There are three types of dams: impoundment, diversion, and pumped storage.

Impoundment dams store water in a reservoir behind the dam. The stored water may be released to a lower elevation to generate electricity. The capacity to produce hydroelectricity from an impoundment dam depends on the amount of water stored and the height from which it falls. This potential energy is transformed into mechanical energy as the water rushes down the sluice and strikes the rotary blades of turbine. The turbine's rotation spins electromagnets that generate current in stationary coils of wire. This current is then directed through a transformer that increases its voltage for long-distance transmission over power lines.

Figure 6 Impoundment dam on the Columbia River in Washington state Source: U.S. Department of Energy