Hydropower

Hydropower converts the energy of flowing or falling water into electricity. In a typical plant, moving water turns a turbine, whose shaft drives an electric generator. A dam and reservoir can create a controlled height difference and regulate when water passes through the turbines, but hydropower does not always require a large dam: some run-of-river installations divert part of a stream through a powerhouse and return it downstream. The amount of electricity a plant can produce depends on factors including water flow, the drop in elevation, equipment and operating conditions.

Reservoir or impoundment plants store water behind a dam and release it through turbines as needed, subject to water availability and operating rules. Run-of-river plants use the flow of a river, often diverting some water through turbines before returning it to the channel; their output is closely tied to river conditions. Pumped-storage hydropower uses two reservoirs at different elevations. When electricity is available, pumps move water uphill; later, the water flows back through turbines to generate electricity. Pumped storage is an energy-storage method rather than a primary energy source, and pumping itself requires electricity.

Hydropower is a major form of renewable electricity generation. Where a plant can control stored water, operators may adjust output to respond to electricity demand; pumped-storage facilities can shift some electricity use from one period to another by pumping water uphill and generating later. Hydropower can also be associated with water supply, irrigation or flood management, although these purposes and benefits depend on the project and its operation. River flow, drought and competing water uses can limit the electricity available from a site.

Hydropower avoids burning fuel at the turbine, but dams and reservoirs are not impact-free. They can change river flow, water temperature, water chemistry and the movement of sediment; dams may obstruct fish migration, while reservoirs can inundate natural areas, farmland or archaeological sites. Projects can also require relocation and affect livelihoods, water availability and cultural heritage. Reservoirs may emit carbon dioxide and methane as organic matter decomposes, but the amounts vary by site and are uncertain. The scale and distribution of these effects depend on local ecology, project design and operation, making assessment and consultation important when projects are planned or managed.