A dam is a barrier constructed to hold back water either for general retainment, reservoir containment, or electricity generation.
Most dams are built to control flood hazards, to store water for irrigation or other uses, or to produce electricity. Along with these benefits come environmental costs including riparian habitat loss, water loss through evaporation and seepage, erosion, declining water quality, and air pollution. Other negative consequences of dams include changes in groundwater flow and the displacement of human populations.
Riparian, or streamside, habitats suffer both above and below dams. Valuable ecological zones that support specialized plants, riparian environments, and nearby shallows provide food and breeding grounds for birds, fish, and many other animals. Upstream of a dam, impounded water drowns riparian communities. Because reservoirs can fill hundreds of miles of river channel, and because many rivers have a long sequence of dams and reservoirs, habitat drowning can destroy a great deal of river biodiversity. Downstream, shoreline environments dry up because of water diversions (for irrigation or urban use) or because of evaporation and seepage losses in the reservoir. In addition, dams interrupt the annual floods that occur naturally on nearly all rivers. Seasonal flooding fertilizes and waters floodplains and clears or redistributes debris in river channels. These beneficial effects of flooding cease once a dam is installed on a river. Sixty percent of the world's 227 largest rivers are significantly fragmented, therefore resulting in damage to the river ecosystems.
As a result of covering streamside vegetation with water in the construction of the reservoir, its decomposition results in the emission of greenhouse gases such as methane gas (CH4). When flooding a large area, the amount of decay can contribute to large emissions of greenhouse gases, thus leading to climate change. However, if the dam is being used for the generation of hydroelectricity to replace other sources of power, there would potentially be a decrease in greenhouse gas emissions to offset those levels generated from decomposition.
Water losses from evaporation can be extreme: Lakes Powell and Mead on the Colorado River lose an average of 3 billion cubic feet (1 billion cubic meters) of water to evaporation each year; Egypt's Lake Nasser on the Nile River loses about 45 billion cubic feet (15 billion cubic meters). Water losses can also be the result of seepage into bedrock. As river water enters groundwater, water tables usually rise around a reservoir. In arid regions, increased groundwater can increase local fertility (sometimes endangering delicate dry-land plant species), but in moister regions, excessive groundwater can cause swamping. Evaporation from exposed groundwater can leave higher salt concentrations in the soil. The most catastrophic results of reservoir seepage into groundwater occur when saturated rock loses its strength. In such events valley walls can collapse, causing dam failure and disastrous flooding downstream.
In August 2010, Brazilian officials announced the impending start of construction on an $11 billion hydroelectric dam across the Amazon. Upon completion, the Belo Monte dam will be the third largest in the world. The reservoir behind the dam will displace thousands of people from cities and towns along the Xingu River basin, which will be flooded by the dam. The Belo Monte dam project is opposed by a broad array of environmental and indigenous rights groups, but advocates of the project argue that it is needed to produce electricity for expanding cities such as Sao Paulo, Brazil. About 200 square miles (520 square km) of land will be flooded by the dam. To mitigate the environmental impacts, the government has pledged $280 million toward a sustainable development plan for the area. In response, some indigenous tribes, including the Xikrin, Kayapo and the Parakan tribes, have withdrawn their formal opposition to the project. Critics of the project counter that there have been insufficient long-term environmental impact studies, especially of potential downstream impacts.
Dams are generally viewed as positive for humans because of the generation potential, assistance in reducing risks of flooding, and provision of drinking water sources, depending on the purpose of the dam. Perhaps the most significant negative effect of dam construction results from the displacement of human populations to previously uninhabited or lower-density areas. The World Commission on Dams estimates that 40 to 80 million people have been displaced because of dam construction. Because people normally settle along rivers, where water for drinking, irrigation, power, and transport are readily available, reservoir flooding can displace huge populations. The Three Gorges Dam on China's Chang Jiang (Yangtze River) displaced more than 1 million people and flooded some of western China's best agricultural land.
See also Air pollution ; Levees ; Water quality .
Brooks, K. B., and W. Cronon. Public Power, Private Dams: The Hells Canyon High Dam Controversy. Seattle: University of Washington Press, 2006.
Hayes, Walter P., and Michael C. Barnes. Dams: Impacts, Stability, and Design. New York: Nova Science, 2009.
International Symposium on Dams in the Societies of the Twenty-first Century and Luis Berga. Dams and Reservoirs, Societies and Environment in the Twenty-first Century. London: Taylor & Francis, 2006.
Leslie, Jacques. Deep Water: The Epic Struggle over Dams, Displaced People, and the Environment. New York: Farrar, Straus and Giroux, 2005.
National Atlas of the United States. “Major Dams of the United States.” http://nationalatlas.gov/mld/dams00x.html (accessed December 1, 2012).
Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20460, (202) 272-0167, http://water.epa.gov .
Tennessee Valley Authority, 400 W Summit Hill Dr., Knoxville, TN, 37902-1499, (865) 632-2101.
Mary Ann Cunningham
Alyson C. Heimer, MA