Kennewick, WA Environmental Rules: Stormwater Management (2026)

Kennewick is designated by Ecology as a regulated Phase II small MS4 community in Eastern Washington and operates under Ecology's Eastern Washington Phase II Municipal Stormwater General Permit, issued pursuant to RCW 90.48 (Water Pollution Control Act) and the federal Clean Water Act NPDES program at 33 USC 1342. The permit requires Kennewick to implement six minimum control measures (public education, public involvement, illicit discharge detection and elimination, construction-site runoff control, post-construction stormwater management, and pollution prevention/good housekeeping), maintain an updated MS4 map, and adopt local code consistent with the Stormwater Management Manual for Eastern Washington (SWMMEW). Kennewick's MS4 includes outfalls to the Columbia River and to U.S. Army Corps of Engineers stormwater ponds, as well as discharges through local irrigation district conveyances. Construction projects disturbing one acre or more must obtain coverage under Ecology's Construction Stormwater General Permit, prepare a Stormwater Pollution Prevention Plan (SWPPP) per the SWMMEW, and file a Notice of Intent before ground disturbance. Permanent post-construction BMPs (bioretention, swales, infiltration) must be maintained by the owner under a recorded maintenance agreement. Illicit non-stormwater discharges to catch basins, ditches, or any tributary of the Columbia or Yakima River are prohibited.