ABSTRACT
A watershed (also known as a basin or catchment) is that portion of the earth’s surface where runoff terminates or accumulates in a common hydrologic feature, such as a lake or river (Fig. 2). At a localized level, the watershed of a pond or stream includes all the land area that contributes runoff to the pond or stream. On a regional level, the watershed of a river includes all the land area that contributes runoff to the river or its tributaries. Examples of regional watersheds would be the Colorado River basin or the Ohio River basin. Finally, at the largest scale, the continental divide separates large, continental watersheds whose runoff ultimately flows to the oceans surrounding continents. Examples of some of the largest watersheds that drain significant portions of continents include the Amazon, Mississippi, and Nile River basins.
