A river is a stream of water that flows over the land surface.
Rivers naturally flow from high to low elevations under the force of gravity.
After a dam is built across a river, the water backs up behind the dam, flooding the land to the elevation of the dam. This forms a lake or pond, commonly referred to as an impoundment. A river gathers most of its water from rain and snowmelt in its watershed, the area of land that captures water and directs it to the river. This water either flows over the land surface or seeps underground as groundwater to the river. The groundwater that flows into a stream is called the stream s baseflow.