How it Works
Water constantly moves through a vast global cycle: the sun evaporates the water from lakes and oceans; the water forms clouds that precipitates as rain or snow; and then flows back to the lakes and oceans. The energy of this water cycle, which is driven by the sun, is tapped most efficiently as hydropower.

Dams provide power by harnessing the kinetic energy of the water as it falls over the dam. Moving water can be used to do work because its potential (stored) energy changes to kinetic energy. When water is elevated (such as on the brink of a waterfall or in a reservoir behind a dam), it has gravitational potential energy. This potential energy changes to kinetic energy when the water falls or is allowed to flow. For example, when holes in the bottom of a container allow water to escape, the water's potential energy becomes kinetic energy. The farther the water falls, the more kinetic energy it has. Penstocks guide flowing water into turbines that generate electricity. The amount of water released can be adjusted to meet the demand of energy needed. Spillways divert excess water that builds up behind the dam.

The energy generated by water can be transferred to other objects, causing them to move and thus accomplish work. Using the energy in water involves locating or creating places where potential energy is changed to kinetic energy (such as a waterfall or dam). Water can also be channeled and diverted to where it is used to do work (such as over a turbine or into a lock).

Locations
Hydroelectric sites are generally located at places on rivers or streams that can be easily dammed to create a reservoir of water. Most of the larger hydroelectric dams in the United States are on sizable rivers, such as the Colorado and Columbia in the West and those in the Tennessee Valley Authority region in the South. Most large useable sites in the U.S. have already been dammed for hydropower. The potential for new hydroelectric power stations is limited.

Many Wisconsin cities use dams to provide their residents and industries with electricity. A number of industries in Wisconsin and the United States are located near large hydroelectric sites so they can use the cheap, reliable electricity these plants provide. Examples include the paper industry in Wisconsin and the aluminum smelting industry in the Pacific Northwest. The world's first hydroelectric power station was built in Appleton, Wisconsin in 1882, only three years after Thomas Edison's invention of the light bulb. This station's output was 12.5 kilowatts, which lit two paper mills and a house. The Wisconsin River, which runs the length of Wisconsin and spills into the Mississippi River, has been described as the "hardest working river in the nation." It has 25 hydroelectric dams. Find out more by visiting the Wisconsin Valley Improvement Company. Most of the hydroelectric dams on the Wisconsin river are located on the upper two-thirds of the river. Ten of these have generating capacities greater than one megawatt.

In 2003, hydropower supplied the United States with seven percent of its electricity generation. Wisconsin has 72 hydroelectric sites with a total generating capacity of 449 megawatts. These sites produced about two billion kilowatt-hours of electricity in 2003. Hydropower accounts for 80% of the nation's total renewable electricity generation. The United States is the second largest producer of hydropower in the world with Canada coming in first and Brazil, third. Canada generates more than 70 percent of its electricity from hydropower. The small country of Norway generates 95 percent of its electricity from hydropower. One fifth of all electricity generated in the world is hydropower. To maintain the current power supply without hydropower, the U.S. would have to burn an additional 121 million tons of coal, 27 million barrels of oil, and 741 billion cubic feet of natural gas combined. The release of harmful greenhouse gases can be avoided by using hydropower.

View Map of Electric Utility Hydroelectric Sites in Wisconsin (.pdf)