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

Portland's stormwater program is one of the most comprehensive in the US, built on a Phase I NPDES MS4 permit issued by Oregon DEQ. Portland City Code Chapter 17.38 (Drainage and Water Quality) and the Stormwater Management Manual (SWMM, last updated 2020 with 2023 amendments) require on-site stormwater management for any project creating or replacing 500 sq ft or more of impervious surface - stricter than the state minimum of 10,890 sq ft (0.25 acre) in OAR 340-041. Approved facilities include rain gardens, eco-roofs, pervious pavement, swales, drywells, and infiltration planters. The hierarchy prioritizes infiltration first, then filtration, then detention. Water quality facilities must treat the 90th percentile storm event. Clean Water Services for Washington County does not apply here, but Multnomah County unincorporated areas are served by Portland's Bureau of Environmental Services (BES) in sewer districts. Gresham holds its own Phase I MS4 permit with similar requirements under GRC 6.16. Columbia Slough watershed has additional TMDL requirements for bacteria and pollutants. Illicit discharge to storm drains (e.g., car wash water, paint, fluids) is prohibited under PCC 17.38.034. Portland's Clean River Rewards program (PCC 17.36) provides stormwater fee discounts up to 35% for residents who manage stormwater on-site.