Hydroelectric means producing electricity from moving water.

Here's how it works: water flowing downhill or released from a dam turns giant turbines, which are like enormous spinning wheels. These turbines connect to generators that convert the spinning motion into electricity. The faster and more powerfully the water flows, the more electricity gets produced.

Hydroelectric dams create large reservoirs by blocking rivers. When operators open the dam's gates, water rushes through with tremendous force, spinning the turbines. The Hoover Dam on the Colorado River can generate enough hydroelectric power for about 1.3 million people.

Hydroelectric power is renewable because rivers keep flowing and rain keeps refilling reservoirs. Unlike burning coal or gas, hydroelectric plants produce no air pollution during operation. However, building dams does change river ecosystems and can affect fish and wildlife.

About 16% of the world's electricity comes from hydroelectric power. Countries with lots of rivers and mountains, like Norway and Canada, rely heavily on hydroelectric energy. Some homes even use small hydroelectric generators in nearby streams, though most hydroelectric power comes from large dams.