Water has long been used as a source of energy, beginning with the Greeks use of water wheels over 2,000 years ago. For over a century, hydropower has been used to generate electricity from falling water. Hydroelectric power stems from the process of using water’s energy as it flows from higher to lower elevation, rotating hydraulic turbines to create electricity. Tidal power, although not widely used, can also generate hydroelectricity by utilizing the same principle.

Hydropower is considered to be a clean, renewable source of energy, emitting a very low level of greenhouse gases when compared to fossil fuels. It has a low operating cost once installed and can be highly automated. An additional benefit is that the power is generally available on demand since the flow of water can be controlled. Using hydroelectric power also has disadvantages. Dams can block fish passage to spawning grounds or to the ocean, although many plants now have measures in place to help reduce this impact. The diversion of water can impact stream flow, or even cause a river channel to dry out, degrading both aquatic and streamside habitats. Hydroelectric plants can also have an impact on water quality by lowering the amount of dissolved oxygen in the water. In the reservoir, sediments and nutrients can be trapped and the lack of water flow can create a situation for undesirable growth and the spread of algae and aquatic weeds.

One incentive for hydroelectric facilities to help mitigate their overall impact on the environment is through green power certification. The Low Impact Hydropower Institute (LIHI) created a voluntary certification program whereby facilities are classified as low impact after passing a series of tests that demonstrate minimal impact. In 2007, less than 30 facilities in the U.S. had that distinction. Certification programs, such as the one set by the LIHI, can benefit hydropower efforts by attracting consumers concerned about energy source impacts.

While the use of water to produce electricity is an attractive alternative to fossil fuels, the technology must still overcome obstacles related to space requirements, building costs, environmental impacts, and the displacement of people. However, within the U.S., possible locations for new hydropower projects are beginning to diminish.

Recommended Resources

How Hydropower Plants Work
HowStuffWorks.com details the basic components of a hydropower plant and how it operates.

Wind & Hydropower Technologies Program: Hydropower Technologies
The U.S. Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy’s website for their hydropower technologies program gives an overview of how hydropower works, advantages and disadvantages of using hydropower, the history of the technology, and the latest in research developments.

Data & Maps

Idaho National Laboratory (INL): State Resource Assessment Reports
INL’s website includes individual reports on the hydropower potential and current capacity of each state (except Delaware).

Laws & Treaties

Water Power
The U.S. Federal Energy Regulatory Commission (FERC) provides information on the licensing, compliance, and safety and inspection of hydroelectric power in the U.S.


New Scientist: Hydroelectric Power’s Dirty Secret Revealed
In this February 2005 article from NewScientist.com, Duncan Graham-Rowe argues that hydroelectric power can damage the climate via greenhouse gas emissions, sometimes emitting more greenhouse gases than fossil fuels.

PBS: Great Wall Across the Yangtze
PBS details the Three Gorges Dam project and presents both sides of the controversy surrounding its construction, including an international perspective on the issue.

For the Classroom

Energy Story: Hydro Power
The California Energy Commission’s Energy Quest website presents a simple chapter about hydroelectric power that describes the technology’s history, its use in the U.S., and how dams operate.

Foundation for Water and Energy Education (FWEE): Hydro Tours
The FWEE website offers a variety of visual tours, including a ‘walk’ through a hydroelectric project. They also provide a list of educational activities and resources, including a hands-on science curriculum for middle school called ?The Nature of Water Power?.

Big Dams, Big Dilemmas
This National Geographic activity examines the effects existing dams have on the environment and predicts possible effects of proposed dams. [Grades 9-12]

Three Gorges Dam: The Biggest Dam in the World
Discovery Education created this activity for students to learn about the background and controversy surrounding the Three Gorges Dam. Students will also build their own dams in order to learn about the engineering principles used in their construction. [Grades 6-8]


Botkin, Daniel B. and Edward A. Keller. Environmental Science: Earth as a Living Planet, Fourth ed., Wiley: 2003.

Great Wall Across the Yangtze from PBS.

Griffiths, Dan. Three Gorges Dam Reaches for the Sky, BBC News, May 19, 2006.

Wind & Hydropower Technologies Program: Hydropower Technologies from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy, August 2005.