dam

A dam is a barrier built across a river or stream to hold back water, creating a reservoir upstream and altering the flow downstream. Dams are constructed for water supply, irrigation, flood control, hydroelectric power, and recreation.

Common dam types include gravity dams, which rely on weight to resist water pressure; arch dams, which transfer load to canyon walls; and buttress dams, which use a framework of supports. Many dams are earthfill or rockfill, with cores of clay or concrete.

A dam typically consists of the dam body, foundation, and abutments, along with a spillway to safely pass floodwater and outlet works or tunnels to release stored water. Hydroelectric dams add a powerhouse and electrical equipment to generate electricity.

Notable dams include the Hoover Dam in the United States, the Three Gorges Dam in China, the Aswan High Dam in Egypt, Itaipu on the Brazil–Paraguay border, and Grand Coulee in the United States. Dams vary widely in size and purpose.

Safety and environmental considerations are central to dam operation. Regular inspections and maintenance address risks such as seepage, overtopping, and foundation problems. Dams also affect ecosystems and sediment transport, alter downstream flows, and can require measures for fish passage and relocation of communities.