Hydro is one of the largest producers of electricity in the United States. Water power supplies about 7-10 percent of the entire electricity that we use. In states with high mountains and many rivers, even more electricity if made by hydro power. In California, for example, about 15 percent of all the electricity comes from hydroelectric.

The state of Washington leads the nation in hydroelectricity. The Grand Coulee, Chief Joseph and John Day dams are three of six major dams on the Columbia River. About 87 percent of the electricity made in Washington State is produced by hydroelectric facilities. Some of that electricity is exported from the state and used in other states.

Some interesting sites:
Hydro Power News and information
Wisconsin Valey Improvement Company
Tennessee Valey Autority