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, and declining water quality. Farther-reaching consequences of dams include changes in groundwater flow and the displacement of human populations.

Riparian, or stream–side, 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...