Lotic ecosystems respond to modified flow regimes through changes in physical habitat availability, water chemistry and temperature, nutrient cycling, biomass/energy relationships, and the population and community dynamics of aquatic biota. A systems perspective is therefore essential in understanding flow-related impacts and in making water management decisions. More retrospective studies and experimental management are needed to provide the necessary design information for environmentally sound hydropower development. The responsibility for these studies must be shared among developers, regulators, and natural resource managers.