Hydropower is currently the largest and least expensive source of renewable electricity produced in the United States. Large and small-scale hydropower projects are most commonly used by clean-power generators to produce electricity.
Water flowing through a hydropower plant. Hydropower produces 10% of the nation's electricity. Photo credit: U.S. Army Corps of Engineers
Most hydropower projects use a dam and a reservoir to retain water from a river. When the stored water is released, it passes through and rotates turbines, which spin generators to produce electricity. Water stored in a reservoir can be accessed quickly for use during times when the demand for electricity is high.
Dammed hydropower projects can also be built as power storage facilities. During periods of peak electricity demand, these facilities operate much like a traditional hydropower plantówater released from the upper reservoir passes through turbines, which spins generators to produce electricity. However, during periods of low electricity use, electricity from the grid is used to spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the water can be stored until the demand for electricity is high again.
A third type of hydropower project, called "run of the river," does not require large impoundment dams (although it may require a small, less obtrusive dam). Instead, a portion of a river's water is diverted into a canal or pipe to spin turbines.
Many large-scale dam projects have been criticized for altering wildlife habitats, impeding fish migration, and affecting water quality and flow patterns. As a result of increased environmental regulation, the National Hydropower Association forecasts a decline in large-scale hydropower use through 2020. Research and development efforts have succeeded in reducing many of these environmental impacts through the use of fish ladders (to aid fish migration), fish screens, new turbine designs, and reservoir aeration. Although funding has been limited, current research focuses on the development of a "next generation turbine," which is expected to further increase fish survival rates and improve environmental conditions.
Where It's Available
By far, the largest use of water for hydroelectric power was in the Pacific Northwest, along the Columbia River, and in New York, on the Niagra and St. Lawrence River systems.