Effects of Selective Withdrawal and Hypolimnetic Releases on Downstream Fish Resourcesby Sue A. Perry, Federal Energy Regulatory, Commission, Washington, DC, USA,
William B. Perry, Federal Energy Regulatory, Commission, Washington, DC, USA,
Abstract: Selective withdrawal (multilevel outlet) structures on dams can be operated to modify reservoir discharges with the goal of improving downstream water quality and fish resources. A study was conducted to compare the downstream effects of releases from a dam with a selective withdrawal system (Libby Dam on the Kootenai River) to the effects of releases from a dam with a hypolimnetic withdrawal (Hungry Horse Dam on the South Fork Flathead River). Both dams have hydropower facilities that operate in a peaking mode, with frequent large fluctuations in outflow. Hypolimnetic releases have radically altered temperatures in the South Fork Flathead River, where the annual thermal regime varied only between 3 degree C and 7 degree C. The thermal regime in the regulated Kootenai River was also characterized by late fall and winter elevation and summer depression of water temperatures, but followed the natural thermal regime much more closely.
Subject Headings: Dams | Rivers and streams | Water quality | Water resources | Fish management | Thermal properties | Water supply | Water discharge
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