I live on one of the main watersheds in the U.S. It is called the Chesapeake Bay. Now what is a watershed? A watershed is basically a large body of water that most of the water in an area flows into. In other words a big body of water, with lots of rivers flowing into it. This is why watersheds get so polluted. All of the runoff from surrounding areas flows into it. If that runoff is dirty then the watershed gets dirty.
In the picture above you can see how all these rivers and tributaries flow into one large body of water that will later flow into the ocean!