Snohomish County is required by federal regulation to take actions that minimize or prevent stormwater pollution under the county’s National Pollutant Discharge Elimination System (NPDES) municipal stormwater permit.

NPDES is a federal permit system based in the Federal Clean Water Act (CWA). NPDES permits were initially developed to regulate sewage treatment plants and industrial discharges. While much pollution was prevented through such permits, many studies found that significant pollution is picked up in stormwater, the water that flows over the ground after it rains or snows.
Brief History
In 1987, amendments to the CWA expanded the NPDES permit system to include stormwater discharges from municipalities (i.e., cities and counties), large construction sites, and many industrial sites.

Within Washington State, the NPDES permit system is administered by the Washington State Department of Ecology (Ecology). In 1995, Ecology issued "Phase 1" permits to the following cities and counties with a population of 100,000 or more: Seattle, Tacoma, Snohomish County, King County, Pierce County, and Clark County. The Washington State Department of Transportation was also brought under the permit. The term "Phase 1" refers to this initial group of permittees.

In February, 2007, Ecology reissued the Phase 1 permit to the permittees listed above, and issued Phase 2 permits for the first time to over 100 cities and counties in Washington State, including 14 cities in Snohomish County: Arlington, Bothell, Brier, Edmonds, Everett, Granite Falls, Lake Stevens, Lynnwood, Marysville, Mill Creek, Monroe, Mountlake Terrace, Mukilteo, and Snohomish.