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King County's Stormwater Management Program (SWMP) Plan is available for public comment and is updated every year. Your comments help King County continue improving programs that deal with water quality and quantity that meet permit requirements.

The plan describes the work the county plans for the coming year to fulfill requirements in the National Pollutant Discharge Elimination System (NPDES) Phase I Municipal Stormwater Permit. The permit is through the Washington State Department of Ecology (Ecology) as part of the National Clean Water Act.

The permit continues to adapt to our growing understanding of how water is moving across our developed landscapes. The focus of 2016 is on how stormwater processes affect natural water systems and is using data for watershed-scale stormwater planning. We collect water quality and quantity data, review the historical changes in hydrology and watershed characteristics to model and predict impacts of stormwater. This information is used to evaluate strategies to address interconnected stormwater issues.

These data include:

  • Collecting water quality data during base and storm flows and conditions
  • Collecting flow data
  • Data on benthic macro invertebrates to correlate with flow data
  • Include the status of the aquatic community within the watershed
  • Data on watershed characteristics (e.g. soil types, existing and future landcover, and MS4 (Large Municipal Separate Storm Sewer System)

The permit also includes direction for partnering and funding for the data collection and assessments. Modeling and evaluation of strategies will also consider changes to codes, rules, standards, plans and structural stormwater controls.

Ongoing permit responsibilities

  • Total Maximum Daily Loads (TMDL) – Ecology identifies water bodies that are impaired by pollutants, develops programs to return the waters to beneficial uses, and writes a plan to reduce the pollutants. TMDLs are estimations of how much of a pollutant such as bacteria or nitrogen, can be added to a water body and still maintain its beneficial uses like swimming, boating, etc. King County conducts stormwater sampling and testing to address pollutants in the Bear-Evans Creek and Puyallup-White basins.
  • Low Impact Development (LID) - Focuses on controlling runoff at or near its source. This can include preserving as much natural area as possible, restoring cleared soils with soil amendments, and replanting. Runoff from developed areas is slowed by using dispersion, infiltration, plant uptake, and small-scale storage and release to help recharge water bodies. LID is used for stormwater management during site development or redevelopment and treatment and flow control responsibility belongs to privately owned and maintained systems.
  • Monitoring - The permit continues the collaborative Regional Stormwater Monitoring Program, or RSMP. King County participates in this shared effort and is helping standardize data collection and reduce costs for participating jurisdictions.

King County is implementing a number of programs to comply with permit requirements:

  • Legal/Land Use Authority – As the local land-use authority for the unincorporated area, King County has appropriate codes, regulations, and enforcement and education components to reduce water-polluting practices and to increase or promote practices that protect water quality.
  • Mapping – King County is mapping its stormwater drainage and treatment systems and studying where stormwater runoff goes and how it is treated.
  • Coordination
  • Public involvement – King County invites public feedback each year to help shape the county's stormwater plan. Each spring a draft plan is posted online and comments are welcome year round. For more information on King County's SWMP, visit our stormwater website.
  • Controlling runoff from new development, redevelopment and construction sites –King County has updated Surface Water Design Manual (SWDM). The manual has drainage design, source control rules, and standards for staff employee training and inspections. The updated manual requires LID approaches to stormwater management when possible. The SWDM applies to new development, redevelopment, and construction projects in unincorporated King County, as well as on county-initiated projects within incorporated areas.
  • Structural stormwater controls – King County identifies and funds structural stormwater control projects to improve stormwater quality and/or quantity.
  • Source control program for existing development – This program enforces the Stormwater Pollution Prevention Manual. King County identifies businesses or activities that contribute pollutants to stormwater and provides inspections, technical support and enforcement to reduce and eliminate those pollutants.
  • Illicit Connections and Illicit Discharges Detection and Elimination (IC/IDDE) –King County has water quality codes, employee training standards, and an inspection and response program addressing spills, illegal dumping,  and screening for and eliminating illicit connections and discharges to the stormwater system.
  • Operation and Maintenance Program – King County inspects and maintains its own properties to minimize stormwater pollution and has stormwater pollution prevention plans in place. Employee training standards are central to this program. King County also has an extensive inspection and enforcement program for private stormwater facilities.
  • Education and Outreach Program – King County's public education and outreach program engages target audiences on stormwater issues, and some of these efforts are measured to determine the program's effectiveness. One of the central pieces of this effort has been participation and leadership in the Puget Sound Starts Here campaign ( and the Don't Drip & Drive campaign ( The county also offers Natural Yard Care workshops to engage the public in reducing stormwater pollution starting at home with their yard and maintenance practices. King County land and facility managers also follow an Integrated Pest Management process to identify the most environmentally sound and economical pest control methods and strategy.

King County Agency Participation

Many King County agencies play a role in the SWMP.

  • The Water and Land Resources Division (WLRD) coordinates SWMP actions and annual reporting. WLRD manages the public involvement, manual equivalency, structural stormwater control, public information about the plan and has a significant role in source control, investigating illicit connections and discharges (IC/IDDE), mapping and operations and maintenance programs.  
  • Many divisions manage and develop properties and facilities that are not covered under other NPDES permits. These divisions include Wastewater Treatment, Solid Waste, Parks, Roads, Transit, the King County International Airport, Facilities Maintenance and the Sheriff's Office. Drainage facilities on those lands must be designed, mapped, and maintained according to permit requirements and King County's Surface Water Design Manual and source control best management practices. An important element is employee training.
  • The Department of Permitting and Environmental Review is responsible for ensuring the permit requirements including the Surface Water Design and Stormwater Pollution Prevention manuals and related codes are applied to new and re-development projects. .
  • Public Health – Seattle & King County's wastewater program oversees onsite sewage and septic systems throughout King County. Public Health regulates and inspects businesses throughout the county, and can identify potential illicit discharges or connections to the stormwater system. If onsite systems are failing and contaminating stormwater systems, Public Health can require corrective actions.

For information about stormwater in King County, please contact us at