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involving physical habitat limitations, fishery management practices, reservoir operations, and behavioral and physiological responses of the trout to the tailwater environment. Habitat Conditions as Functions of Lifestage and River Discharge Physical habitat simulation (PHABSIM) models were employed to evaluate seasonal habitat conditions. Microhabitat electivity data for rainbow and cutthroat trout, juveniles and adults, were collected during summer and winter at different levels of discharge. These data were then integrated with physical models of three winter habitat types located at the Tailrace, Pipe Creek, and Indian Crossing. The resulting model projections strongly suggested habitat for trout less than 250 mm (10 inches) TL was relatively scarce in winter, at least at the three habitat simulation stations. Conditions were apparently not limiting for either adult or juvenile trout in summer or for the adult life stage in winter. Results and observations of other study elements appeared to verify the PHABSIM projections. In general, there appeared to be more usable winter habitat for trout at lower levels of discharge. The habitat simulation stations at Pipe Creek and Indian Crossing were, together, probably the most representative of the tailwater's winter habitats. The output from these two stations produced an inverse relationship between total weighted usable area and discharge. Overall winter habitat was greater at Pipe Creek and Tailrace than at Indian Crossing. There was a positive relationship between weighted usable area and discharge at Tailrace. These model projections were corroborated by the substantially higher tag return rates (reflecting higher winter survival) for trout tagged prior to winter at and near the tailrace. -4-