A reservoir is a body of water held by a dam on a river or stream, usually for use in irrigation, electricity generation, or urban consumption. By catching and holding floods in spring or in a rainy season, reservoirs also prevent flooding downstream. Most reservoirs fill a few miles of river basin, but large reservoirs on major rivers can cover thousands of square miles. Lake Nasser, located behind Egypt's Aswan High Dam, stretches 310 miles (500 km), with an average width of 6 miles (10 km). Utah's Lake Powell, on the Colorado River, fills almost 93 miles (150 km) of canyon.

Because of their size and their role in altering water flow in large ecosystems, reservoirs have a great number of environmental effects, positive and negative. Reservoirs allow more settlement on flat, arable flood plains near the river's edge because the threat of flooding is greatly diminished. Water storage benefits farms and cities by...