other words, within a 23 kilometre-long

reach below the dam. The former flood plain in this river fragment transformed into a

floodplain terrace. The subject of flood plain development on dammed rivers has been

discussed in numerous research papers. (Chiwei, 1990; Kondolf, 1997; Wyżga, 1999;

Juracek, 2002; Ruleva, Zlotina, Berkovich, 2002; Starkel, 2001).

During the Włocławek dam operation flood waves entered the surface of flood

plain for an average of 7.3 days a year (fig. 9). In the years 1983-1992, inundation lasted

merely 6 days, which prevented, among others, entrance of vegetation and stabilization

of forms on its surface. The best developed section of the new low flood plain is located

below the dam (from the dam to Łęg Witoszyn). It was emerging in the conditions of

peak-intervention mode of hydro power plant operation (from 1970 to 2002) – currently

it operates in the intervention-flow mode.

116

Fig. 48. Formation of the new Vistula flood plain level in Włocławek at the reach

between river kilometre 681 and 683. Explanation: a and b – fragments of

Krzywogórska Island, 1-6 – river groynes (photo: 1988 – Z. Babiński; 2008 – author).

The first regime – peak-intervention – featured large frequency of water stages

fluctuation occurrence in the range of 1.5-2.0 m and 2.0-2.5 m (fig. 30 – a and 31 – A),

which resulted in frequent, short-term inundation of areas that currently belong to the

new flood plain. Z. Babiński (1982) points out that, especially in the first years of the

dam operation, the number of days when the flood plain was inundated quadrupled. B.

Wyżga (1993), during his research on changes in the regulated bottom of the Raba river

valley (tributary of the upper Vistula), observed that a decrease in the mean annual and,

117

in consequence, maximum water stages in relation to the height of river banks limited

the frequency of flooding. Morevoer, B. Wyżga (1993) noted that, as the channel

became deeper, given water stages were reached at increasingly larger and less frequent

flows. Similarly, this particular observation can be related to the Vistula reach under

study, where one can note a drastic change in mean water stages at Włocławek (fig. 27).

For instance, water level of 230 m (mean value estimated from the years 1969-2000), in

the period preceding the construction of the dam, was observed at the flows ranging

from 320 to 380 m3∙s

- 1. In recent years, however, the same water level tended to occur

at flows exceeding 1200 m3∙s

- 1 (three times larger channel capacity of the Vistula in

Włocławek). In consequence, the new flood plains becomes inundated at flows

exceeding 1400 m3∙s

- 1.

The newly formed flood plain at the right bank of the Vistula, width of which

ranges from 120 to 300 m, reaches 0.1-0.3 m above the average water table elevation

(depending on local conditions), and tends to be lower by 1.0-2.0 m than the surface of

the old flood plain. It developed into lath-shaped forms on both sides of the channel in

Włocławek and surrounds the following islands: Włocławska, Grodzka, Krzywogórska,

Korabnicka, Rachocin and Bógpomóż.

Field research showed that the new level is composed mainly of sedimentary

channel facies, thickness of which is diverse and ranges from 0.1 to 3.0 m (fig. 48,

photo 14). The surface, on the other hand, is covered with a 0.3-0.8 metre-thick layer of

flood sediments. In the lower parts of the channel embankments and post-flood basins

one may encounter channel sediment, tills and Pliocene clays, often covered with

boulder pavement. (fig. 49, photo 1). At a number of reaches, the emerged bottom of the

Vistula constitutes the proximal zone of flood level.

The decrease in frequency of water occurrence on the surface of the new flood

plain contributed to strengthening of vegetation and preservation of accumulated flood

sediments in the form of several grass-covered levees, height of which ranges from 0.5

to 0.8 m (fig. 49, fig. 50). They are located at the distance of 120 m away from the

current edge of the active channel. The said levees are likely to mark consecutive

episodes of the Vistula channel incision, thus contributing to the process of widening

the new flood plain (fig. 49). Studying their sedimentary structure allowed for

determining qualitative and quantitative changes in transported debris. According to K.

Teisseyre (1988), morphology and lithological structure of near-channel levees reflect

118

river's hydrologic regime, in particular, the frequency of high and low floods, as well as

concentration of suspension and grain size of suspended material carried by the floods.

Sediments of the grass-covered levees (formation type identified by K.

Teisseyre, 1988) form a characteristic set of laminate with alternating layers of fine

sands, silts and clayey silts (fig. 50). Also, minor longitudinal natural levees and

willow-covered levees tend to occur in the direct vicinity of the channel. However,

these formations are not as enduring as near-channel levees and are being washed out

during higher water stages.

Fig. 49. Morphologic outline of a right-bank fragment at the new flood plain in

Włocławek (river kilometre 679-681). Explanation: 1 – grass and willow-covered old

natural levees and their sediments (fig. 50); 2 – near-channel young natural levees and

their sediments; 3 – boulder pavement on the surface; 4 – backswamp and their

sediments; 5 – crevasse channels of backswamps; 6 – distal part of the new plain; 7 –

groynes; 8 – flood embankment; 9 – floodplain terrace; 10 – higher terrace beyond the

reach of flood waters.

119

Fig. 50. Natural levee (A) at the distal part of the new flood plain in Włocławek

(approx. 80 meters away from the Vistula channel) with lithological cross-section (B)

with structure of sediments that form them and lithofacial profile (C). Explanation: 1 –

levee, 2 – proximal part flood plain, 3 – distal part of flood plain, C – organic matter,

Fm – massive mud lithofacie, SFm – massive sandy silt lithofacie, Sm – massive sand

lithofacie, Gm – massive gravel.

120

8. Summary

Geomorphologic, hydrologic and sedimentologic research conducted of the

Vistula valley floor below Włocławek reservoir indicated that after 40 years since the

dam was commissioned, it has undergone considerable transformations. The changes in

the valley floor are very anthropogenic and included, among others:

– increased diversity of channel typology,

– noticeable changes in the hydrologic regime of the river on over 230 km-long

reach,

– prevalence of erosion over river debris aggradation within a 60 km-long reach

below the dam.

The maximum range of impact of the dam operation is impossible to define, as

the direct influence of the dam on the character of sedimentation structure of river bed

sediments is markedly modified by factors that may locally occur within the river

channel.

One may add that in the period from 1969 to 2011, at the reach under study, no

works were conducted to obtain aggregate from the channel, nor dredging Thus, the

results of morphologic research conducted by the author may be regarded as highly

credible.

I. The operation of the Włocławek dam is believed to disturb the natural

hydrologic regime of the lower Vistula, which finds its reflection in daily and hourly

water stages fluctuation. The largest hourly water stages fluctuation is observed within

the 60 km-long reach below the dam (i.e. down to Toruń). In the direct vicinity of the

dam (up to 5 km below) their maxima amount to 49 cm∙h-1

and 20 cm∙h-1

in Toruń. At

the gauging station in Tczew, which is located 230 km away from the dam, they reach

the maximum of 5 cm∙h-1

. The maximum daily water stages amplitudes, on the other

hand, amount to approximately 200 cm in Włocławek, 130 cm in Toruń, and 90 cm in

Tczew – that is 200 km away from the dam. Based on this premise one may assume

that the actual influence range of the dam on the course of hydrologic conditions

stretches 230 km downriver, and it is highly probable that it extends to the very mouth

of the Vistula, i.e. 260 km below the dam.

121

II. The analyses of movement rate of erosion and deposition sections within the

contemporary channel (in terms of depth changes both in cross sections and in

longitudinal profiles) show that after 20 years, the erosion zone stretches over an

approximately 30 km-long reach below the dam, and its mean rate of movement was

estimated to 1.1 km∙year-1

(Babiński, 1992). Upon reaching the regulated reach of the

Vistula, narrowed to a constant width, the propagation rate of the erosion zone increased

nearly fivefold (to 5.2 km∙year-1

). In the years 1980-1990, an intensive aggradation of

debris was observed between Ciechocinek and Toruń (40-60 km below the dam), which

was related to the intensive development of debris erosion processes in the higher-

located reach. Movement rate of the deposition zone downriver in the years 1970-1990

amounted to mean 2.5 km∙year-1

. For instance, debris aggradation in the reach between

Silno and Toruń was so intensive that the bottom in thalweg was on average covered

with a 60 cm-thick layer of sediments. At present, the erosion zone was found to mark

its presence as far as below Toruń, that is over 60 km away from the dam.

III. In the years 1970-1995 the development of Vistula's valley floor was observed to

progress in two directions. On the one hand, the active channel incision (at the rate of

approx. 8.7 cm∙year-1

within a 10 km-long reach below the dam). On the other hand,

debris was deposited in the overbank zone (in lateral channels and in between groynes,

as well as on the flood plain). Currently, the development appears to be unidirectional,

as the active channel zone tends to deepen further down the river (at an annual rate of

5.7 cm∙year-1

, for example at the reach between km 10 and 20 below the dam) and

simultaneously reduces its width (by 0.4 m∙year-1

). In the last 40 years, the Vistula

channel in Włocławek lost 40% of its width, that is an average of 221 m. After over 40

years, the average channel depth in a cross-section increased on average by 3.5 m at the

reach stretching up to 10 km away from the dam, by 2.1 m at the reach between 10 and

20 km below, by 1.6 m at the reach from 20 to 30 km further down the river and

approximately by 0.5 m in Toruń, that is 55-60 km below the dam.

IV. In the initial period of dam operation, at the reach located in the gorge section of the

Vistula valley (from Włocławek to Nieszawa), the channel incision to the point of

reaching the erosion-resilient upper part of the fossil valley. At that time, the zone

outside the active channel was being overlain with sediments, which resulted in the

122

formation of a new flood plain. Currently, the process of bed erosion further progresses.

In addition, the material deposited within the overbank zone also undergoes erosion.

V. The river section under study, prior to the construction of the Włocławek dam, was

divided into two types of channels: 1 – braided-anastomosing and 2 – regulated,

straight. After more than 40 years of functioning of the reservoir, the river reach under

discussion became even more diverse in terms of typology. At present we can

distinguish three types of channels: 1 – from the dam to Bobrowniki (20 km-long reach)

– straightened channel, confined vertically by the river bed that varies in terms of

resilience to erosion and vertically, by chaotically occurring hydrotechnical structures

(groynes); 2 – from Bobrowniki to Silno (20 to 45 km away from the dam) – braided

channel, which becomes braided-anastomosing during high water stages; 3 – from Silno

to Toruń (45 to 60 km below the dam) regulated channel, straight, with sinusoidal

course of midstream during low water flows.

VI. As demonstrated in the research, the deficiency of bed load in total debris transport

below the dam bears an adverse effect on the hydrotechnical structures located within

the erosive reach from Włocławek to Toruń. The development of erosion may prove to

be threatening, as one may conclude from, among others, destabilization of the devices

located at the lower station of the dam, i.e. more intensive flow of infiltration water

through the earthen face dam, scoured road bridge span in Włocławek, or uncovering

the crown of the PERN pipelines and the Yamal–Europe natural gas pipeline now

buried below the river bed. In December 2007, a PERN oil pipeline was unsealed,

which led to an ecological catastrophe on the Lower Vistula. Further development of

bed erosion may severely damage devices located on the valley floor below the dam

(electric wires, oil pipeline and gas pipelines, as well as regulatory structures –

groynes).

VII. Analysing the present and forecast course of erosion-deposition processes on the

Vistula river below Włocławek reservoir, one may conclude that in fifteen or so years,

current deposition zone in Solec Kujawski, which lies 72 km away from the dam, will

cease its development, giving way to erosion processes. Aggradation of the river bed is

bound to continue its progress 100 km away from the dam. One may also expect that

thresholds, constituting the culmination of the alluvial bed, will play the decisive role in

123

shaping vertical and horizontal changes in the Vistula valley floor. Thus, the most

considerable changes in the Vistula channel are expected to occur up to 10 km below

the dam. Such situation will coincide with lateral migration of the Vistula channel at

river kilometre 683 – current location of a threshold that transversally dams the channel.

At present the said threshold lifts water in the reach that stretches 10 km below the dam.

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