Virtually all biological and ecological processes are affected by water temperature. Not only does temperature directly influence chemical equilibria, but invertebrate and fish communities are also extremely sensitive to temperature. In terms of effects on biota, water temperature has direct but often subtle impacts on life history timing, habitat suitability, growth rates, rates of infection, mortality from disease and toxic chemicals, and increased exposure to both native and non-native aquatic predators better adapted to altered water temperatures.
Effects of hydroelectric project operations on the natural temperature regime of riverine sites may sometimes be related to changes in riparian shading due to tree clearing for roads, power lines,and other facilities. However, the primary effects on temperature are related to alterations in water surface area, depth, and velocity due to water diversions into or out of the stream corridor, including reservoir impoundments, and conveyance through pipelines or penstocks. Changes in the water prism along a stream reach influence the balance of heat flux into (e.g., solar radiation, air convection, ground conduction) and out of (e.g., nighttime re-radiation, evaporative cooling, and ground conduction) the reach. These effects are even more pronounced at reservoir sites, where the ratio of water surface area to reservoir volume is much smaller than that found in riverine sites, which alters the rates and balance of heat exchange with the surrounding environment. Methods to assess these effects are described below.
The following reference is recommended for additional information on instream water temperature:
Poole, G. C., and C. H. Berman. 2001. An ecological perspective on in-stream temperature:natural heat dynamics and mechanisms of human-caused thermal degradation. Environmental Management 27:787-802.
