It is defined as an area which has a common point of drainage. A watershed is a self-contained system. The Four Mile Run watershed is just under 20 square miles in area. Much of the rain that falls into this watershed will enter the Potomac River via Four Mile Run. It is eventually used up by the plants or evaporates.
Watersheds are good examples of nested systems.
Although the Four Mile Run watershed is a distinctive hydrologic unit in its own right, it is also a subset of the larger Potomac River watershed. The Potomac River, in turn, is part of the very large Chesapeake Bay watershed. Last updated June 16, 1997.