f the 80,000 dams in the United States, only about 2400, or 3%, are used for hydropower. The others are used in the following
ways: recreation, 35%; stock and farm ponds, 18%; flood control, 15%; public water supply, 12%; irrigation, 11%; and other uses
such as trade navigation, 6%. Dams that are not used to produce electricity can have as many environmental impacts as the
hydroelectric dams. Existing dams also have significant undeveloped hydropower potential.
A good example of a dam with more than one use is the Grand Coulee Dam in Washington. It not only generates power but also pumps water up from Lake Roosevelt to supply the state's irrigation system.
There are three types of hydropower projects: impoundment dams, diversion hydropower, and pumped storage. Impoundment dams store water, which may be released at a later time to meet rising demands for electricity.
Diversion hydropower facilities rely on part of a river being channeled through a canal or pipe for carrying water to a downstream powerhouse. They may or may not require a dam. The original hydropower facilities at Niagara Falls were of this type. Pumped storage requires an upper reservoir and a lower one. High-volume peaking power is generated when water flows through a turbine from the upper to the lower reservoir. The turbine is reversed to pump water back to the upper reservoir during the off-hours using cheaper electricity from another power station, for example, from a baseload nuclear plant. This seemingly inefficient approach to power generation is practical because many nuclear plants are more efficient if they run constantly to meet a certain level of power demand. Because considerably less power is normally needed at night, the nuclear plant's electricity can be used then to pump water from the lower reservoir to the upper one for later use in helping to meet unusually high, and often costly, electrical demand.
