As a case in point, a study of water pollution in the Maryland portion of the Chesapeake Bay watershed would have to take into account pollution sources in the upper Chesapeake Bay watershed of New York and Pennsylvania. The physical condition of a watershed, therefore, directly affects the health and well being of natural and social systems within its divides and indirectly affects those systems beyond its divides. A watershed is the area drained by a stream and all of its tributaries. Any rain that falls within the watershed will pass through the main stream channel. A divide separates each basin from the surrounding drainage basins.
Divides follow ridges and hill tops.
If a raindrop falls on one side of a divide, it will flow down one side of the hill, and into one drainage basin. If the raindrop falls on the other side of a divide, it will flow into a different drainage. Watersheds are composed of many smaller drainage basins . In the diagram, a sub-basin has been drawn for every tributary.