Water that collects on impervious surfaces like roofs, roads and parking areas, whether from rain, snowmelt, or irrigation sources, becomes runoff.
Runoff carries pollutants like oil, grease and heavy metals from vehicles, chemicals and animal waste from lawns, and physical debris like soils and trash into our waterways and wetlands, compromising the health of our watershed. Urban runoff is the leading source of water pollution in Washington State.
In areas of smaller annual rainfall like the Columbia Basin, pollutant load can be even greater because pollutants have a greater time to collect and concentrate on surfaces between storm events.