Humans have been harnessing the energy of river currents for centuries, using water wheels spun by rivers initially to process grains and cloth. Today, hydropower provides about 16 percent of the world's electricity, generating power in all but two U.S. states.

Hydropower became an electricity source in the late 19th century, a few decades after British-American engineer James Francis developed the first modern water turbine. In 1882, the world’s first hydroelectric power plant began operating in the United States along the Fox River in Appleton, Wisconsin.

A typical hydroelectric plant is a system with three parts: a power plant where the electricity is produced, a dam that can be opened or closed to control water flow, and a