Diagram of a watershed. (Produced by Lane Council of Governments. Source: U.S. EPA)

The term watershed is used (especially in North America and Europe) to indicate an area of land from which all water falling as rain or snow would flow toward a single point. This includes both surface water flow, such as rivers, streams and creeks, and the underground movement of water. The boundaries and the area of such a watershed are determined by first specifying geographic point on land. A line is then drawn which connects all of the points of highest elevation immediately adjacent to that point. The watershed area would be the land area within those boundaries. The watershed of the Amazon River would include all of the tributaries that flow into it so it would actually contain several hundred smaller watersheds. The watershed is thus defined hydrologically, that is, by the specific river or stream. Watershed and drainage basin or catchment are used synonymously and all of them refer to the area of land drained by a river system. Three hydrological types of watershed can be distinguished:

• Exorheic watershed, which empty to the sea and represent the major part of the drainage of all of the continents except Australia.
• Endorheic watershed, which discharge inland, into closed lake basins, and are mainly (but not exclusively) restricted to the arid and semi-arid regions.
• ArheicRegion which generates no significant surface water runoff regions, which is the region within which no rivers arise (the lower part of the Nile, Oranje and Niger, all in Africa, are a good examples of this category of basin).

Watershed identification is now a primary tool for environmental planning. Called watershed management areas, these geographic units are utilized to gain an understanding of what happens to the surface or groundwater in the upper elevations of a watershed because that is imperative for the interpretation of local phenomenon. In some locales, the term "watershed" actually refers to the height of land between two catchment areas.

Further Reading

• O'Sullivan, P. E.& C. S. Reynolds, 2004. The Lakes Handbook. Limnology and limnetic ecology. Blackwell Publishing, Malden, MA, USA.
• Wetzel, R. G., 2001. Limnology: Lake and river ecosystems. Academic Press, San Diego.