The importance of hydropower as a source of electricity generation varies by geographic region. While hydropower accounted for 8% of total U.S. electricity generation in 2011, it provided over half of the electricity in the Pacific Northwest. Because hydroelectric generation relies on precipitation, it varies widely from month to month and year to year.

Hydropower is our largest renewable electricity source.
bar graph showing hydropower and other renewable electricity generation, 1990-2011, in million megawatthours.
Source: U.S. Energy Information Administration, Electric Power Monthly, (July 2012).

Did You Know?

The U.S. Army Corps of Engineers was the largest operator of U.S. conventional hydroelectric generating capacity in 2011, followed by the U.S. Bureau of Reclamation.
The Nation's oldest power facilities are hydroelectric plants.
line graph showing, in gigawatts, the 2010 hydro capacity by initial operating year. See Today in Energy, July 8, 2011 for data.

Did You Know?

The Nation's 25 oldest operating power facilities are hydroelectric, the oldest of which began operating in 1891.

Hydroelectric generation is highly variable because it depends on precipitation.
line graph showing U.S. hydroelectric net generation, in thousand MWh. See Electric Power Monthly for data.
Source: U.S. Energy Information Administration, Electric Power Monthly, (July 2012).

Did You Know?

The Grand Coulee Dam, operated by the U.S. Bureau of Reclamation, is the fifth-largest power plant operating in the world and the largest in the Nation, with a net summer capacity of 7,079 Megawatts.

Conventional hydroelectric generators of varying capacity operated in 48 states in 2011. Operating expenses for hydroelectric generators are lower than for most other forms of electricity generation but facilities are limited by geography and operations are subject to seasonal constraints. There is a large concentration of capacity in the Pacific Northwest, contributing to low wholesale and retail electricity prices in that region, especially in the spring runoff season.

A United States map showing conventional hydroelectric capacity as a percent of total capacity by state. See Form EIA-860, Annual Electric Generator Report for data.
Source: U.S. Energy Information Administration, Form EIA-860 Annual Electric Generator Report, (July 2012).

Types of Hydroelectric Plants

Conventional hydroelectric generators were among the oldest of the Nation's power plants operating in 2011. The vast majority of hydroelectric generators were built before 1980 and recent changes to hydroelectric capacity have been small.

Conventional hydroelectric plants come in two broad categories: run-of-river and storage. A run-of-river plant utilizes the flow of a waterway (usually a river) to turn a turbine, while a storage plant creates a reservoir using a dam that controls water flow over a turbine.

A run-of-river plant has little control over generator output. A storage plant has some control over generation by controlling spillway water flow at intake through the dam, but is still constrained by total reservoir water levels.

There are several other types of non-conventional hydroelectric generators including pumped-storage, hydrokinetic axial flow and wave buoy turbines. Pumped-storage generators represent the only non-conventional form of hydroelectric generation currently in wide commercial use. These systems pump water to high elevations during low load periods then run the same water through the turbines to produce electricity during high demand times. Other hydroelectric technologies, such as wave buoys, are being developed and demonstrated but not in wide use at this time.

A map showing hydroelectric generators in and around the United States. Data from Energy Velocity.

Hydroelectric Generation Varies Considerably

Depending on the season and precipitation, the hydroelectric share of total generation varies from 4% to 10%. Precipitation, snowpack, drought conditions, and other meteorological factors contribute to water availability for generation through hydroelectric dams. For example, early snow melt runoff in the Pacific Northwest, elevated snowpack levels throughout much of the Western river basins, and significant rainfall in March in areas of high hydropower capacity resulted in a large increase in hydroelectric generation in 2011.

Hydropower and the Environment

Most hydroelectric generators in the United States were co-located at dams originally built for other purposes, like flood control, municipal water supply, and irrigation. Operations are affected by environmental considerations associated with water use, fish populations, and impact on wildlife in surrounding areas. For example, fish ladders and lifts have been constructed at many dams to help protect migrating populations.