What is a Watershed? Beyond the Definition

A watershed is an area of land that drains into a body of water. When rain falls it lands on trees, grass, homes, roads, farms, parking lots, gardens, schools and more. Natural surfaces absorb that water, but paved surfaces, buildings, and landscaped plots send most of the water flowing over land, downhill to nearby streams. We call that water runoff, and it carries with it anything that’s on the land – soil, fertilizer, trash and more. In our region, all of those streams eventually meet the Potomac River. You live in the Potomac Watershed and what you do on land affects the river!

In the diagram above, the watershed effect is illustrated. It is also important to realize that the boundaries of a watershed are defined by the points of highest elevation (designated here with a doted line labeled DIVIDE). We all know that water flows downhill. Here we see that if the water lands on our side of the hill, it becomes part of our watershed; if it falls on the opposite side, it becomes part of a neighboring watershed.