Water Cycle and Human Intervention
Energy is exchanged at all stages of the earth's natural water cycle, in which water evaporates from the oceans, forms into clouds, falls back to earth as precipitation, then flows downhill back into the oceans. By intervening in this natural water cycle, people can tap the energy naturally generated by flowing water and use it for their own needs, everything from grinding grain to creating electricity.
Hydropower Facilities
Hydropower facilities send water through a pipe, or penstock, which then moves the blades of turbines, spins a generator and makes electricity. Some facilities produce electricity continuously, as water flows past; while others store the water, then release it later to generate electricity.
Importance of Hydropower
Hydropower accounted for 6 percent of all energy generated in the United States in 2008, according to the U.S. Energy Information Administration. Among renewable energy sources, it accounted for 67 percent of all energy generated.