Watershed is an area of land that drains into the same body of water. An example of a watershed might include all the land that ultimately drains into a particular river, such as the Mississippi River. A watershed is also known as a drainage basin or catchment. The water may come from natural sources, such as rain or snow. It may also include water pumped by people.
Watersheds have many shapes and sizes. Large watersheds often include smaller watersheds within them. For example, the Missouri River drains into the Mississippi River. Land that drains into the Missouri River is therefore part of the Missouri River watershed as well as the larger Mississippi River watershed.
A watershed may drain into any body of water, including a lake, bay, or estuary (flooded coastal river valley). For instance, any land that ultimately drains into Chesapeake Bay is part of the Chesapeake Bay watershed.
In managing a body of water, it is often useful to consider the entire watershed. Changes to the environment within a watershed can affect the amount or quality of the water delivered to the water body. The paving of roads and parking lots, for example, greatly increases the amount of water runoff. The runoff increases because the hard surfaces do not absorb rain and snow as well as does the soil beneath them. The excess runoff may cause flooding downstream within the watershed. Furthermore, any pollutants deposited on the paved areas may be carried in the runoff, reducing water quality downstream. Monitoring the environmental changes to a watershed can help control the amount and quality of runoff.
The term watershed may also be used to refer to the boundary between two drainage basins, also called a drainage divide. A drainage divide generally occurs along a topographic high, such as a ridge, hill, or mountain range.
See also River (Drainage basins) .
