Washington County, OR Environmental Rules: Erosion Control (2026)

Erosion and sediment control on construction sites in Washington County is governed by multiple layers. Projects disturbing 1 acre or more of ground require Oregon DEQ's NPDES General 1200-C Construction Stormwater Permit under ORS 468B.050. Projects under 1 acre fall under Clean Water Services (CWS) Design & Construction Standards and require Erosion Prevention and Sediment Control (EPSC) plans reviewed by CWS and the county/city building department. Standard best management practices: silt fencing at downslope perimeters, straw wattles on slopes, erosion control blankets on graded surfaces, stabilized construction entrances (crushed rock), sediment basins or traps for significant grading, inlet protection on storm drains, dust control, and concrete washout containment. Disturbed areas must be stabilized with temporary seeding, mulch, or erosion blankets within 14 days of the last disturbance during the rainy season (October 1-May 31) or 7 days during high-erosion-risk periods. CWS inspectors conduct periodic site visits with authority to issue stop-work orders. Permanent stabilization through vegetation or hardscape required at project completion. In Tualatin Valley hillsides (Bull Mountain, Cooper Mountain, Chehalem Mountain), slope and soil erodibility trigger enhanced controls. Violations discharging sediment to protected streams can trigger DEQ penalties under the Clean Water Act and Oregon's own enforcement.