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