Powered by Water

A Resource Guide to Hydroelectricity

In light of the modern environmental movement, the concept of renewable energy may seem like a recent innovation. However, the idea of using natural resources to support society is anything but new. In fact, humans have been utilizing hydropower for over 2,000 years, when the Greeks used water wheels to produce flour. Just as ancient societies harnessed the power of falling water for mechanical output, people today are using water to produce electricity. In fact, hydroelectricity is the most common form of renewable energy in the United States.

How is Hydroelectricity Generated?

Hydroelectricity is generated by the force of falling water. The amount of electricity produced depends on the height, volume and speed of the water flow. Increasing each of these factors will generate more electricity. Therefore, a typical hydroelectric power plant is located near a highly elevated water source. The water is stored in a reservoir behind a dam, which stops the water flow. By restricting the flow of water at a high elevation, the dams maximize the stored potential energy. When the water is allowed to flow, the potential energy gets converted to kinetic energy. The force of the water strikes the blades of the turbines, causing them to rotate. The turbine's rotation causes the magnets within the generator to rotate as well, inducing a current. Finally, the current is put through a transformer where voltage is increased for long distance transmission over power lines.

Hydroelectricity vs. Non-Renewable Energy

Hydroelectricity has considerable advantages over traditional non-renewable energy sources. By definition, a non-renewable resource is not able to be replenished, and is thus limited. Not only does this diminish the availability of natural resources, but the methods used to obtain these resources is often harmful to the environment. Oftentimes, companies drill deep into the ground for resources such as oil, coal or gas. The power plants used to produce this type of energy also tend to create a significant amount of pollution. Compared to these methods, hydroelectric power is more beneficial to the environment. Water is a resource provided free by nature; it is constantly renewed through precipitation. From an economic perspective, hydroelectric plants tend to have high start-up but low maintenance costs. A hydroelectric power plant doesn't burn any fuel, so there is minimal air pollution; this reduces the amount of greenhouse has emissions. Although hydroelectricity is a step towards a more sustainable planet, it is not a perfect system.

The Environmental Impact of Hydropower

Although hydropower does not pollute, it can still have environmental impacts

Despite all the benefits of a hydroelectric system, there are still several disadvantages to consider. Constructing dams and reservoirs can cause destruction to the land and local wildlife, especially fish. Construction may disrupt fish migration patterns, impacting their population. The presence of a hydroelectric plant might also change the temperature and flow of the nearby water source is also a concern. This can have significant impacts on the surrounding eco-system. In some cases, reservoirs can actually produce greenhouse gases. Additionally, reservoir construction can cause flooding to upstream, and possibly downstream, areas. Reservoir construction might also relocate people from their homes.

Reservoir construction is "drying up" in the U.S.

Reservoir construction has slowed down considerably in the United States. This mostly because the construction requires a large volume of water and a large amount of land; this involves a massive amount of time, energy and money. In fact, most of the good locations for hydro plants have already been utilized. As this chart shows, reservoir construction increased rapidly towards the middle of the 20th Century, when urbanization was occurring at a rapid rate. However, this statistic has reached a steady plateau within the past 20 years. Future construction plans will most likely be based on small-scale hydro plants that generate electricity for a single community.

Hydroelectric Power in the United States and around the World

In the United States, most of our energy comes from non-renewable resources, such as fossil fuels. However, hydroelectricity still plays a significant role in the nation's energy production, accounting for 7 percent of the country's total energy consumption. As the Electric Utility Hydroelectric Net Generation by State chart shows, the use of hydroelectric power varies between states. States with high topographic relief, such as Washington, Idaho and Oregon, use hydroelectricity as their primary power source. Meanwhile, states such as Florida and Kansas, which do not have the appropriate topography, produce very little hydroelectric power.

As the World Net Hydroelectric Power Generation chart shows, the United States is not the only country making use of hydroelectricity. In fact, hydroelectric power makes up 19 percent of the world's total electricity production. According to the Energy Information Administration (EIA), the world's largest producers of hydroelectricity in descending order are: China, Canada, Brazil and the United States. About two-thirds of the possible econimic potential still remains to be constructed and there is still an abundance of untapped resources available in India, China, Central Africa and Latin America.

For more information about hydroelectricity, visit the following resources:

  • The History of Hydropower Development in the United States
    The U.S. Department of the Interior, Bureau of the Reclamation provides a time line and describes the history of how hydroelectric power developed in the United States.


  • How Hydroelectric Energy Works
    This site explains how hydro plants work, their role in the United States' energy consumption, addresses environmental concerns, and also discusses Low Impact Hydropower. Sponsored by the Union of Concerned Scientists.


  • Renewable Energy Sources in the United States
    The National Atlas of the United States discusses all forms of renewable energy sources, along with their respective advantages and disadvantages.


  • Hydroelectric Reservoirs: The Carbon Dioxide and Methane Emissions of a "Carbon Free" Energy Source (PDF)
    A graduate student term paper submitted by Claudia Farrer examining the environmental impacts, specifically carbon emissions, of hydroelectricity.


  • The Energy Story
    A resource directed towards kids and adolescents that explains energy, renewable energy, fossil fuels and other relevant information. Sponsored by the California Energy Commission.


  • Hydroelectric Energy
    National Geographic discusses hydroelectric power in the form of an encyclopedic entry, with a series of images and a link to relevant vocabulary.


  • Hydrothermal Power Systems
    The U.S. Department of Energy provides an extensive resource about renewable energy data in the United States (Hydropower is listed in Section VIII).


  • Hydroelectricity
    The U.S. Environmental Protection Agency describes the basics of a hydroelectric system and discusses its environmental impacts.