Hydroelectric energy is made by moving water. Hydro comes from the Greek word for water.

Hydroelectric energy has been in use for thousands of years. Ancient Romans built turbines, which are wheels turned by flowing water. Roman turbines were not used for electricity, but for grinding grains to make flour and breads.

Water mills provide another source of hydroelectric energy. Water mills, which were common until the Industrial Revolution, are large wheels usually located on the banks of moderately flowing rivers. Water mills generate energy that powers such diverse activities as grinding grain, cutting lumber, or creating hot fires to create steel.

The first U.S. hydroelectric power plant was built on the Fox River in 1882 in Appleton, Wisconsin. This plant powered two paper mills and one home.

Harnessing Hydroelectricity

To harness energy from flowing water, the water must be controlled. A large reservoir is created, usually by damming a river to create an artificial lake, or reservoir. Water is channeled through tunnels in the dam.

The energy of water flowing through the dam's tunnels causes turbines to turn. The turbines make generators move. Generators are machines that produce electricity.

Engineers control the amount of water let through the dam. The process used to control this flow of water is called the intake system. When a lot of energy is needed, most of the tunnels to the turbines are open, and millions of gallons of water flow through them. When less energy is needed, engineers slow down the intake system by closing some of the tunnels.

During floods, the intake system is helped by a spillway. A spillway is a structure that allows water to flow directly into the river or other body of water below the dam, bypassing all tunnels, turbines, and generators. Spillways prevent the dam and the community from being damaged. Spillways, which look like long ramps, are empty and dry most of the time.

From Water Currents to Electrical Currents

Large, fast-flowing rivers produce the most hydroelectricity. The Columbia River, which forms part of the border between the U.S. states of Washington and Oregon, is a big river that produces massive amounts of hydroelectric energy.

The Bonneville Dam, one of many dams on the Columbia River, has 20 turbines and generates more than a million watts of power every year. Thats enough energy to power hundreds of thousands of homes and businesses.

Hydroelectric power plants near waterfalls can create huge amounts of energy, too. Water crashing over the fall line is full of energy. A famous example of this is the hydroelectric plant at Niagara Falls, which spans the border between the United States and Canada.

Hydroelectric energy generated by Niagara Falls is split between the U.S. state of New York and the Canadian province of Ontario. Engineers at Niagara Falls cannot turn the falls off, but they can severely limit the intake and control the amount of water rushing over the waterfall.

The largest hydroelectric power plant in the world is the enormous Three Gorges Dam, which spans the Yangtze River in China. It is 185 meters (607 feet) tall and 115 meters (377 feet) thick at its base. It has 26 turbines and will be able to generate more than a billion watts of power. The Three Gorges Dam is operating, but engineers are still working on the system. They are adding even more turbines and generators to the project.

Hydroelectric Energy and the Environment

Hydroelectricity relies on water, which is a clean, renewable energy source. A renewable source of energy is one that will not run out. Renewable energy comes from natural sources, like wind, sunlight, rain, tides, and geothermal energy (the heat produced inside the Earth). Non-renewable energy sources include coal, oil, and natural gas.

Water is renewable because the water cycle is continually recycling itself. Water evaporates, forms clouds, and then rains down on the Earth, starting the cycle again.


Reservoirs created by dams can provide large, safe recreational space for a community. Boaters and water skiers can enjoy the lake. Many reservoirs are also stocked with fish. The area around a reservoir is often a protected natural space, allowing campers and hikers to enjoy the natural environment.

Using water as a source of energy is generally a safe environmental choice. Its not perfect, though. Hydroelectric power plants require a dam and a reservoir. These man-made structures may be obstacles for fish trying to swim upstream. Some dams, including the Bonneville Dam, have installed fish ladders to help fish migrate. Fish ladders are a series of wide steps built on the side of the river and dam. The ladder allows fish to slowly swim upstream instead of being totally blocked by the dam.

Dams flood river banks, destroying wetland habitat for thousands of organisms. Aquatic birds such as cranes and ducks are often at risk, as well as plants that depend on the marshy habitat of a river bank. Operating the power plant may also raise the temperature of the water in the reservoir. Plants and animals near the dam have to adjust to this change or migrate elsewhere.

The OShaughnessy Dam on the Tuolumne River in the U.S. state of California was one of the first hydroelectric energy projects to draw widespread criticism for its impact on the environment. The dam, constructed in 1913, flooded a region called Hetch Hetchy Valley, part of Yosemite National Park. (The lake created by the OShaughnessy Dam is called the Hetch Hetchy Reservoir.) Environmental coalitions opposed the dam, citing the destruction of the environment and the habitats it provided. However, the power plant provided affordable hydroelectric energy to the booming urban area around San Francisco.

The Hetch Hetchy Reservoir is still a controversial project. Many people believe the OShaughnessy Dam should be destroyed and the valley returned to its native habitat. Others contend that destroying a source of energy for such a major urban area would reduce the quality of life for residents of the Bay Area.

There are limits to the amount of hydroelectric energy a dam can provide. The most limiting factor is silt that builds up on the reservoirs bed. This silt is carried by the flowing river, but prevented from reaching its normal destination in a delta or river mouth by the dam. Hundreds of meters of silt build up on the bottom of the reservoir, reducing the amount of water in the facility. Less water means less powerful energy to flow through the systems turbines. Most dams must spend a considerable amount of money to avoid silt build-up, a process called siltation. Some power plants can only provide electricity for 20 or 30 years because of siltation.

Hydroelectric Energy and People

Billions of people depend on hydroelectricity every day. It powers homes, offices, factories, hospitals, and schools. Hydroelectric energy is usually one of the first methods a developing country uses to bring affordable electricity to rural areas.

Hydroelectricity helps improve the hygiene, education, and employment opportunities available to a community. China and India, for instance, have built dozens of dams over the past decade, as their development has quickly grown.

The United States depended on hydroelectric energy to bring electricity to many rural or poor areas. Most of this construction took place during the 1930s. Dams were a huge part of the New Deal, a series of government programs that put people to work and brought electricity to millions of Americans during the Great Depression. The Bonneville Dam on the Columbia River, the Shasta Dam on the Sacramento River, and the Hoover Dam on the Colorado River are some dams constructed as part of the New Deal.

The most famous hydroelectric power project of the New Deal is probably the Tennessee Valley Authority (TVA). The TVA constructed a series of dams along the Tennessee River and its tributaries. Today, the TVA is the largest public power company in the U.S., providing affordable electricity for residents in the states of Alabama, Georgia, Kentucky, Mississippi, North Carolina, Tennessee, and Virginia.

However, hydroelectricity often comes at a human cost. The huge dams required for hydroelectric energy projects create reservoirs that flood entire valleys. Homes, communities, and towns may be relocated as dam construction begins.

Egypt began construction of the Aswan Dam complex on the Nile River in 1960. Engineers realized that ancient temples of Abu Simbel were going to be flooded by the reservoir, called Lake Nasser. These monuments were built directly into cliffs several stories tall. The Abu Simbel temples are a part of Egypts cultural heritage and a major tourist destination. Rather than have the monuments flooded, the government of Egypt relocated the entire mountainside to an artificial hill nearby. Today, Abu Simbel sits above the Aswan Dam.

Chinas massive Three Gorges Dam project will bring safe, affordable electricity to millions of people. It will allow hospitals, schools, and factories to work longer, more reliable hours. It will also allow people to maintain healthier lifestyles by providing clean water. Construction of the dam directly benefited workers, too. More than a quarter of a million people have found work with the project.

However, the project has forced more than a million people to relocate. Lifestyles were disrupted. Many families were relocated from rural towns on the banks of the Yangtze River to Chongqing, a major urban area with 31 million residents. Other people were relocated out of the province entirely.

Hydroelectric Energy: The Power of Running Water

Hydroelectric engineers just dam it.

Hoover Dam
The Hoover Dam was built during the Great Depression, a period when most people had little money and jobs were very scarce. Building the dam seemed like an impossible task. Many people said it could not be built.

Workers labored long, hard days for two years, building tunnels that are 15 meters (50 feet) widebig enough to fit a commercial airplane without its wings. The Hoover Dam is 221 meters (726 feet) tall52 meters (171 feet) taller than the Washington Monument in Washington, D.C.

Building the dam gave hope and dignity to many victims of the Great Depression. It gave people a job and a way to earn money. The Hoover Dam is still in use, providing power to 1.7 million people in Arizona, California, and Nevada. It is often considered an engineering milestone and is named for Herbert Hoover, the U.S. president who helped make the project happen.

Washington's Energy
The state of Washington is the largest consumer of hydroelectric power in the United States. The state used almost 58 million watts of hydroelectricity in 2009, more than double the next-largest state consumer, Oregon.

Hydroelectric Nations
Hydroelectric power provides almost all the energy for some nations. Norway, Brazil, and the Democratic Republic of Congo all get more than 90 percent of their electricity from hydroelectric power plants. Plans for a new hydroelectric plant in the Democratic Republic of Congo may link homes and businesses in Europe with the African power supply.

adjust
Verb

to change or modify something to fit with something else.

affordable
Adjective

reasonably priced, not expensive.

ancient
Adjective

very old.

ancient Rome
Noun

civilization founded on the Mediterranean Sea, lasting from the 8th century BCE to about 476 CE.

aquatic
Adjective

having to do with water.

bank
Noun

a slope of land adjoining a body of water, or a large elevated area of the sea floor.

Bay Area
Noun

region surrounding San Francisco Bay in the U.S. state of California.

block
Verb

to prevent something from happening.

Bonneville Dam
Noun

series of hydroelectric dams and locks across the Columbia River in the U.S. states of Oregon and Washington.

bypass
Verb

to go around or skip.

cite
Verb

to give as an example.

Noun

steep wall of rock, earth, or ice.

Noun

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

coal
Noun

dark, solid fossil fuel mined from the earth.

continual
Adjective

ongoing or repeating frequently.

controversial
Noun

questionable or leading to argument.

crane
Noun

large wading bird.

cultural heritage
Noun

traditions and customs of a specific population.

dam
Noun

structure built across a river or other waterway to control the flow of water.

Noun

the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.

destination
Noun

place where a person or thing is going.

developing world
Noun

nations with low per-capita income, little infrastructure, and a small middle class.

diverse
Adjective

varied or having many different types.

dozen
Noun

a group of 12.

duck
Noun

aquatic bird.

electricity
Noun

set of physical phenomena associated with the presence and flow of electric charge.

employment
Noun

job or work.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

environmental coalition
Noun

group of people or organizations who are united in defense of the environment or an environmental issue.

evaporate
Verb

to change from a liquid to a gas or vapor.

factory
Noun

one or more buildings used for the manufacture of a product.

Noun

imaginary line along which parallel rivers plunge, or fall.

fish ladder
Noun

series of steps overflowing with water, where fish can migrate upstream around a barrier such as a dam.

Noun

overflow of a body of water onto land.

flour
Noun

ground grain, usually of wheat.

gallon
Noun

unit of volume equal to four quarts (3.79 liters).

generate
Verb

to create or begin.

generator
Noun

machine that converts one type of energy to another, such as mechanical energy to electricity.

Noun

heat energy generated within the Earth.

Noun

harvested seed of such grasses as wheat, oats, and rice.

Great Depression
Noun

(1929-1941) period of very low economic activity in the U.S. and throughout the world.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Herbert Hoover
Noun

(1874-1964) 31st president of the United States.

Noun

energy generated by moving water converted to electricity. Also known as hydroelectricity.

hydroelectricity
Noun

power generated by moving water converted to electricity. Also called hydroelectric energy or hydroelectric power.

hygiene
Noun

science and methods of keeping clean and healthy.

impact
Noun

meaning or effect.

Industrial Revolution
Noun

change in economic and social activities, beginning in the 18th century, brought by the replacement of hand tools with machinery and mass production.

intake system
Noun

process engineers use to control the flow of water through a dam.

lumber
Noun

precisely cut pieces of wood such as boards or planks.

Noun

wetland area usually covered by a shallow layer of seawater or freshwater.

migrate
Verb

to move from one place or activity to another.

Noun

place where a river empties its water. Usually rivers enter another body of water at their mouths.

Noun

type of fossil fuel made up mostly of the gas methane.

New Deal
Noun

(1933-1938) series of U.S. government programs intended to provide economic "relief, recovery, and reform" to Americans during the Great Depression.

Noun

energy resources that are exhaustible relative to the human life span, such as gas, coal, or petroleum.

obstacle
Noun

something that slows or stops progress.

oil
Noun

fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.

organism
Noun

living or once-living thing.

power
Noun

ability to do work.

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division of a country larger than a town or county.

public
Adjective

available to an entire community, not limited to paying members.

quality of life
Noun

satisfaction with the material, cultural, and technological conditions of a region or population.

Noun

liquid precipitation.

recreational
Adjective

having to do with activities done for enjoyment.

recycle
Verb

to clean or process in order to make suitable for reuse.

reduce
Verb

to lower or lessen.

relocate
Verb

to move a residence or business from one place to another.

rely
Verb

to depend on.

renewable energy
Noun

energy obtained from sources that are virtually inexhaustible and replenish naturally over small time scales relative to the human life span.

Noun

natural or man-made lake.

reuse
Verb

to use again.

Noun

large stream of flowing fresh water.

Noun

regions with low population density and large amounts of undeveloped land. Also called "the country."

severe
Adjective

harsh.

Noun

small sediment particles.

siltation
Noun

process where sediment and silt build up on the bottom of a reservoir, reducing the amount of water it can hold.

spillway
Noun

dam structure that allows excess water to flow directly into the river or other body of water below the dam.

steel
Noun

metal made of the elements iron and carbon.

stock
Verb

to supply.

sunlight
Noun

visible radiation from the sun.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

temple
Noun

building used for worship.

Tennessee Valley Authority (TVA)
Noun

largest public utility in the U.S.

Three Gorges Dam
Noun

electrical power plant along the Yangtze River in China.

Noun

rise and fall of the ocean's waters, caused by the gravitational pull of the moon and sun.

tourist
Noun

person who travels for pleasure.

Noun

stream that feeds, or flows, into a larger stream.

turbine
Noun

machine that captures the energy of a moving fluid, such as air or water.

upstream
Adjective

toward an elevated part of a flow of fluid, or place where the fluid passed earlier.

Noun

developed, densely populated area where most inhabitants have nonagricultural jobs.

Noun

movement of water between atmosphere, land, and ocean.

Noun

flow of water descending steeply over a cliff. Also called a cascade.

water mill
Noun

large mechanism powered by flowing water.

watt
Noun

unit of power. Abbreviated w.

Noun

area of land covered by shallow water or saturated by water.

widespread
Adjective

affecting a large area or community.

wind
Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.