Water that does not infiltrate the soil runs off across the surface of the watershed. Runoff begins as a sheet of water and then turns into small trickles and then streams. Runoff may contribute water directly to lakes or it may flow first to steams (streamflow) and then to lakes or the ocean.

Runoff can erode the land surface, leading to pollution of lakes and rivers. If precipitation is intense and/or the soil is saturated, flooding can occur. Watershed managers try to reduce erosion and flooding by using good land use practices and by building control structures.