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A watershed refers to land that is drained by an interconnected system of rivulets, streams, rivers, lakes, and groundwater. Water from a watershed eventually drains into a common destination. Both rain and snow contribute to the watershed. Coastal watersheds that begin as rivulets often end as large rivers that empty into a lake or an ocean. Watersheds can range in size from just a few square miles to many hundreds, even millions of square miles.

A catchment or drainage basin, which is the total area of land that drains into a water body, is usually a topographically delineated area that is drained by a stream system. River basins are large watersheds that contribute to water flow in a river. The watershed of a lake is the total land area that drains into the lake. In addition to being hydrologic units, watersheds are useful units of land for planning and managing multiple natural resources. By using the watershed as a planning unit, management activities and their effects can be determined for the land area that is directly affected by management. The hydrologic effects of land management downstream can be evaluated as well. Sometimes land use and management can alter the quantity and quality of water that flows to downstream communities. By considering a watershed, many of these environmental effects can be taken into consideration.

Watersheds are important as habitats for many creatures, and as a source of drinking and recreational water for many communities. As well, because one watershed can often by connected to another watershed that lies "downstream," the environmental quality of one watershed can affect other watersheds. As more communities rely on watersheds for their drinking water, the preservation of watersheds is becoming more urgent.

To function properly, a watershed needs to be maintained in a fairly undisturbed state, especially near watercourses. This undisturbed habitat helps to keep unwanted pollutants and excess soil and runoff from reaching the water course. The preservation of watershed habitats is recognized as a priority by local, regional, and national governments.



Grossman, E. Watershed: The Undamming of America. Boulder: Counterpoint Press, 2002.


Center for Watershed Protection. 8391 Main Street, Ellicott, MD, 213043-4605. (410) 461–8323. <http//www.cwp.org/>.

United States Environmental Protection Agency, Office of Wetlands, Oceans, and Watersheds (4501T). 1200 Pennsylvania Avenue, NW, Washington, DC 20460. <http://www.epa.gov/owow/>.

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