Dams are massive barriers built across rivers and streams to confine and utilize the flow of water for human purposes such as irrigation and generation of hydroelectricity. This confinement of water creates lakes or reservoirs.

The first known dam was built in 2900 B.C. across the Nile River to protect the city of Memphis from flooding. Dam build was continued into the time of the Roman empire, after which dam construction was literally lost until the 1800s. Dams are a structure also seen in nature - beavers build dams to keep the water deep enough to cover the openings to their homes, protecting them from predators.

According to Patrick McCully, campaigns director of the International Rivers Network, over 800,000 dams have been constructed worldwide for drinking water, flood control, hydropower, irrigation, navigation, and water storage. But since the 1950s, the peak of the big dam era, perceptions of dams and dam building have changed. Once symbols of development, dams today symbolize, as shown in this website, not progress but environmental and social devastation.