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 32 turbines and is able to generate more than two billion watts of power.

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 Earth). Nonrenewable 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 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 artificial 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 riverbank. 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 O'Shaughnessy 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 O'Shaughnessy 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 O'Shaughnessy 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 reservoir's 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 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 recently, as they have quickly industrialized.

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 its citizens 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 Egypt's 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.

China's massive Three Gorges Dam project brings safe, affordable electricity to millions of people. It allows hospitals, schools, and factories to work longer, more reliable hours. It also allows 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.