Large Hydropower

Large hydroelectric dams are the fourth largest source of electrical power in the U.S. after coal, nuclear, and natural gas. Most large dams in the U.S. were constructed in the 60 year period between 1910 and 1970. The primary purpose for the construction of dams, both large and small, within the U.S. was for flood control. The generation of electricity from the dams and the diversion of water for agriculture and drinking water were secondary benefits of the public works projects.

Hydroelectric power in the U.S. is centered in the Pacific Northwest and California which generate approximately 60% of the energy produced by hydroelectric dams in an average year. Other regions with notable hydroelectric resources include New York, Tennessee, Alabama, Arizona, Arkansas, and North Carolina.

How It’s Used

The water stored behind a dam has potential energy that is proportional to the difference in water levels behind and in front of the dam. This potential energy can be converted into electricity by funneling the dammed water through large pipes, known as penstocks, into large turbines. The force of the dammed water turns the turbine, generate electricity which can then be transported to end-use consumers hundreds of miles away.

Environmental Impact

Deriving energy from dams has a number of economic and environmental benefits. Since there are no fuel and minimal maintenance costs, the cost of hydropower is consistent and low once the cost of a dam’s construction has been recovered. Hydroelectricity also doesn’t have any radioactive waste, carbon dioxide emissions, other harmful air emissions associated with it.

But despite these advantages, no further large hydroelectric dams are likely to be built anytime soon in the U.S. In part this is because of the tremendous environmental impact that dams have on regional watersheds and the needs of the plants and wildlife that live within them. The most significant impact that large dams have is that they flood substantial amounts of land behind the dam to create reservoirs. While these reservoirs make for good recreational areas, they destroy the natural environments and ecosystems that resided in the area previously.

Creating an artificial barrier to the flow of rivers also presents a major problem to a number of different types of wildlife, especially migratory fish such as salmon. Salmon populations on both the Pacific and Atlantic coasts of the U.S. have seen their numbers drop dramatically in part because of the increased difficulty of getting between oceans and spawning grounds in the upper reaches of watersheds. Downstream wildlife can also be disturbed by the oftentimes sudden and significant fluctuations in water levels caused by dams as water is held behind the dam or flooded through in order to meet regional energy demands.

Last, dams slow the current of water behind the dam, resulting in silt and organic nutrients being carried in the river to settle out before the water is passed through the dam. While in the reservoir, the oxygen content of the river water may also drop before passing through the dam. These changes in the water quality typically have negative environmental effects on downstream plants and wildlife. The filling of reservoirs with silt will ultimately degrade the value of the dam to serve its primary purpose as a means of flood control.