There are over 77,000 dams of significant size (over 6 ft
[1.8 m] tall) in the United States, and tens of thousands of additional uncharted smaller dam structures. Constructed for the purposes of harnessing water resources for irrigation , water supply, and hydroelectric power, dams can be a useful ally for human needs; they also have a major and long-term impact on the entire river ecosystem .
Since 1912, over 460 dams have been removed from United States waterways for both safety and environmental reasons. Dams associated with hydroelectric power projects are decommissioned after their useful life and removal is often recommended to return river ecosystems back to their natural state.
Damaged or obsolete dam structures may present a safety hazard, particularly if they are no longer regularly maintained. Upkeep of a dam is expensive, and local taxpayers may bear the brunt of caring for a dam that no longer serves any practical function. Finally, abandoned or unused dams can be aesthetically unpleasant in an otherwise scenic natural area.
Dams and their associated reservoirs can significantly impact river ecosystems and alter their natural course. The structures raise water temperatures, obstruct debris and nutrient flow, and prevent sediment dispersal. And they have an enormous impact on runs of salmon , steelhead, and other migratory fish, often in a relatively short period of time. For example, according to the National Fish and Wildlife Service the first dam to be built across the Connecticut River in 1798 resulted in the extinction of native Atlantic salmon stock in the river just a few years later. Despite the integration of fish lifts and fish ladders into modern dams, populations of native fish stocks are still greatly depleted by dam structures. Currently, several dams in the northwest are being removed to replenish many of these species which are protected under the Endangered Species Act .
There can be some short-term negative environmental issues associated with dam removal projects. Contaminated sediment that may collect under a dam can disperse throughout the area, and populations of non-native species that have settled in the dam-altered habitat may decline once the dam has been removed. However, in most cases dam removal encourages re-establishment of the native ecosystem and organisms.
Depending on its location and the ownership of the dam, removal may be governed by a variety of federal, state, and local authorities. For federal dam projects, the U.S. Army Corps of Engineers and/or the Bureau of Reclamation are charged with planning and completion of removal projects. However, only 3% of dams included in the U.S. Army Corps of Engineers national inventory of dams are owned by the federal government, compared with 58% of privately-owned dams.
Even if ownership of a dam is private, the waterways the structure harnesses are public and there is still a significant amount of regulatory oversight on all levels of government. The U.S. Environmental Protection Agency (EPA) and some state authorities typically request a full environmental assessment and written report called an "Environmental Impact Statement" (or EIS). The EIS outlines different scenarios for completion of a dam removal project, from no action (letting the dam naturally deteriorate over time) to full dismantling and removal with additional clean-up of dam sediments. It then describes the impact each approach will have on the ecosystem surrounding the dam site.
[Paula Anne Ford-Martin ]
McNully, Patrick. Silenced Rivers: The Ecology and Politics of Large Dams. 2nd edition. London: Zed Books, 2001.
Baish, Sarah K., et al. "The Complex Decision-Making Process for Removing Dams." Environment 44, no. 4 (May 2002): 20.
American Rivers, Friends of the Earth, & Trout Unlimited. Dam Removal Success Stories: Restoring Rivers through Selective Removal of Dams that Don't Make Sense. December 1999.
U.S. Army Corps of Engineers and the Federal Emergency Management Agency. National Inventory of Dams. [June 2002]. <http://crunch.tec.army.mil/nid/webpages/nid.cfm>.
American Rivers, 1025 Vermont Ave., N.W. Suite 720 , Washington, DC USA 20005 (202)347-7550, Fax: (202)347-9240, Email: [email protected], <http://www.amrivers.org/damremoval/default.htm>