Hydro means water in Latin – so hydro power is the power of water.

People have used water power for more than 2,000 years. Ancient Egyptians used water wheels for grinding grain

and early Americans learned how to use them for sawing wood.

Hydroelectric power makes electricity by using the energy from falling water.

The water comes from big dams across rivers, and flows down great tubes to turn electricity generators.

Most of the worlds biggest rivers are already used for this.

Workers can change the amount of water flowing through the dam depending on the weather and how much electricity people need.

This is the biggest hydroelectric dam in the United States. It is the Grand Coulee Dam in Washington.

The first hydroelectric power plant in the U.S. opened in 1882.

People also use tides to produce electricity.

A barrier is built across the river.

When the tide moves up the river, the energy in the water turns turbines and makes electricity.

Wave power can also drive generators but this is still a very new idea. Just a few experimental machines are used today.