Today hydro electric power is in use on almost every major river in the world. We receive only a small amount of energy from hydroelectric sources. In the US there are many hydro electric plants but not all of them produce enough electricity to be useful. The major hydroelectric plants in the US are on the Mississippi River, the Colorado River, The Colombia River and the Snake River. The most common type of hydroelectric plant uses a dam on a river to store water in a reservoir. Water released from the reservoir flows through a pipe and into a turbine, which spins to drive a generator, producing electricity.

Pumped storage plants use a two-reservoir system, pumping water from a river or a reservoir to a higher reservoir during the night. During the day, water is released from the upper reservoir, spinning the turbines to produce electricity.