Large-scale hydropower plants are generally developed to produce electricity for government or electric utility projects. These plants are more than 30 MW in size, and there is more than 80,000 MW of installed generation capacity in the United States today.
Most large-scale 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 plant—water 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.