Flowing water has energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower.

Photograph of a large hydropower dam in the Pacific Northwest, showing water cascading down the side of the dam.

Large hydropower plants, such as the Bonneville power plant on the Columbia River, provide most of the electricity consumed in the northwestern United States.

There are different types of hydroelectric power plant. The most common type uses a dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. But hydroelectric power doesn't necessarily require a large dam. Smaller hydroelectric power plants may use a low dam or weir to raise the water level a little; others just use a small canal or penstock to channel the river water through a turbine.

Today's hydropower plants range in size from small, local projects producing a few kilowatts to huge dams and reservoirs that generate 10,000 MW or more and supply energy to millions of people. Although output varies from year to year depending on the amount of precipitation received in upstream watersheds, hydropower is currently the largest source of electricity from renewable resources, generating roughly 10% of the electricity used in the United States.

Photograph of a slow-moving river showing water pouring over a low dam, about four feet in height. To the right side of the dam is a small concrete building that houses a hydroelectric power plant.

Some run-of-river hydropower projects, such as this one in Kankakee, Illinois, use a small dam to raise the head so that the energy in the flowing water can more easily be captured.

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