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Run-off, composed of a mixture of water and soil along with any other organic or inorganic substances that may exist in the land, is the product of precipitation, snowmelt, or other water coming in contact with the earth and carrying matter to streams, rivers, lakes, and other surface water bodies.
In some cases it is spoken not about the runoff of the water, but of the carried substances, e.g., fertilizer runoff, when the latter ones are the major concern of the runoff.
The area within which runoff flows into a given body of water is a watershed, usually bounded by ridges of hills or mountains.
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.
A common context of run-off deals with agriculture. When farmland is tilled and bare soil is revealed, rainwater carries billions of tons of topsoil into waterways each year, causing loss of valuable topsoil and clogging of valuable waterways.
Urbanization is another 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.
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