Reservoirs

Various Uses  Irrigation, electricity generation, flood control, water supply (urban), improved navigation, fish culture, recreation, combination

Most are small  Built for irrigation by damming rivers in low rainfall (low runoff) areas

Reservoirs vs. Natural Lakes  Much in common  Reservoirs usually have larger drainage basins for their size  Reservoirs usually younger  Reservoirs have greatest depth at dam

Large Reservoirs  3 types  Mainstream run-of- river  Tributary storage  Mainstream storage

Mainstream run-of-river  Used to generate electricity  Rapid flushing  Little or no stratification  No drawdown  Water release from deep outlet  Most river-like

Columbia River Grand Coulee Dam

Tributary storage reservoirs  Built on smaller rivers  Used for irrigation, flood control  Slower flushing  Stratified if deep enough  Periodic drawdowns  Surface outlet  Most like natural lakes in area

Acton Lake, Ohio

Mainstream storage reservoirs  Intermediate in “position” between other types  More lotic during periods of high runoff  More lentic during other times

Mainstream storage reservoirs  Lie in well-defined river valleys  Exhibit pronounced longitudinal zonation

Mainstream storage reservoirs

Flushing rates  If flushing rate is greater than the doubling time of planktonic organisms, washout will occur  Poor development environment for plankton

Impacts on river  Lentic system built into a lotic environment  Interferes with migration

Prickett Dam, Michigan

Impacts on river  Modified timing, volume of discharge Interferes with life histories of biota  Trapping materials behind dams Creates erosional habitats downstream

Prickett Dam, Michigan

Impacts on river  Largest threat – large- scale water diversion for agriculture, especially in dryland areas  Reduced or zero discharge, loss of downstream wetlands, biota

Drawdowns  Necessary for dam maintenance  Increase flood capacity  Remove accumulated sediments

Drawdowns  Exposure, desiccation of littoral zone  Pronounced effects on biota

Benefits and Costs  Benefits Development Development Agriculture Agriculture Fisheries Fisheries  Costs Flooding Flooding Modified river reaches Modified river reaches Above, belowAbove, below