Jocassee Reservoir (3,063 ha) is the upper pool for a 61O·MW pumped storage hydroelectric plant in northwestern South Carolina. Trout (200-225 mm long) have been stocked annually since 1972. The volume of trout habitat during summer has decreased annually from 1973 to 1976, and is associated with increases in the temperature and volume of water pumped from the lower reservoir. The top of the thermocline during September was lowered from about 9 m in 1973 to nearly 19 m in 1976, and trout habitat was reduced by about 65%. On the basis of a regression presented, we predict that trout habitat will be reduced to nil when plant operation reaches 63% of capacity (perhaps by 1981).