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.