A dam is a barrier constructed across a waterway to impound water for storage, flood control, hydropower generation, or water supply. Dams are among the most significant human modifications to the hydrologic cycle.
A dam is an engineered structure built across a river or stream to obstruct, direct, or slow the flow of water, creating an upstream reservoir that stores water for various purposes including municipal and agricultural water supply, hydroelectric power generation, flood control, navigation improvement, and recreation. Dams are classified by construction material (earth fill, rock fill, concrete gravity, concrete arch, and roller-compacted concrete) and by purpose (single-purpose or multipurpose). The world's approximately 58,000 large dams (over 15 meters tall) store an estimated 8,300 cubic kilometers of water, fundamentally altering river systems and sediment transport on a global scale. While dams provide critical water security and clean energy benefits, they also have significant environmental impacts including disruption of fish migration, alteration of downstream flow regimes and sediment transport, changes in water temperature and quality, inundation of terrestrial habitats and displacement of communities, and greenhouse gas emissions from decomposition of organic matter in reservoirs. Dam safety is a major concern, as failures can cause catastrophic flooding and loss of life. In the United States, the National Inventory of Dams catalogues over 91,000 dams, many of which are aging and require significant maintenance or removal. Dam removal has become an increasingly common practice for restoring river ecosystems, particularly for obsolete or unsafe structures.