Stormwater runoff is excess rainwater that runs off from land surfaces during and following rainfall. A certain amount of rainfall will naturally soak into the ground, particularly if the land cover is vegetated or has a tree canopy. Before land is developed for beneficial purposes, a significant portion of rainfall soaks (infiltrates) into the ground, thereby resulting in much less surface runoff.
In developed conditions, however, less of the rainfall can soak into the ground and surface runoff generally increases. Increased runoff contributes to higher peak flows in drainage systems and receiving streams, as well as a greater volume of runoff. Higher peak flows and greater volume contribute to urban stormwater management concerns, including flooding safety hazards, property damage, soil and stream erosion, and impacts to surface water quality and stream health.
Stormwater flows UNTREATED from our roofs and roads into our streams, lakes, and rivers — the same bodies of water we use for swimming, fishing, and providing drinking water.