Why is watershed management important to maintaining good water quality in a large river or lake?

Watershed management is important because surface and storm water runoff always end up draining to other bodies of water. Everything upstream always flows downstream so this has to be taken into account for lakes and rivers. Managing what water goes into the drains and through open sewers is important. Pollutants could easily make their way into rivers, streams and lakes if the watershed is not managed properly by businesses and people alike.