Stormwater falling on land that has been disturbed for construction flows rapidly off the site to surface waters, carrying large amounts of eroded soil, plus pollutants from vehicles and construction processes. After construction is finished, parts of the site are usually covered by pavement, buildings, and other impervious surfaces. Water can no longer be absorbed into these areas, so more stormwater remains on the land surface, to run off quickly overland or through storm drains.
Runoff from developed sites typically carries soil and sediments, road salts, nutrients and pesticides, fluids from motor vehicles, and toxic chemicals in amounts that are damaging to natural resources. Generally speaking, damage to resources from development is directly proportional to the amount of impervious surface on the developed site. Studies show that water resources are damaged whenever impervious surface area within a watershed exceeds 25 to 30%, and degradation can be detected with as little as 10% impervious surface.