What is a dam?

A dam is defined as a structure built across a river to control its flow. This is primarily done to manage water resources for various purposes, such as generating hydroelectric power, providing water for irrigation, flood control, and creating reservoirs for drinking water or recreational use. Dams serve several functions, including regulating water levels, enhancing the reliability of water supplies, and supporting local ecosystems by maintaining river flow patterns.

Other choices describe different concepts. A natural barrier that blocks rivers is not a dam but rather refers to natural formations like rocks or sediment deposits. An area that collects rainwater typically refers to a catchment area or watershed, not specifically a dam. A type of aquifer, which stores water underground, is unrelated to the function of a dam, which focuses on the surface management of water flow. The emphasis on controlling river flow and providing numerous benefits highlights why the definition of a dam as a man-made structure is the most accurate choice.