The USGS Water Science School
As urbanization occurs, more and more land is developed into parking lots, roads, housing subdivisions, and buildings. These are examples of how impervious areas are replacing the natural environment. In a natural landscape, much of the precipitation that falls soaks into the ground. When urbanization occurs, rainwater lands on pavement and roads, and no longer soaks into the ground. It is collected by storm sewers and is directed into local creeks. If the increased runoff is full of sediment, then the water quality of the creek can be harmed. Sediment ponds, like the one in this picture, are now being built to trap runoff water. Sediment settles to the bottom of these ponds rather than accumulating in local creeks and streams.