A dam is a large structure built across a river or stream to hold back water. It creates a reservoir, which can be used for various purposes, such as water supply, irrigation, and recreation. Dams also help control flooding by regulating water flow downstream. In addition to these benefits, dams are often used to generate electricity through hydropower. This process involves water flowing through turbines, which convert the energy of moving water into electrical energy. Notable examples of dams include the Hoover Dam in the United States and the Three Gorges Dam in China.