What is Stormwater Runoff?
“Stormwater runoff” occurs when water from rain or snowmelt flows over the ground. When land is developed, the natural landscape is typically replaced with impervious surfaces (such as roads, rooftops and parking lots) which prevent water from soaking into the ground. This means that both the amount (volume) and speed (velocity) of water flowing across the ground surface (stormwater runoff) will increase.
Where Does Runoff Go?
Most stormwater runoff flows either:
- directly into waterways such as streams, rivers, ponds, lakes and wetlands, or
- down the nearest stormdrain or drainage ditch, which leads into the nearest waterway without any treatment.
Why is Stormwater Runoff a Problem?
Stormwater runoff may become a problem in three ways:
- Runoff often picks up pollutants from lawns, roads and other surfaces, carrying those pollutants into the nearest waterbody. These pollutants can harm fish and wildlife populations, kill native vegetation, foul drinking water supplies, and make recreational areas unpleasant and/or unsafe. Some of the pollutants commonly carried by runoff are:
- Sediment
- Oil, grease, and toxic chemicals from motor vehicles
- Pesticides and nutrients from lawns and gardens
- Viruses, bacteria, and nutrients from pet waste and failing septic systems
- Heavy metals from roof shingles, motor vehicles, and other sources
- Road salts
- Land development-related increases in impervious surfaces restrict water infiltration into the ground. This means an excessive amount (volume) of water runs off the land, overwhelming natural and man-made drainageways and causing flooding.
- Land development typically leads to smooth, impervious surfaces, which speed up the velocity of water running off the land. This causes erosion of land surfaces, erosion of the bed and banks of waterways, and subsequent sedimentation in downstream waters.