MODELING OF CLIMATE-CHANGE EFFECTS ON STREAM TEMPERATURES AND FISH HABITATS BELOW DAMS AND NEAR GROUNDWATER INPUTS

A deterministic heat transport model was developed to calculate stream water temperatures downstream of reservoir outlets (tailwaters) and g roundwater sources. The model calculates heat exchange between the atm osphere, the water and the sediments and is driven by climate and stre am hydrologic parameters. Past and projected climate conditions were u sed as input to the stream water temperature model. To produce a proje cted future weather scenario, output from the Columbia University Godd ard Institute for Space Studies (GISS) global circulation model (GCM) for a doubling of atmospheric CO2 were used to adjust past (1955-1979) weather parameters. Stream reach lengths, within which water temperat ures are suitable for survival or good growth of 28 fish species, were determined for four selected streams. Several alternative upstream in flow conditions were chosen: Discharges from surface (epilimnion) and bottom (hypolimnion) outlets of reservoirs, and two groundwater inflow scenarios. By applying water temperature criteria for fish survival a nd good growth (Stefan et al., 1993) to simulated stream temperatures, it was possible to estimate stream lengths with suitable habitat. Whe n simulated suitable habitat was compared to actual fish observations, good agreement was found. For projected climate change, the simulatio ns showed how much of the available stream habitat would be lost. In t he examples presented the effect of cold hypolimnetic water release fr om a reservoir or groundwater discharges is felt as far as 48 km (30 m iles) downstream from its source, especially in smaller shaded streams . The impact of climate change on stream temperatures below dams is mo re pronounced when the water release is from the epilimnion (reservoir surface) rather than the hypolimnion (deep water). Examples used for this study show elimination of coldwater habitat for rainbow trout whe n the upstream release is from the surface of a reservoir, but only re ductions of coldwater habitat when the upstream release is from a rese rvoir hypolimnion.