Claytor Lake, Virginia, an 1820 ha main stem hydroelectric reservoir, was studied for one year to determine the effect of water discharge on reservoir phytoplankton. Number/liter, areal units/liter, average cell size, and chlorophyll a content were used as measures of photoplankton abundance. Rate of water discharge had an adverse effect on reservoir phytoplankton during spring and summer. Although increased rate of water discharge caused increased phytoplankton loss, the adverse effect of rate of water discharge on reservoir phytoplankton populations was probably at least partially due to additional discharge of nutrients. The inverse relationship between rate of water discharge and reservoir phytoplankton populations decreased in the uplake direction.