Hydroelectric power in the United States

Hydroelectric Power 
   Electricity generated by moving water from dams is called hydroelectric power. The sun provides heat energy that evaporates surface water from oceans, lakes, and streams. The water returns to the earth as rain or snow. Reservoirs behind dams built across river valleys collect and store the water.
The mechanical energy of the moving water as it leaves the dam is converted to electricity. The amount of kinetic energy in the falling water depends on the speed and the volume of the water available to turn the generators. About 10 percent of the electricity used in the United States is produced by hydroelectric power.
Generating electricity with water power is efficient and clean. Little energy is lost, and no pollutants are produced. Another advantage of hydroelectric power is its low cost. Although dams are expensive to build, hydroelectric plants have low operating costs.
   The lakes, or reservoirs, behind the dams also provide water storage for irrigation and serve as recreational areas. However, building a dam changes a river forever. It floods low-lying areas upstream, destroying farmland and wildlife habitats. Dams may also block the migration and survival of some kinds of fish, such as salmon.