The biggest hydroelectric dam in the United States is the Grand Coulee Dam on the Columbia River in central Washington. Started in 1933 and completed in 1942, it is the largest concrete structure ever built: 5,233 feet long and 550 feet high. The dam has four power plants with 33 generators; it makes enough electricity for more than two million homes.

U.S. hydroelectric generation expanded until about 1975, but its share of the national electrical energy mix steadily declined from a peak of about 40 percent in the 1930s to about 7 percent today.

Figure 5 Renewable energy total consumption and major sources, 1949-2005 Source: U.S. Department of Energy