Project Watershed Central New York

Runoff is water precipitation or snowmelt that contributes to streams, rivers, lakes, and other surface water bodies.

Excessive runoff causes flooding, typically locally during a thunderstorm, or more widely during an extended period of heavy rain. Rain causing melting snow also causes major runoff, especially in the springtime.

Urbanization is also a major culprit in causing excessive runoff, as impermeable surfaces such as pavement and buildings do not allow percolation of the water down through the soil to the water table. It is instead forced directly into streams, where erosion and siltation can be major problems, even when flooding is not. It may also lower the water table, making droughts worse, especially for farmers and other who depend on wells for groundwater.

The area within which runoff flows into a given body of water is a watershed, usually bounded by ridges of hills or mountains.