Rainwater flowing off the land can be used to power turbines linked to electricity generators. To ensure a steady flow, the water is stored in reservoirs created by building huge dams across upland river valleys. This means that the power plants can be built only in suitable terrain. The dams can cause serious problems for wildlife, and the reservoirs often silt up. But hydroelectric power is a proven, dependable technology that currently supplies 15 percent of the world's electricity.
Water flows out of dam
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