Run-of-the-river hydroelectricity is a type of hydroelectric generation whereby the natural flow and elevation drop of a river are used to generate electricity. Power stations of this type are built on rivers with a consistent and steady flow, either natural or through the use of a large reservoir at the head of the river which then can provide a regulated steady flow for stations down-river (such as the Gouin Reservoir for the Saint-Maurice River in Quebec, Canada).
Power stations on rivers with great seasonal fluctuations require a large reservoir in order to operate during the dry season, resulting in the necessity to impound and flood large tracts of land. In contrast, run of river projects do not require a large impoundment of water. Instead, some of the water is diverted from a river, and sent into a pipe called a penstock. The penstock feeds the water downhill to the power station's turbines. Because of the difference in altitude, potential energy from the water up river is transformed into kinetic energy while it flows downriver through the penstock, giving it the speed required to spin the turbines that in turn transform this kinetic energy into electrical energy. The water leaves the generating station and is returned to the river without altering the existing flow or water levels.
Most run-of-river power plants will have a dam across the full width of the river to utilize all the river's water for electricity generation. Such installations will have a reservoir behind the dam but since flooding is minimal, they can be considered "run-of-river".