Once a sign of modernization and growth, dams are often seen today as symbols of environmental and social devastation. Over 800,000 dams have been built worldwide to provide drinking water, flood control, hydropower, irrigation, navigation, and water storage.
Dams change the chemical, physical, and biological processes of river ecosystems.
Large dams have contributed to huge declines in migratory species of fish. Things are improving in the United States, where environmental law, public pressure, and technological advances have combined to restore some of the natural order of parian ecosystems. But developing countries cannot afford such expensive solutions. And in drought-ridden countries, reservoirs, although a source of environmental problems and human health risks, are vital for community and urban water storage, and even more are needed to respond to population expansion and the push for agricultural development. Reservoirs can trap nutrient-laden sediments behind the dam, reducing reservoir volumes and accelerating a cycle of eutrophication.