Washington County, OR Environmental Rules: Stormwater Management (2026)

Stormwater regulation in Washington County is administered by Clean Water Services (CWS), the regional utility formed in 1970 as the Unified Sewerage Agency. CWS operates under an NPDES Phase I MS4 permit from Oregon DEQ and enforces Design and Construction Standards (R&O 19-05 and updates) that govern both new development and post-construction runoff. Trigger: any project creating or replacing 1,000 sq ft or more of impervious surface requires water quality and flow-control treatment on-site, using approved BMPs — rain gardens, bioswales, filter strips, porous pavement, detention ponds, or underground detention tanks. CWS Vegetated Corridor Rules protect riparian buffers 15-200 ft along streams, wetlands, and sensitive water features — Tualatin River, Fanno Creek, Bronson Creek, Rock Creek, Beaverton Creek, and others throughout the county. Erosion and sediment control required during construction per CWS Erosion Prevention and Sediment Control Manual. CWS permits are required for most plumbing/sewer connections. Phase I MS4 requires public education, illicit discharge detection and elimination (IDDE), construction site runoff control, post-construction stormwater management, and pollution prevention/good housekeeping. Post-construction facilities (rain gardens, detention ponds) must be maintained by property owners in perpetuity per recorded maintenance covenants. DEQ enforces the MS4 permit and can fine CWS and local co-permittees for violations.