Hydropower is one of the oldest sources of energy. The flow of rivers has been used for thousands of years to turn paddle wheels that grind grain. It was the first industrial method for generating electricity, having begun in 1880. When it comes to dams, most in the U.S. were originally built for flood control, irrigation and municipal water supply. It was only later that generation of electricity was incorporated into the functionality of dams.

Hydropower is regarded as a renewable energy source. It is based on the energy available in flowing water and, therefore, is constantly replenished by the Earth's water cycle, which is powered by the sun. However, the amount of precipitation that replenishes water sources can vary, producing excess water or droughts.

As of 2017, hydropower was the largest renewable energy source in the U.S. In that year hydro-generated electricity accounted for approximately 7.5% of all the utility scale electricity generation in the U.S. Of electricity generated from renewable energy sources, hydropower accounted for 44% of the total.

The water cycle is the natural process whereby water is circulated over the Earth - on land, in the oceans, and in the atmosphere.

• The sun heats water on land and in the oceans, which then evaporates and enters the atmosphere.
• Water vapor in the atmosphere condenses into clouds and forms rain and/or snow.
• Rain or snow falls to earth and collects in either the ocean, land-based lakes or rivers. By drainage from land it often winds up in the ocean. The water then evaporates, starting the cycle over again.

Hydroelectric dams and other land-based hydropower sources are greatly dependent on the amount of precipitation in the area. Droughts can markedly diminish the power performance of land-based systems; profuse rain can increase the potential of that performance.

In 2017 most of the hydropower generated in the U.S. originated in 5 states:

Washington—28%

Oregon—14%

California—12%

New York—10%

Montana—4%