Opinion ID: 411109
Heading Depth: 3
Heading Rank: 3

Heading: Temperature Changes

Text: 11 In a thermally stratified reservoir, the lower hypolimnion layer will generally be colder than the upstream river, while the upper epilimnion layer will be warmer. Some species of fish can survive only in warm water; others can survive only in cold water. Thus, cold hypolimnion water, even if fully oxygenated, will harm or kill warm water fish but benefit cold water fish; conversely, warm epilimnion water will harm or kill cold water fish and benefit warm water fish. In some cases, cold water discharges may be desirable--to create a trout fishery, for example. 12 Also, colder water has higher capacity to assimilate wastes, both because decomposition is slower and because oxygen is more soluble in cold water. 13 In short, dams cause changes in the temperature of downstream water, and some of the time, but not all of the time, those changes are undesirable. 12 Changes in the temperature of downstream water can be prevented in dams with multiple outlet levels by release of an appropriate mix of epilimnion and hypolimnion water. For some dams without multiple outlet levels, destratifying the reservoir may be feasible. However, the goal of maintaining downstream water temperature, because it requires a mix of warm and cold water, may conflict with the goal of maintaining downstream oxygen levels, which calls for release of warm epilimnion water.