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

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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


Exceedence probability

Annual mean discharge of the Crowsnest River at Frank




90 percentile

75 percentile


50 percentile

25 percentile

10 percentile




We can now plot these data on cumulative probability paper

Exceedance probability (percentiles)

99.9 99 90 70 50 30 20 10 1 0.1











Non-exceedence probability (percentiles)

Recurrence interval is the reciprocal of the probability

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


1 yr out of 4 discharge is above this line

Annual average 6.0 m3/s

2 yr out of 4 discharge above this line

Annual average 4.1 m3/s

3 yr out of 4 discharge above this line

Annual average 2.7 m3/s


Before Ottawa R dams

After Ottawa R dams

River Management can significantly alter the seasonal hydrograph


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 hydrograph

Short-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

How does deforestation affect the hydrological balance

[P-E]DA – DS/Dt

Fig 5-2 in your text


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 (DS 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 plains depend on seasonal variability in river discharge

  • 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.

River flood plains often undergo extensive urban development

Oxbow lake and the Chippewa River. Eau Claire, Wisconsin.


Meander pattern of the Sacramento River, CA, note old abandoned channels and oxbows, and the encroachment of agriculture on the river channel.


Green River, Wyoming

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



In 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.

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


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 lake forming from a river meander


Oxbow lakes and braided channels on the Upper Amazon River


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

An important reference on River Meanders

Leopold, L.B., and M.G. Wolman, 1960,

River Meanders:

Geol. Soc. America, Bull., v. 71,

pp. 769-794.


Rivers often simultaneously occupy several of their historical channels at once.

We call this type of river channel braided

A good example of braiding in the river channel


Oldman R



Showing old river channels


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


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

Geography 3035

Fluvial Geomorphology—Hester Jiskoot

Other courses of relevence are

Geography 4015

Integrated Watershed Management—Jim Byrne


Geography 4012

Hydrology—Stefan Kienzle


Sutcliffe, W.H. 1973

Correlations between seasonal river discharge and landings of American Lobster and Atlantic halibat in the Gulf of St. Lawrence.

J.Fish. Res. Bd. Can. 30:856-859.