Runoff is the water that flows over the surface of the land or a building or road downstream to a lake, wetland or stream.
Before the land was altered, much of the water that fell on it soaked directly into the ground. Water was then used by vegetation, recharged stores of ground water, or was discharged to lakes, wetlands and streams. As we farmed the land and built our cities we re-routed runoff via ditches, stormwater pipes and culverts directly to local lakes, wetlands and streams. As a result, we are overwhelming many of our water bodies today.