A simplified illustration of a drainage basin.
A watershed, also called a drainage basin, is an area of land in which all precipitation will eventually drain out through a single stream outlet. The shape and size of a watershed are products of the local topography. The mouth of the watershed is the lowest point in elevation and the watershed divide is composed of ridges separating the watershed from adjacent watersheds. Any drop of rain that falls within the perimeter of the drainage basin will drain through its mouth; any drop of rain that falls outside of the perimeter will not.