A water reservoir is a large natural or artificial lake used to store water for various purposes, such as drinking, irrigation, and hydroelectric power generation. These reservoirs can be created by damming rivers or by excavating land to hold water. They play a crucial role in managing water supply and ensuring availability during dry periods. Reservoirs also help in flood control by capturing excess rainwater and releasing it gradually. They can support local ecosystems and provide recreational opportunities, such as fishing and boating. Examples of well-known reservoirs include the Hoover Dam and the Lake Mead reservoir in the United States.