recent evolution of a mediterranean deltaic coastal zone...

Journal of the Geological Society, London, Vol. 162, 2005, pp. 897–908. Printed in Great Britain.

897

Recent evolution of a Mediterranean deltaic coastal zone: human impacts on the

Inner Thermaikos Gulf, NWAegean Sea

VASILIOS KAPSIMALIS 1, SERAFIM E. POULOS 2, ARISTOMENIS P. KARAGEORGIS 1,

PETROS PAVLAKIS 1 & MICHAEL COLLINS 3

1Hellenic Centre for Marine Research, Institute of Oceanography, P.O. Box 712, 19013, Anavyssos, Greece

(e-mail: [email protected])2University of Athens, Faculty of Geology & Geoenvironment, Department of Geography & Climatology,

Panepistimiopolis, Zografou 157 84, Athens, Greece3School of Ocean and Earth Science, Southampton Oceanography Centre, University of Southampton,

European Way, Southampton SO14 3ZH, UK

Abstract: The Inner Thermaikos Gulf is located in the northwestern Aegean Sea, receiving water and

sediment fluxes from the Axios, Aliakmon, Gallikos and Loudias Rivers. The geomorphological and

sedimentological evolution of the system is reconstructed for the last 150 years (1850–2000), on the basis of

detailed analysis of historical bathymetric charts. Late Holocene history is considered within the context of

changing riverine sediment supply and human activities. Three evolutionary stages are identified. Stage I

(1850–1916) corresponds to a natural phase of rapid deltaic progradation and sea-floor deposition, with an

average sediment accumulation rate of 6.5 3 106 m3 a�1. During Stage II (1916–1956), human interference

(e.g. artificial changes in river delta plains, realignment of channels and land reclamation schemes) to the

deltaic system increased sediment delivery to the coastal waters by a factor of three; this, in turn, enhanced

the progradation of the active river mouth areas. In contrast, Stage III (1956–2000) is characterized by

significant coastline (deltaic) retreat and erosion of the adjacent sea floor (net loss of 2.5 3 106 m3 a�1); this

was as a result of extensive river damming, which trapped a significant part of the sediment moving seaward.

Furthermore, these human impacts have affected the character of the surficial sea-bed sediments of the Gulf,

by reducing the proportion of mud. The response of the deltaic margin of the Inner Thermaikos Gulf to

various anthropogenic interventions seems to be analogous to that of other deltas in the Mediterranean region

where large drainage projects, the development of irrigation networks and dam construction have taken place

within their river basins.

Keywords: Thermaikos Gulf, Aegean Sea, delta evolution, human impact, sediment budget.

Deltaic coastal systems form a complex interface between

continental and marine environments, with high levels of spatial

and temporal variability in the prevailing natural processes

(Wright & Coleman 1973). Superimposed upon the natural

evolution, deltaic coasts have undergone considerable develop-

ment, such as intensive agriculture, industrialization and urbani-

zation. Thus, human interference incorporates: (1) changes in

fluvial freshwater and sediment fluxes; (2) canalization and flood

control works; (3) land reclamation schemes; (4) channel dred-

ging for aggregates; (5) more recently, structures for coastal

protection. Within this context and against a background of

minimal tidal range (in the case of the Mediterranean Basin),

within the Holocene, the two principal factors controlling the

morphological and sedimentary evolution of any deltaic coastal

system are the fluvial water and sediment supply and the wave-

derived longshore drift (McManus 2002).

Globally, riverine sediment fluxes have been modified by

various anthropogenic activities (Vorosmarty et al. 2003). Since

late prehistoric times, long periods (centuries to millennia) of

increased sediment delivery to the coast have been attributed to

changes in land use of the catchment area (mainly as a result of

deforestation and crop farming) (Van Andel et al. 1990; Walling

1999), whereas short periods (over the order of a few decades) of

significant increase in the detrital sediment discharge have been

attributed to the canalization and straightening of lower river

reaches. The former situation has been documented by Wood-

ward (1995), in the case of the Mediterranean rivers, where

human impact upon the Ebro, Po and Axios rivers, for example,

extended back to 2500 years bp (Milliman 2001). The latter

situation has been reported for the River Axios (Poulos et al.

1996). Since the beginning of the 20th century, a dramatic

reduction in sediment supply to the coastal zone has occurred

globally, following the construction of dams for irrigation and

hydroelectric power schemes (Vorosmarty et al. 2003). In addi-

tion, a further reduction in sediment supply has been attributed

to sand and gravel extraction along river channels (Marchetti

2002). This dramatic reduction has not only stopped delta-front

progradation, but has initiated coastal erosion, which includes

coastline retreat and/or the removal of nearshore sediments

(McManus 2002; Yang et al. 2003), together with the landward

intrusion of saline groundwaters (Oueslati 1995; Zalidis 1998).

In the case of the Mediterranean Basin, it has been estimated

that the potential (natural) sediment supply has been reduced by

almost 50% since the middle of the 20th century (Poulos &

Collins 2002). Such sediment reduction, exceeding 90% in the

case of the larger river systems, has already initiated annual rates

of coastal erosion ranging from tens of metres (e.g. Ebro River,

Jimenez et al. 1997; Rhone River, Bird 1988; Po River, Simeoni

& Bondesan 1997), to hundreds of metres (e.g. Nile River, Fanos

1995; Stanley 1996).

This study examines the geomorphological and sedimentologi-

cal evolution of the deltaic coastal zone of the Inner Thermaikos

Gulf, NW Aegean Sea, located between 408259 and 408409N and

228359 and 238009E. Although a preliminary analysis of changes

in the deltaic coastline has been presented previously by Poulos

et al. (1994, 2000), the present study, with the use of appropriate

geographic information system (GIS) tools, not only examines

coastal changes, but extends this investigation to the adjacent sea

bed. Such an approach aims to provide: (1) a detailed description

(qualitatively and quantitatively) of the overall pattern of deltaic

coastal zone evolution during historical times (from 1850 to

2000) in relation to human-induced changes in freshwater and

sediment supply, in response to various anthropogenic construc-

tions and interventions either within the upstream basin (i.e. dam

construction) and/or on the coastal deltaic plain (canalization,

land reclamation etc); (2) an estimation of nearshore and off-

shore (extending to the southern limit of the Inner Thermaikos

Gulf) changes in bathymetry, defining areas of accretion or

erosion; (3) a volumetric assessment of sediments that have

entered the Gulf over the last 150 years; (4) the identification of

changes in the sedimentary (grain-size) character of the surficial

sea-bed sediments, providing an indication of their mobility.

The study area: the Inner Thermaikos Gulf

Physical geographical setting

The Inner Thermaikos Gulf is located in the southern Balkan

Peninsula and incorporates the mouths of the Axios, Aliakmon

and Gallikos Rivers (Fig. 1a). The subaerial deltaic plains of

these rivers (Table 1), together with the artificially drained lake

of Yiannitsa and the River Loudias (operating now as a drainage

canal for the Yiannitsa Lake), form the Plain of Thessaloniki, c.

1500 km2 in area (Fig. 1b). Today, the Plain hosts a population

in excess of 1 million; it is one of the most important socio-

economic areas of the southern Balkans and the second most

important zone in Greece. The seaward boundary of the Inner

Thermaikos Gulf is taken here as an imaginary west–east line,

some 20 km in length, connecting the promontories of Atheris

and Epanomi (Fig. 1b). Water depths vary from a few metres,

Fig. 1. Location maps: (a) the Inner Thermaikos Gulf catchment; (b) the Yiannitsa–Thessaloniki Plain and its coastal system. F.Y.R.O.M., Former

Yugoslav Republic of Macedonia.

Table 1. General characteristics of the main rivers discharging along thecoastline of the Inner Thermaikos Gulf deltaic coast (after Poulos et al.2000)

Axios Aliakmon Gallikos

GeomorphologyCatchment area (km2) 23747 9250 1230Maximum relief (m) 2800 2200 2180Relief ratio (10�2) 1.15 1.7 0.25Deltaic plain (km2) ~1500LithologyAcid þ mafic (%) 51.2 23.7 57.5Carbonates (%) 11.3 15.7 1.0Clastic deposits (%) 37.5 60.6 41.5ClimateMean annual temperature (8C) 14.5 16.5 16.5Mean annual rainfall (mm) 650 750 480Type Terrestrial MediterraneanLand useForest (%) 16.9 19.0 3.4Uncultivated area (%) 5.9 40.0 38.9Arable land (%) 75.4 39.3 55.6Urban area (%) 1.7 1.3 1.7Wetland (%) 0.1 0.4 0.4

V. KAPSIMALIS ET AL .898

adjacent to the delta fronts, to c. 35 m at the southerly limit (Fig.

2). At the northeastern extension of the Gulf lies the Bay of

Thessaloniki, where the harbour of Thessaloniki is located (Fig.

1b).

The morphology of the deltaic coastline, in the essentially

tideless environment of Greek waters (Tsimplis 1994), results

mainly from interaction between the fluvial water and sediment

discharge and the prevailing wave activity; the latter is relatively

low (mean monthly wave power approaching the Inner Gulf

is ,30 W m�2), when compared with other Greek deltas (i.e.

70–1454 W m�2 for the Pinios River (Poulos et al. 1993) and

.25 000 W m�2 for the Alfios River (Poulos et al. 2002)). This

pattern is related to the restricted wave fetches, caused by the

semi-enclosed and shallow character of the Inner Thermaikos

Gulf, leading to the formation of a birdfoot-type delta (Galloway

1975).

Fluvial water and sediment fluxes

Most of the freshwater inputs to the Inner Thermaikos Gulf

coastline originate from the Axios, Aliakmon and Gallikos

Rivers (Table 2), which together have been shown to represent an

overall annual mean discharge of 276 m3 s�1, between 1926 and

1970 (Therianos 1974). However, recent flow measurements

(1997-1998) have identified a substantial decrease in the fresh-

water supply (to some 130 m3 s�1), attributed mainly to the

increasing consumption of freshwater because of agricultural and

urban development (Karageorgis & Anagnostou 2001).

The principal rivers (Axios, Aliakmon and Gallikos) supply

annually some 28 3 106 tonnes of total sediment load (Table 2).

This sediment flux, associated with high sediment yields

(.500 t km�2), results from intensive weathering processes

induced by: (1) highly variable climatic conditions (i.e. large

daily and seasonal air temperature fluctuations and relatively

high precipitation levels); (2) erodible lithologies; (3) the

mountainous terrain of the region, with high relief ratios (Table

1); (4) sparse vegetation cover (Woodward 1995). During the

second half of the 20th century, such high sediment discharges

have not reached the coastal zone, as they accumulate behind a

number of irrigation and hydroelectric dams (Poulos & Collins

2002; Karageorgis et al. 2004).

Delta evolution

The rapid progradation of the deltaic complex since the 5th

century bc (Fig. 3) has been established (Sturck, 1908) from

archaeological data and historical records from ancient travellers.

At that time, the ancient towns of Skydra (to the west) and Pella

(to the north) were located adjacent to the sea; today they are

located some 30 km from the coast. During the second half of

the 20th century, the evolution of the deltaic coast of the Inner

Thermaikos Gulf has been investigated by Evmorphopoulos

(1961), Kotoulas (1984), Poulos et al. (1994) and Albanakis et

al. (1993). An assessment of the observed subsidence rates of the

plain (10 cm a�1), for the period 1960–1999, has been provided

by Stiros (2001).

Marine sediments

Modern sedimentary processes in the Thermaikos Gulf have been

studied extensively in recent decades by Lykousis et al. (1981),

Lykousis & Chronis (1989), Poulos et al. (1996) and Karageorgis

& Anagnostou (2001, 2003). Similarly, Georgas & Perissoratis

Fig. 2. Recent bathymetric chart of the Thermaikos Gulf (Hydrographic

Service of the Greek Navy 2000), based upon the Greek Geodetic

Reference System (1987). Water depths in metres.

Table 2. Freshwater and sediment fluxes of the main rivers discharginginto the Inner Thermaikos Gulf

Axios Aliakmon Gallikos

Freshwater discharge (106 m3 a�1)*Mean annual 5030 2292 1.2†

Monthly maximum 8800 4320 ?Monthly minimum 1545 662 ?Overall total 7323High water period (months) Dec.– Jan. Dec.– May ?Low water period (months) Jul.– Nov. Jun.– Nov. ?Sediment fluxes (106 t a�1)a. Suspended load 13.2‡ 6.2§ 1.0‡

b. Dissolved load† 1.7 1.2 0.6c. Bed loadk 2.6 1.3 0.3Total sediment load (a þ b þ c) 17.5 8.7 1.9Overall total 28.1

*Measured values (after Therianos 1974).†Measured values (after Skoulikidis 1993).‡Estimated value (after Poulos et al. 1996).§Extrapolated value, from the total catchment area and based upon in situ

measurements over part of the drainage basin.kEstimated value, based upon the assumption that bed load represents some 15% ofthe total sediment load (after Poulos & Chronis 1997).

Fig. 3. Palaeogeographical evolution of the Thessaloniki Plain during

historical time, based upon information provided by ancient Greek

writers (modified from Sturck 1908).

HUMAN IMPACT ON A DELTAIC COASTAL ZONE 899

(1992) and Poulos et al. (1994) have provided a socio-economic

appraisal of the ranges of human activities over the area. In

general, the bottom sediments consist mainly of silt- and clay-

sized particles, with increasing amounts of clay occurring off-

shore. Sand is abundant near the coastlines, adjacent to the river

mouths and, particularly, over the eastern part of the Gulf (as

relict deposits). Furthermore, Lykousis & Chronis (1989), inter-

preting high-resolution (3.5 kHz) seismic profiles taken from the

Inner Thermaikos Gulf, have indicated that the thickness of the

Holocene sedimentary cover exceeds 20 m near the river mouths

and decreases to ,4 m over the eastern part of the Gulf (Fig. 4).

Materials and methods

A series of bathymetric charts were obtained from the UK Hydrographic

Office (Taunton), dated 1850, 1916, 1947 and 1956, and the Hydrogra-

phical Service of the Greek Navy (Athens, Greece), published in 2000.

All the charts were projected onto the Greek Geodetic Reference System

(1987) and processed with the use of the ArcView GIS (Version 3.2).

Quantitative estimates of the volumetric changes in the offshore water

depths between the time periods of the available charts were derived

using the SURFER# software package. The weight of the sediments

accumulated in front of the river mouths was extrapolated from the

sediment volume and bulk density; the latter is equal to c. 1.5 g cm�3,

according to Lykousis et al. (2002).

Comparison between the various charts requires consideration of

various potential errors, which include: (1) vertical and horizontal datum

inconsistencies; (2) the absence or use of different georeference systems;

(3) computer-gridding errors. To minimize such errors, standard terres-

trial reference points (such as churches, capes, lighthouses), were

selected; these were used for an intercomparison of shoreline position

and offshore bathymetric contours, for all of the charts. Furthermore, a

�0.5 m range in sea-floor change was designated as a ‘moderate’ degree

of error in the bathymetric comparisons; as such, this has been

characterized as a zone of ‘no significant’ change (List et al. 1997).

The datasets used for the study of the grain-size characteristics of the

surficial sea-bed sediments were collected in 1978 (Chronis 1986) and

1997–1998 (Karageorgis & Anagnostou 2003).

Results and discussion

The reconstruction of the historical charts (Fig. 5) has revealed

major changes in the evolution of the coastline and the

associated bathymetry of the Inner Thermaikos Gulf. These

include significant spatial trends in erosion and accretion

dynamics over the past 150 years. Three evolutionary stages have

been identified: an early stage, from 1850 to 1916 (Stage I); an

intermediate stage, from 1916 to 1956 (Stage II); and a late

stage, from 1956 to 2000 (Stage III). Spatial changes in the sea-

bed elevations, accompanied by a volumetric estimation of the

sediment deposited and/or eroded from the sea floor of the Inner

Thermaikos Gulf, are presented in Table 3. The thickness of the

deposits and the areas affected by river mouth evolution are

summarized, on the basis of the earlier evolutionary stages (see

above), in Table 4.

Stage I

The coastal and bathymetric changes shown between the 1850

and 1916 charts reveal progradation of the deltaic coast and, in

particular, the area incorporating the active mouths of the Axios

and Aliakmon Rivers. Over this period, the position of the active

mouth of the Axios River varied. In particular, before the 1890s,

the river mouth was located at the Kavoura Cape, opposite the

Megalo Emvolo Cape. Subsequently and until 1934, the mouth

was located near Palaiomana, opposite the Mikro Emvolo Cape,

some 10 km to the NE (Konstantinidis 1989). However, it can be

assumed that these mouths (Kavoura and Palaiomana) were

active concurrently, especially during high freshwater discharges.

During high river discharges, overbank flows flooded extensive

areas of the delta plain, creating swamps, coastal lakes and

ephemeral channels. Conversely, under the influence of southerly

winds, the sea invaded the coastal zone of the deltaic platform,

infilling the salt marshes and lagoons.

Although some of the sediment load transported by the Axios

River was deposited on the deltaic plain during floods, the

greater proportion was discharged into the Thermaikos Gulf.

Most of the coarse-grained riverine sediments are likely to have

been trapped on the seaward side of the mouth, forming a

prodeltaic lobe. The fine-grained suspended sediment load would

have been dispersed offshore, covering almost the whole of the

sea bed over the inner part of the Gulf (including the navigation

channel to the port of Thessaloniki).

Sediment deposited during Stage I (Fig. 6), over the Kavoura

mouth of the Axios River, covered an area of 25 3 106 m2, with

a maximum thickness of 22 m; this corresponds to a sediment

volume of 85 3 106 m3. Similarly, at the Palaiomana mouth, the

new prodeltaic deposit covered an area of 23 3 106 m2, with a

maximum thickness of 16 m and a total volume of 60 3 106 m3

(Table 4).

The two main river mouths were active simultaneously during

Stage I (over 66 years), when the accumulation rates for the

Kavoura and Palaiomana mouths were of the order of

1.3 3 106 m3 a�1 and 0.9 3 106 m3 a�1, respectively (Table 4).

However, Konstantinidis (1989) has mentioned that the active

mouth of the Axios River was located near the Kavoura Cape

from 1850 to 1900 (50 years) and for the following 16 years

(1900–1916) in Palaiomana. If this is correct, then the accumula-

tion rates for the Kavoura and Palaiomana mouths become

1.7 3 106 m3 a�1 and 3.8 3 106 m3 a�1, respectively (Fig. 6b).

The much lower accumulation rate at the Kavoura mouth could

be attributed to the entrapment of large water and sediment

fluxes within the Loudias Swamps (some 80 km2).

Along the deltaic coast, between the Axios River mouths, no

significant deposition is observed (Fig. 6a), and this implies that

the secondary drainage networks played only a minor role in

sediment dispersion, except during extreme floods.

Fig. 4. Thickness of the Holocene sedimentary cover in the Inner

Thermaikos Gulf, in milliseconds (where 10 ms is equivalent to c. 8 m),

based upon the interpretation of geophysical data (after Lykousis &

Chronis 1989).


Over the same period of time (Stage I), the Aliakmon River

was in a phase of rapid progradation, incorporating a meandering

channel system. Some of the sediment load was trapped within

the meanders, forming extended point-bars. The remainder of the

load reached the coast, contributing to the progradation of the

mouth and the offshore deposition of fine-grained sediments.

During this period, some 340 3 106 m3 of sediments accumulated

over an area of 55 3 106 m2 with a maximum thickness of 20 m

(Table 4). Furthermore, a limited amount of the fine-grained

sediments (mostly muds) was transported by a cyclonic surface

circulation pattern (Poulos et al. 2000) to the west and deposited

in the Methoni Bay (Figs 1b and 6). Overall, the derived rate of

deposition is 5.2 3 106 m3 a�1, which is considerably higher than

either of those attributed to the Axios River mouths.

In general, the Axios River carried substantially less sediment

load than the Aliakmon River, although it has a much larger

Fig. 5. Digital output of the various (dated) bathymetric charts provided by the UK Hydrographic Office (Taunton). The projection system used is the

Greek Geodetic Reference System (1987). Water depths in metres.

Table 3. Estimation of morphological changes of the sea bed of the Inner Thermaikos Gulf over various timeintervals

1850–1916 1916–1956 1956–2000

Spatial changes in the sea bed (106 m2)*Net gain 120 270 60Unchanged 180 70 150Net loss 80 40 170Volumetric changes in the sea bed (106 m3)Net gain 560 1020 190Net loss 130 120 300Balance 430 900 �110Weight of deposited or eroded sea-bed sediment (106 t)Deposition 860 1570 300Erosion 200 180 460Net sediment supply 660 1390 �160Accretion or erosion rates(106 m3 a�1) 6.5 18 �2.5(106 t a�1) 10 28 �4

*Areas of net gain (.þ0.5 m), no significant change (�0.5 m) and net loss (,� 0.5 m).


Table 4. Estimates of morphological changes of the subaqueous deltaic wedges in front of the active river mouths

1850–1916 1916–1956 1956–2000

Maximum thickness of the subaqueous deltaic wedges (m)Axios (Kavoura) mouth 22 20 16Axios (Palaiomana) mouth 16 21 5Aliakmon mouth 20 22 10Accretional areas, in front of the active river mouths (106 m2)Axios (Kavoura) mouth 25 39 18Axios (Palaiomana) mouth 23 30 7Aliakmon mouth 55 59 24Volume of deposits at the active river mouth (106 m3)Axios (Kavoura) mouth 85 220 60Axios (Palaiomana) mouth 60 140 10Aliakmon mouth 340 270 35Accretion rates of the subaqueous deltaic wedges (106 m3 a�1)Axios (Kavoura) mouth 1.3 5.5 1.4Axios (Palaiomana) mouth 0.9 3.5 0.2Aliakmon mouth 5.2 6.8 0.8Sediment budget, in front of the active river mouth (106 t)Axios (Kavoura) mouth 130 340 90Axios (Palaiomana) mouth 90 215 15Aliakmon mouth 520 415 55Accretion rates of the subaqueous deltaic wedges (106 t a�1)Axios (Kavoura) mouth 2.0 6.8 1.3Axios (Palaiomana) mouth 1.4 4.3 0.2Aliakmon mouth 7.8 8.3 1.2

Fig. 6. (a) Spatial distribution of

accretional and erosional patterns in the

Inner Thermaikos Gulf during Stage I

(1850–1916), derived from the comparison

of bathymetric charts; (b) schematic

representation of the estimated sediment

influx, at the active river mouths.


drainage basin (Table 2). According to Poulos et al. (2000), this

difference in sediment yield could be attributed to: (1) differ-

ences in lithology, with the Axios River draining mostly resistant

mafic and acidic rocks whereas the Aliakmon River drains

mostly weaker clastic formations (Table 1); (2) the geomorpholo-

gical characteristics of the two catchments, with the Axios River

having a much lower relief ratio (1.1) than the Aliakmon River

(1.7), incorporating a drainage network with a large number of

primary and secondary branches and meanders, together with a

more gentle slope to its deltaic plain; (3) soil degradation,

associated with land use changes.

Over this period, the Loudias River drained the Yiannitsa Lake

and Loudias Swamps to the west of the Kavoura mouth (Fig. 1b).

These systems discharged into the Thermaikos Gulf either

directly, as an independent river with its mouth located between

those of the Axios (Kavoura) and Aliakmon, or indirectly, as a

tributary of the western channel of the Axios River (Konstantini-

dis 1989). Finally, the Gallikos River, although its mouth was

located very close to the Palaiomana mouth of the Axios River

(Fig. 6), was active independently. The Gallikos River drains a

much smaller catchment area (1230 km2) and is characterized by

ephemeral (seasonal) flows.

During Stage I, the sea floor of the Inner Thermaikos Gulf,

excluding the areas of active deltaic progradation, does not

appear to have undergone any significant changes (Fig. 6a).

Limited sediment accumulation can be observed over the north-

eastern part of the Gulf and close to the port of Thessaloniki;

this could be associated with the Palaiomana mouth of the Axios

River. Over some parts of the central area of the Gulf, the sea

bed appears to have undergone erosion (.0.5 m) and this is

probably related to near-bed current activity.

Stage II

Within this period (1916–1956), two substages can be identified:

the first (1916–1930) is characterized by natural coastal evolu-

tion, and the second (1931–1956) by increasing human interven-

tions on the deltaic plain in the form of canalization and land

reclamation.

During the first sub-period (1916–1930), the deltaic plain was

evolving naturally. The active mouth of the Axios River was that

of Palaiomana (Fig. 7a), causing siltation of the port of

Thessaloniki. Such siltation led to rerouting of the Axios River,

diverting its active mouth to its previous location near the



Inner Thermaikos Gulf during Stage II

(1916–1956), derived on the basis of the

comparison of bathymetric charts; (b)

schematic representation of the estimated

sediment influx, at the active river mouths.


Kavoura Cape (Fig. 7a). Construction commenced in 1930 and

was finalized in 1934. Accompanying works, undertaken within

the deltaic plain, were: (1) the artificial drainage of the Yiannitsa

Lake; (2) associated construction of the Loudias drainage

channel (39 km long, 50 m wide and 5 m deep, along the former

route of the Loudias River); (3) land reclamation works on the

Loudias Swamps (1933–1935), located to the NW of the

Kavoura mouth (Fig. 1b). Following diversion of the main

tributary of the Axios River (1934), an extended birdfoot delta

developed near the Kavoura mouth. During the second sub-

period (1931–1956), the Axios River delta developed farther to

the south and SW. Erosion occurred over the eastern part of its

active mouth, affecting the earlier deposits of the abandoned

Kaboura mouth (Fig. 7). At this time, the old route of the Axios

River, ending at the Palaiomana mouth, functioned episodically

during floods.

Over the same sub-period, the lower meandering course of the

Aliakmon River was aligned artificially; this decreased its length

from 52.2 km to 30.4 km (Konstantinidis 1989), leading to

further progradation of its mouth area to the NE and to the

development of an extended birdfoot delta. Similarly, works were

carried out along the lower course of the Gallikos River, focusing

upon the protection of its subaerial delta from flood events

(Konstantinidis 1989). Overall, Stage II (1916–1956) was char-

acterized by substantial sediment deposition within the Inner

Thermaikos Gulf and this is estimated to be of the order of

900 3 106 m3 (Table 3). Although sediments were deposited over

the adjacent sea floor, the highest rates of accretion occurred in

front of the active river mouths (Fig. 7b). Thus, at the

Palaiomana mouth, some 140 3 106 m3 of sediments settled over

an area of 30 3 106 m2 corresponding to a maximum thickness

of 21 m and an accumulation rate of 3.5 3 106 m3 a�1 (Table 4).

However, on the basis of the assumption that the majority of

these sediments had been deposited before the artificial diversion

of the main Axios River channel (i.e. from 1916 to 1934), the

accumulation rate could be increased to 7.8 3 106 m3 a�1. At the

Kavoura mouth during Stage II, if only a small amount of

sediment was deposited prior to 1934, then sediment volume

of 220 3 106 m3 was associated with an accumulation rate of

10.0 3 106 m3 a�1 (between 1934 and 1956).

A general increase in sediment supply can be attributed,

mainly, to alignment of the Axios River and straightening of its

lower course; this caused elevated flow velocities and (possible)

erosion of older terrestrial deposits (Konstantinidis 1989). Like-

wise, the bulk of the sediment load was transported directly to

the sea, limiting the deposition of sediment on the deltaic plain.

Furthermore, such high sediment discharge rates also reflect the

absence of any major dams along the river channel. However, a

small reservoir (capacity 3.6 3 106 m3) was constructed, in

1938, on a tributary of the Axios River (the Treska River) some

300 km upstream of the river mouth.

Artificial cut-off of meanders in the Aliakmon River deltaic

plain would also have increased sediment supply into Methoni

Bay (Fig. 1a). The volume of the deposits that accumulated in

front of the Aliakmon River mouth was 270 3 106 m3, corre-

sponding to an average accumulation rate of 5.4 3 106 m3 a�1.

Over the same period, it proved impossible to estimate the

progradation rate of the Gallikos River, especially following

the engineering works. This is due primarily to its proximity to

the Paliomana mouth of the Axios River, but also to its low

sediment discharge.

Earlier investigations of sediment supply, based upon changes

in the coastline (from 1850 to 1952), have revealed an annual

sediment input of 16 3 106 m3 a�1 (Evmorphopoulos 1961) or

16.4 3 106 m3 a�1 (Kotoulas 1984). Calculations undertaken

here, on the basis of comparison of the 1850 and 1956

bathymetric charts, show a net sediment accumulation of

1350 3 106 m3 or 12.8 3 106 m3 a�1. This difference between

the results of the present study and the abovementioned investi-

gations can be attributed, mainly, to: (1) the different approaches

used, and especially to the fact that the present investigation

takes into account not only coastline displacements but also

changes in offshore water depths; (2) the escape seawards of part

of the very fine-grained suspended and dissolved material (c.

10–15% of the total sediment load), which does not contribute

to long-term coastal change.

In general, sea-floor changes in the Inner Thermaikos Gulf

reveal an accretional pattern, associated with high river sediment

discharges. Net sediment deposition is of the order of

900 3 106 m3 (Table 3), with the impact of such sedimentation

being most pronounced at the river mouths. In some places

erosion has also taken place, including, for example, to the east

of the Kavoura Cape, from both sides of the Mikro Emvolo

Cape, and to the south of the Megalo Emvolo Cape. These

erosional areas are related to near-bed currents and the overall

water circulation pattern within the Inner Thermaikos Gulf

(Balopoulos et al. 1986).

For the whole of the Inner Thermaikos Gulf, the overall

accretion of sediment (1020 3 106 m3) (Table 3), in comparison

with that deposited in front of the river mouths (630 3 106 m3),

implies that some 40% of the terrestrial sediment inputs were

dispersed seawards.

Stage III

The period between 1956 and 2000 witnessed an intensification

of human intervention, resulting in substantial alterations to the

natural systems. This followed, in turn, the construction of dams

in the various catchments, for irrigation and/or hydroelectric

power generation. Two irrigation dams were constructed along

main channels of the rivers Axios (Prochoma) and Aliakmon

(near Verroia), in 1958; these were located some 30 km and

50 km inland from their river mouths, respectively (Fig. 1b).

Such construction was followed by the development of an

extensive irrigation network over the Thessaloniki Plain, from

1958 to the end of the 1960s, associated with intensive

agricultural activities such as rice cultivation. This development

caused a significant reduction in the area of wetlands, together

with massive consumption of river water. Furthermore, 11 dams

have been constructed over the last 50 years along the Axios

River, within the part of the catchment that belongs to FYROM

(Former Yugoslav Republic of Macedonia), with a total capacity

of 803 3 106 m3 (Psilovikos & Psilivikos 1997). This develop-

ment has resulted in a considerable reduction in the volumes of

freshwater reaching the Gulf. Mean annual water discharge,

estimated from historical data, is 5.0 3 109 m3 a�1 (Therianos

1974). This contrasts with recent (1995–2000) measurements,

which have revealed a considerable decrease to around

3.4 3 109 m3 a�1 (Karageorgis et al. 2004). In addition, three

more hydroelectric dams were constructed along the Aliakmon

River between 1974 and 1989.

Entrapment of the bed load and the majority of the suspended

material, within the reservoirs, has resulted in a dramatic

reduction in sediment supply to the deltaic coastal plain. The

mean annual sediment discharges were estimated from historical

data (1970–1987) to be (1–2) 3 106 t a�1. Recent (1995–2000)

estimates are 10- to 20-fold lower (0.1 3 106 t a�1) (Karageorgis

& Anagnostou 2001). Initially, such a reduction in sediment


supply inhibited deltaic progradation, causing coastline retreat.

Furthermore, reduction in the natural freshwater flow levels,

through the retention of large quantities of water within the

reservoirs and/or increasing consumption, has caused: (1) saline

intrusion into the coastal deltaic areas (Stiros 2001); (2) dete-

rioration of the wetlands; (3) seawater intrusion along the lower

courses of the rivers, during storm surges (Konstatinidis 1989).

To control coastal erosion, especially along the abandoned

Palaiomana mouth of the Axios River, coastal protection meas-

ures have been undertaken (from 1960 to 1974). These include

the construction of 22 km of seawall (2.5 m in height) and five

pumping stations that drain the agricultural plain. Parts of the

seawall have been destroyed by rapid subsidence of the deltaic

plain, which in some places reaches 10 cm a�1 (Stiros 2001).

This subsidence is related to the decomposition of organic matter

and compaction of fine-grained material accompanied by syn-

sedimentary deformation. Such an effect has been enhanced by

the extended, often illegal, overpumping of the deltaic aquifers

(Andronopoulos et al. 1991). Coastal erosion has also been

caused by the extraction (not always appropriately licensed) of

aggregates (sand and gravel), from the lower courses of the

rivers. Such activity has reduced bed load transport rates and,

consequently, the process of deltaic infilling and progradation.

This is a common situation in many Greek rivers and deltas,

such as has been reported for the lower reaches Alfios River in

the Peloponnese (Nicholas et al. 1999).

Since 1956, the Inner Thermaikos Gulf has entered into an

erosional phase, at an annual rate of c. 2.5 3 106 m3 (Table 3).

However, in front of the active river mouths and near the port of

Thessaloniki, limited amounts of sediment are still deposited

(Fig. 8).

Sediment deposited seawards of the Axios River mouth has

accumulated at a rate of 1.4 3 106 m3 a�1; this compares with

an estimated sediment load for the Aliakmon River of

0.8 3 106 m3 a�1 (Table 4). Such rates are approximately four

(Axios River) and eight (Aliakmon River) times less than those

estimated during Stage II (Tables 3 and 4). Most of the ‘missing’

sediment load has been retained behind the dams. Moreover, the

remainder of the load that eventually reaches the sea could

represent: (1) dissolved load; (2) very fine-grained suspended

sediment; (3) material originating from the erosion of the deltaic

plain. Furthermore, it is likely that a significant amount of

suspended particulate matter and the majority of the dissolved

load are transported farther offshore, extending even to the



Inner Thermaikos Gulf during Stage III

(1956–2000), derived on the basis of the

comparison of bathymetric charts; (b)

schematic presentation of the estimated

sediment influx at the active river mouths.


continental shelf edge of the Outer Thermaikos Gulf (Lykousis

& Chronis 1989; Karageorgis & Anagnostou, 2001, 2003).

Sediment discharges associated with the Loudias drainage chan-

nel (with average discharges ranging from 0 to ,10 m3 s�1) and

the Gallikos River (dry during most of the year, but undergoing

subsidence over the coastal part of the deltaic plain) are consid-

ered to be minimal. A comparison of the surficial sediment

distributions within the Inner Thermaikos Gulf in 1978 (Fig. 9a)

and 1997–1998 (Fig. 9b) reveals that they are essentially similar,

but that there is a distinct change in the northeastern part of the

Gulf, where muds have been replaced by sandy muds and muddy

sands. Hence, the percentage of sand has increased in places up

to 15%, where it was previously minimal (,1%). Such a change

is likely to be the result of: (1) a decline in sediment supply (i.e.

eventual cessation of supply from the Axios River mouth

(Palaiomana) and the Gallikos River); (2) sea-bed erosion,

induced by local near-bed current activity, and probably en-

hanced by the navigation activities related to the port of

Thessaloniki. This change in the sedimentary facies of the sea

bed has also led to alteration of the associated benthic habitat

(Voutsinou-Taliadouri & Varnavas 1995).

Conclusions

The recent sedimentary evolution of the Inner Thermaikos Gulf

reflects anthropogenic impact on the natural evolution of the

coastal system, through human-induced modifications to the

sediment supply and water discharge (Fig. 10). This influence is

most pronounced along the deltaic coastal zone, where two main

rivers (Axios and Aliakmon) and two smaller rivers (Gallikos

and Loudias) discharge into the marine waters.

In general terms, over the past 150 years, the Gulf has

accumulated a net sediment volume of 1230 3 106 m3, at an

average rate of accretion of 8 3 106 m3 a�1 or 12 3 106 t a�1.

Some 85% of this load has been deposited around the active

river mouths, with approximately 15% being dispersed offshore.

Over this period of time, three evolutionary ‘stages’ have been

identified on the basis of changes in sediment supply and

associated human interference.

(1) From the middle of the 19th century to the early 20th

century (Stage I: 1850–1916), the coastal system of the Gulf was

Fig. 9. Distribution patterns of the sedimentary facies on the sea bed of

the Inner Thermaikos Gulf: (a) in 1978 (modified from Chronis 1986);

(b) in 1997–1998 (modified from Karageorgis & Anagnostou 2001).

Sampling stations are shown in both parts of the figure. Sedimentary

facies are defined according to the classification of Folk (1974).

Fig. 10. A summary of the human activities

affecting the evolution of the deltaic system

of the Inner Thermaikos Gulf in relation to

sediment supply between 1850 and 2000.

Horizontal bars indicate the duration of

each activity; lighter and darker shading

indicates lower and higher impact of human

activities, respectively. (For details, see

text.)


controlled naturally, with a net marine sediment supply of some

430 3 106 m3, at an average fluvial discharge of 6.5 3

106 m3 a�1 or 10 3 106 t a�1. This input resulted in progradation

of the delta complex, especially near the active river mouths.

(2) Subsequently, during Stage II (1916–1956), especially in

its second part (1934–1956), human intervention to the natural

system of the deltaic plain was at a maximum through: (a)

artificial realignment of the main river channels (1934); (b)

drainage of the Yiannitsa Lake and the Loudias Swamps (1935);

(c) the instigation of other land reclamation projects (Fig. 10).

The net riverine sediment supply increased considerably to

900 3 106 m3; this corresponds to an average input of 18

3 106 m3 a�1 or 28 3 106 t a�1. Over this period, rapid progra-

dation occurred at the active mouths of the Axios and Aliakmon

Rivers. At the same time, some of the fine-grained components

of the sediment load were dispersed over the Inner Thermaikos

Gulf.

(3) Stage III (1959–2000) is strongly characterized by a

further ‘cycle’ of human interference. This incorporates the

construction of irrigation reservoirs and hydroelectric dams.

Such structures have caused a significant reduction in sediment

supply, leading to an overall erosional phase over the Gulf and a

mean sediment loss of 2.5 3 106 m3 a�1 or 4 3 106 t a�1. During

this period, the river mouths underwent rapid retreat. The ‘active’

rivers prograded only minimally, with the lower reaches of the

deltaic plain being subjected to coastal flooding. Furthermore,

human activities have affected the texture of the offshore sea-bed

sediments by reducing the proportion of the mud fraction.

The authors gratefully acknowledge the helpful and critical revision of an

earlier draft of the manuscript by J.C. Woodward (University of

Manchester). We also appreciate the constructive comments and sugges-

tions made by two anonymous reviewers.

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Received 9 February 2004; revised typescript accepted 27 May 2005.

Scientific editing by Jamie Woodward


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