Water moves constantly through a vast global cycle. It:
The energy of this water cycle, which is driven by the sun, is tapped most efficiently with hydropower.
Hydropower plants capture the energy of falling water to generate electricity. A dam impounds water to form a reservoir and raises the water level to create head. Head is the vertical distance the water falls as it passes through the dam (i.e. the difference in water level between the reservoir and the river below the powerplant). The water is directed through penstocks to turbines, which drive generators to produce electric power.
Just having water in the river isn’t enough. A good hydropower site must have enough streamflow as well as enough head. For example, the Mississippi River has tremendous streamflow, but only a few of its dams are high enough to provide the head needed for efficient hydropower development. Dams in the arid West may have plenty of head, but not enough streamflow. Where the right combination of streamflow and head exists, the Corps has included powerplants at its dam.