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?
Short-term response to a precipitation event
•The hydrograph
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.
http://www.uoregon.edu/~millerm/meander.html
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
Green
River,
Wyoming
Such restoration can involve breaching of levees and removal of dams with the
aim of.re-establishing natural floodplain vegetation and habitat
http://vernalfishandwildlife.fws.gov/vernalfloodplain.jpg