Hydroelectric power uses the energy of moving water to make electricity. Fuel for a hydro plant is renewable and costs nothing. Another benefit is that hydro plants do not affect air quality.
Hydro plants generated 33 percent of the electricity in the US in the 1920s. Today they generate more electricity than 60 years ago, but account for only 13 percent of the US total. The percentage is smaller because total electric generation from other sources has increased over time.
In a hydropower system, dams on a river capture its power and direct the fast-flowing water through turbines and turning generators to produce electricity. The difference between the water levels above and below the turbine and the rate of water flow determine the amount of power generated.
Run-of-the-river plants use the natural flow of the stream. This greatly limits their potential to produce electricity in a controlled manner because the flow usually varies during the year. To avoid this, some dams store water upstream in a reservoir and then release it as needed.