Pervious material will let water through them. Impervious materials will not. When new houses, office buildings, and shopping centers are built, the amounts of impervious surfaces often go up. The concrete and asphalt used to build roads and parking lots are impervious. Water runs on top of them, collecting pollutants as it goes. When impervious surfaces are constructed right next to streams and creeks, polluted runoff goes straight into the water.
Some builders and homeowners plant or keep what is called a buffer between developed areas and the water. A buffer is an area with plants that help do two things. First of all, they slow the water down. This makes it less likely for the soil in that area to erode, or wash into the water. Second, plants in the buffer area actually help soak up some pollutants in the runoff before they make it into the stream.