Hydroelectricity accounted for an estimated 10 percent of the nation's energy supply and 96 percent of its renewable energy as of July 2005, according to the U.S. Bureau of Reclamation. The federal agency that manages the nation's water resources operates 58 hydroelectric dams producing about 42 billion kilowatt-hours annually. That is enough to meet the residential power needs of 14 million people and replace approximately 72 million barrels of crude oil. In Washington state, home to Grand Coulee Dam, 80 percent of the power needs were met by hydroelectricity in 2002.
Impoundment dams are the most common of the three types of hydroelectric dams, usually used as part of a large power supply system. They create lakes or reservoirs in the river. When the dam's gates are opened, water flows through turbines, which spin to activate generators producing electricity. Water also can be released over a spillway to reduce the reservoir level or increase the river's stream flow for fish management or other purposes.
Diversion dams redirect a section of the river through a pipe or canal. Electricity is produced from the water's current, which is increased by flowing through a narrower space. The faster flowing water spins turbines, which power generators to produce energy. These also are called "run of the river" dams because the water is not actually blocked.
The third type of hydroelectric dams are called pumped storage. They create upper and lower reservoirs. Energy is stored by pumping water up to the upper reservoir for storage. Then electricity is created by releasing the water back down to the lower reservoir through turbines.
Hydroelectric dams are classified as micro, small or large depending upon their electricity output. Micro hydroelectric dams produce up to 100 kilowatts of electricity, enough for a village, farm, ranch or home. Some utilities allow these small producers to sell back excess electricity, which is returned to the power grid. Small hydroelectric dams produce between 100 kilowatts and 30 megawatts, according to the U.S. Department of Energy's definition. Large dams produce more than 30 megawatts. The Bonneville Power Administration, the federal agency that markets energy from 31 federal hydroelectric dams, estimates one megawatt can power 700 typical homes.