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Hydropower Basics

Energy from moving water

Image of the water cycle. Solar energy heats water on the surface, causing it to evaporate.

This water vapor condenses into clouds and falls back onto the surface as precipitation.

The water flows through rivers back into the oceans, where it can evaporate and begin the cycle over again.

Source: Adapted from National Energy Education Development Project (public domain)

Image of how a hydropower plant works.
The water flows from behind the dam through penstocks, turns the turbines, and causes the generators to generate electricity.
The electricity is carried to users by a transmission line.
Other water flows from behind the dam over spillways and into the river below.

Source: Tennessee Valley Authority (public domain)

bar graph showing hydropower and other renewable electricity generation, 1995-2015, in million megawatthours.

Hydropower generates electricity

Hydropower is the largest renewable energy source for electricity generation in the United States. In 2015, hydropower accounted for about 6% of total U.S. electricity generation and 46% of electricity generation from all renewables.

Because the source of hydroelectric power is water, hydroelectric power plants are usually located on or near a water source.

Hydropower relies on the water cycle

Understanding the water cycle is important to understanding hydropower. The water cycle has three steps:

  • Solar energy heats water on the surface of rivers, lakes, and oceans, which causes the water to evaporate.
  • Water vapor condenses into clouds and falls as precipitation (rain, snow, etc.).
  • Precipitation collects in streams and rivers, which empty into oceans and lakes, where it evaporates and begins the cycle again.

The amount of precipitation that drains into rivers and streams in a geographical area determines the amount of water available for producing hydropower. Seasonal variations in precipitation and long-term changes in precipitation patterns, such as droughts, have a big impact on hydropower production.

Hydroelectric power is produced from moving water

The volume of the water flow and the change in elevation (or fall) from one point to another determine the amount of available energy in moving water. Swiftly flowing water in a big river, like the Columbia River that forms the border between Oregon and Washington, carries a great deal of energy in its flow. Water descending rapidly from a high point, like Niagara Falls in New York, also has substantial energy in its flow.

At both Niagara Falls and the Columbia River, water flows through a pipe, or penstock, then pushes against and turns blades in a turbine to spin a generator to produce electricity. In a run-of-the-river system, the force of the current applies pressure on a turbine. In a storage system, water accumulates in reservoirs created by dams and is released as needed to generate electricity. Watch a video about hydropower on the Bonneville Power Administration website.

History of hydropower

Hydropower is one of the oldest sources of energy for producing mechanical and electrical energy. Hydropower was used thousands of years ago to turn paddle wheels to help grind grain. Before steam power and then electricity were available in the United States, grain and lumber mills were powered directly with hydropower. The first industrial use of hydropower to generate electricity in the United States occurred in 1880, when 16 brush-arc lamps were powered using a water turbine at the Wolverine Chair Factory in Grand Rapids, Michigan.

The first U.S. hydroelectric power plant opened on the Fox River near Appleton, Wisconsin, on September 30, 1882.

For more information about hydropower, see Hoover Dam, a hydroelectric facility completed in 1936 on the Colorado River between Arizona and Nevada. this dam created Lake Mead, a 110-mile-long national recreational area that offers water sports and fishing in a desert setting.

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Where Hydropower is Generated

Most U.S. hydropower capacity is in the West

More than half of U.S. hydropower capacity1 for electricity generation is concentrated in Washington, California, and Oregon. Washington has the most hydroelectric generating capacity of any state and is the site of the Grand Coulee Dam, the largest hydropower facility in the United States. New York has the largest hydroelectric capacity of all states east of the Mississippi River, followed by Alabama.

Most hydropower is produced at large facilities built by the federal government. The West has many of the largest hydropower dams. Many smaller facilities also operate around the country.

The share of U.S. total hydropower generation capacity by the top five states in 2015:

  • Washington—27%
  • California—13%
  • Oregon—11%
  • New York—6%
  • Alabama—4%

Because the amount of electricity generation from hydropower depends on precipitation, the ranking of each state in annual hydroelectricity generation may be different from its ranking in generation capacity.

The share of U.S. total hydropower generation by the top five states in 2015:

  • Washington—38%
  • Oregon—13%
  • New York—11%
  • California—6%
  • Alabama—4%

Most dams were not built for electricity generation

Only a small percentage of dams in the United States produce electricity. Most dams were constructed for irrigation and flood control and do not have hydroelectricity generators. The U.S. Department of Energy (DOE) estimated that in 2012, non-powered dams in the United States had a total of 12,000 megawatts (MW) of potential hydropower capacity.

1Net summer capacity for conventional hydroelectric power; does not include pumped storage hydropower.

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Hydropower & the Environment

Hydropower generators produce clean electricity, but hydropower does have environmental impacts

Most dams in the United States were built mainly to control floods and to help supply water for cities and irrigation. Although many of these dams have hydroelectric generators, only a small number of dams were built specifically for hydropower generation. Although hydropower generators do not directly produce emissions of air pollutants, dams, reservoirs, and the operation of hydropower electric generators can affect the environment.

Fish ladder at the Bonneville Dam on the Columbia River separating Washington and Oregon
Fish Ladder at the Bonneville Dam on the Columbia River Separating Washington and Oregon.

Source: Stock photography (copyrighted)

Did you know?

The Safe Harbor Dam on the Susquehanna River in Pennsylvania has elevators that lift migrating shad from the base of the dam to the top of the reservoir.

A dam that creates a reservoir (or a dam that diverts water to a run-of-river hydropower plant) may obstruct fish migration. A reservoir and dam can also change natural water temperatures, water chemistry, river flow characteristics, and silt loads. All of these changes can affect the ecology and the physical characteristics of the river. These changes may have negative impacts on native plants and on animals in and around the river. Reservoirs may cover important natural areas, agricultural land, or archeological sites. A reservoir and the operation of the dam may also result in the relocation of people. The physical impacts of a dam and reservoir, the operation of the dam, and the use of the water can change the environment over a much larger area than the area covered by a reservoir.


Although no new hydropower dams have been built recently in the United States, they are being built in other countries like China. Manufacturing the concrete and steel used to construct these dams requires equipment that may produce emissions. If fossil fuels are used as the energy source to make these materials, then the emissions from the equipment could be associated with the electricity that hydropower facilities generate. However, given the long operating lifetime of a hydropower plant (50 years to 100 years) these emissions are offset by the emissions-free electricity that is generated. {/EE}

Carbon dioxide and methane may also form in reservoirs and be emitted into the atmosphere. The exact amount of greenhouse gases produced in hydropower reservoirs is uncertain. The emissions from reservoirs in tropical and temperate regions, including the United States, may be equal to or greater than the greenhouse effect of the carbon dioxide emissions from an equivalent amount of electricity generated with fossil fuels. Scientists at Brazil's National Institute for Space Research designed a system to capture methane in a reservoir and burn it to produce electricity.

Fish ladders help salmon reach their spawning grounds

Hydropower turbines kill and injure some of the fish that pass through the turbine. The U.S. Department of Energy has sponsored the research and development of turbines that could reduce fish deaths to lower than 2%, in comparison to fish kills of 5% to 10% for the best existing turbines.

There are many species of fish, such as salmon, that swim from the sea up rivers and streams to reproduce in their spawning grounds in the beds of rivers and streams. Dams block their way. Different approaches to fixing this problem have been used, including the construction of fish ladders and elevators that help fish move through or around dams to the spawning grounds upstream.

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Tidal Power

The gravitational pull of the moon and sun along with the rotation of the earth cause the tides. In some places, tides cause water levels near the shore to vary up to 40 feet. People harnessed this movement of water to operate grain mills more than a 1,000 years ago in Europe. Today, tidal energy systems generate electricity. Producing tidal energy economically requires a tidal range of at least 10 feet.

Barrage of the tidal power plant on the estuary of the Rance River in Bretagne, France
Dam of the tidal power plant on the estuary of the Rance River, Bretagne, France

Source: Stock photography (copyrighted)

Tidal barrages

One type of tidal energy system uses a structure similar to a dam called a barrage. The barrage is installed across an inlet of an ocean bay or lagoon that forms a tidal basin. Sluice gates on the barrage control water levels and flow rates to allow the tidal basin to fill on the incoming high tides and to empty through an electricity turbine system on the outgoing ebb tide. A two-way tidal power system generates electricity from both the incoming and outgoing tides.

A potential disadvantage of tidal power is the effect a tidal station can have on plants and animals in estuaries of the tidal basin. Tidal barrages can change the tidal level in the basin and increase turbidity (the amount of matter in suspension in the water). They can also affect navigation and recreation.

Several tidal power barrages operate around the world. The Sihwa Lake Tidal Power Station in South Korea has the largest electricity generation capacity at 254 Megawatts (MW). The oldest operating tidal power plant is in La Rance, France, with 240 MW of electricity generation capacity. The next largest tidal power plant is in Annapolis Royal in Nova Scotia, Canada, with 20 MW of electricity generation capacity. China, Russia, and South Korea all have smaller tidal power plants.

Diagram of tidal turbine.

Source: Adapted from National Energy Education Development Project (public domain)

The United States does not have any tidal power plants, and it only has a few sites where tidal energy could be economical to produce. France, England, Canada, and Russia have much more potential to use tidal power.

Tidal turbines

Tidal turbines look similar to wind turbines. They can be placed on the sea bed where there is strong tidal flow. Because water is about 800 times denser than air, tidal turbines have to be much sturdier and heavier than wind turbines. Tidal turbines are more expensive to build than wind turbines, but capture more energy with the same size blades. Strangford Lough, Scotland, and Uldolmok, South Korea, both have 1.5 MW tidal turbines. A tidal turbine project is under development in the East River of New York. Another project with up to 400 MW of electricity generation capacity is under development in northern Scotland.

Tidal fences

A tidal fence is a type of tidal power system that has vertical axis turbines mounted in a fence or row placed on the sea bed, similar to tidal turbines. Water passing through the turbines generates electricity. As of the end of 2016, no tidal fence projects are currently operating.

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Wave Power

Waves have a lot of energy

Pelamis wave power device off the coast of Portugal
The Pelamis wave power device in use in Portugal

Source: Marine and Hydrokinetic Technologies Program, U.S. Department of Energy, Energy Efficiency and Renewable Energy (public domain)

Wave energy site
Diagram of wave energy site.

Source: Adapted from National Energy Education Development Project (NEED)

CETO underwater wave energy device
CETO Underwater Wave Energy Device

Source: Tuscanit, Wikimedia Commons author (GNU free documentation license) (public domain)

Waves form as wind blows over the surface of open water in oceans and lakes. Ocean waves contain tremendous energy. The theoretical annual energy potential of waves off the coasts of the United States is estimated to be as much as 2.64 trillion kilowatthours, or the equivalent of about 65% of U.S. electricity generation in 2015. The west coasts of the United States and Europe, and the coasts of Japan and New Zealand, have potential sites for harnessing wave energy.

Different ways to channel the power of waves

One way to harness wave energy is to bend or focus waves into a narrow channel to increase their size and power and to spin the turbines that generate electricity. Waves can also be channeled into a catch basin or reservoir where the water flows to a turbine at a lower elevation, similar to the way a hydropower dam operates.

Many other methods of capturing wave energy are under development. These methods include placing devices on or just below the surface of the water and anchoring devices to the ocean floor. The U.S. Department of Energy's Marine and Hydrokinetic Technology Database provides information on marine and hydrokinetic renewable energy, both in the U.S. and around the world.

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Ocean Thermal

Ocean thermal energy conversion system
A Diagram of an Ocean Thermal Energy System
An experimental OTEC plant on the Kona Coast in Hawaii
An aerial photo of OTEC Plant on the Kona Coast of Hawaii

Source: U.S. Department of Energy (public domain)

Energy from the sun heats the surface water of the ocean. In tropical regions, surface water can be much warmer than deep water. This temperature difference can be used to produce electricity. The Ocean Thermal Energy Conversion (OTEC) system uses a temperature difference (of at least 77o F) to power a turbine to produce electricity.

The United States became involved in OTEC research in 1974 with the establishment of the Natural Energy Laboratory of Hawaii Authority. The laboratory is one of the world's leading test facilities for OTEC technology. The Natural Energy Laboratory and the U.S. Navy's Office of Naval Research supported the development of a 105 kilowatt demonstration OTEC plant in Hawaii.

No large-scale operation of OTEC exists today because of the many challenges associated with the technology. The U.S. Energy Information Administration does not project any generation of electricity with OTEC systems in the Annual Energy Outlook 2016, which has projections out to 2040.