Fish and other aquatic biota that live in rivers and streams have to contend with the variability of the flow regime.

How variable is runoff/discharge?

From year to year?

From month to month

From day to day

Crowsnest River –4th order, around 3-4 m3/sec—10-20 m wide DA around 400 km2

0

2

4

6

8

10

12

14

1982 1987 1992 1997 2002 2007

Annual mean discharge of the Crowsnest River at Frank

m3/s

20

0

2

4

6

8

10

12

14

1982 1987 1992 1997 2002 2007

Annual mean discharge of the Crowsnest River at Frank

m3/s

Exceedence probability

0.90

0.50

0.25

0.10

0.75

90 percentile

75 percentile

50 percentile

25 percentile10 percentile

1995 12

1991 9.8

2005 8.7

1993 6.3

2002 6.2

1996 5.9

1992 5.5

2004 4.6

1999 4.4

1994 4.2

1986 4.1

1990 3.8

1988 3.7

1989 3.5

1998 3.1

1987 2.5

1997 1.7

2003 1.5

2000 1.4

2001 1.2

10.0

1.0

100

oo

o

We can now plot these data on cumulative probability paper

Non-exceedence probability (percentiles)

Based on this plot we can estimate that 1 year in 100 there will be a year wetter than 20m3/s and a year drier than 0.8 m3/s, if there is no-longterm climate change

Dischargem3/sec

Exceedance probability (percentiles)

99.9 99 90 70 50 30 20 10 1 0.1

o

o

Recurrence interval is the reciprocal of the probability

1 yr out of 4 discharge is above this line

3 yr out of 4 discharge above this lineAnnual average 2.7 m3/s

2 yr out of 4 discharge above this lineAnnual average 4.1 m3/s

Annual average 6.0 m3/s

River Management can significantly alter the seasonal hydrograph

Before Ottawa R dams

After Ottawa R dams

St.Lawrence River Flood Plain at Lac St. Pierre—has rarely been flooded since the late fifties—What effect might this have on the river?

The hydrographShort-term response to a precipitation event

How would you expect deforestation to influence this response?---Consider the terms of the hydrological balance equation

Human activities can markedly influence the shape of the hydrograph

Deforestation •reduces the lag time •raises the peak• reduces the base flow

Fig 5-2 in your text

How does deforestation affect the hydrological balance[P-E]DA – S/t

We can explain the effects of deforestation on the hydrograph in terms of the hydrological balance, mainly in terms of changes in Evaporation and storage.

Evaporation directly from the soil surface increases when plant cover is reduced, but usually transpiration from plant leaves is decreased more than surface evaporation increases.

•leads to increased runoff—more area under the hydrograph curve

Storage is reduced—water stored in plant tissues is removed, and the water holding capacity of soils is reduced by the death of roots.

•Thus less of the water from the precipation event goes into recharging storage pools (S is less), & more water enters the stream directly, reducing the lag time on the rising limb, and giving a higher and sharper peak in the deforested watershed.

•Recession will also be faster and base flow lower in the deforested watershed since there will be less storage to sustain the stream flow after the rain stops.

Biological Impacts that generally occur when watersheds are hydrologically disturbed by deforestation

Scouring—increased flashiness of the hydrograph can scour and shift gravels.Siltation—shortening of hydrological lag time (usually associated with increased overland flow vs percolation--influx of fine particulate matter to streams.

•plug up interstitial spaces in gravels & reduce water percolation and oxygen supply to interstitial benthic organisms & fish eggs in the gravels.

•Siltation + increased dissolved organic material can reduce light penetration

Nutrient enrichment—hydrological changes usually also increase the influx of Nitrogen & Phosphorus to the stream leading to increased algal growth on the substrate.

Increased temperature & illumination—reduced forest cover, depending on how near to the stream bank trees are cut, can increase light penetration, warming the stream & changing the composition of benthic communities, by reducing survival of cold-water species.

•increased illumination can also contribute to increased benthic algal growth on rocks.

Patterns in stream flow and how they generate habitat diversity in streams

•Annual discharge patterns—floodplain habitats

•Stream meander—pool/riffle alternations

•Stream channel changes over time—oxbow lakes, braided streams

Flood plain habitats greatly increase biodiversity in river systems. They contribute mostly temporary wetlands, but some remain as permanent because of groundwater inputs.•Important for•Waterfowl, mammals, amphibians, fish, aquatic insects, molluscs etc.

•Most of the species found in flood plain wetlands would be lost from the river if the floodplain were not allowed to be flooded regularly.

Flood plains depend on seasonal variability in river discharge

Oxbow lake and the Chippewa River. Eau Claire, Wisconsin.http://www.uoregon.edu/~millerm/meander.html

River flood plains often undergo extensive urban development

Many are also strongly impacted by agriculture

Meander pattern of the Sacramento River, CA, note old abandoned channels and oxbows, and the encroachment of agriculture on the river channel.http://www.uoregon.edu/~millerm/meander.html

http://vernalfishandwildlife.fws.gov/vernalfloodplain.jpg

Such restoration can involve breaching of levees and removal of dams with the aim of.re-establishing natural floodplain vegetation and habitat

Green River, Wyoming

meandersIn streams and rivers of different sizes

Rivers meander when they are traveling on top of a relatively flat surface.

•a straight line is not the most efficient path for water to flow. A meandering river will dissipate more energy through friction with the riverbed than one that flows straight

•The meander pattern shifts constantly since the river is cutting the bank on the outside of meander loops and depositing alluvial sediment on the inside.

http://www.kented.org.uk/ngfl/rivers/River%20Articles/meander.htm

Streams flow down hill and take the path of least resistance, however the path is usually a meandering instead of straight

It takes more velocity to suspend and move larger particles

http://waterknowledge.colostate.edu/meander.htm

Oxbow lake forming from a river meander

The erosion that occurs during a flood even may cause the river to take a shortcut from one loop to the next--thus cutoff loop will remain as an oxbow lake.

Oxbow lakes and braided channels on the Upper Amazon River

http://muller.lbl.gov/travel_photos/AmazonWebPages/AmazonWebPages-Thumbnails/1.jpg

http://waterknowledge.colostate.edu/meander.htm

An important reference on River MeandersLeopold, L.B., and M.G. Wolman, 1960,River Meanders: Geol. Soc. America, Bull., v. 71, pp. 769-794.

The physical stresses on the river bed during floods often cause a river to change course.

The river valley contains many old channels superimposed on each other

A good example of braiding in the river channel

Rivers often simultaneously occupy several of their historical channels at once.We call this type of river channel braided

Oldman RBelow Summerview

Showing old river channels

When rivers cut deeply into the landscape the meander pattern becomes entrenched and from then on shifts downward only.

http://courses.missouristate.edu/EMantei/creative/WeathStrem/entrenched.jpg

If you want to learn more about the landforms and deposits created by running water, with particular reference to western Canadian watersheds, take

Geography 3035Fluvial Geomorphology—Hester Jiskoot

Other courses of relevance are

Geography 4015Integrated Watershed Management—Jim Byrne

AndGeography 4012Hydrology—Stefan Kienzle