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Like all other urban waterways, the biggest ongoing source of pollution in the Duwamish today comes from stormwater runoff that carries chemicals from our cars, buildings, roads, businesses and yards. King County is working with the City of Seattle and the State Department of Ecology to find ways to control these sources of pollution.

King County’s Lower Duwamish Waterway Source Control Implementation Plan

The Lower Duwamish Waterway Source Control Implementation Plan builds on over 50 years of investments made by King County to clean up the Duwamish River. The plan documents ongoing and priority source control actions and programs, and the County’s coordinated source control efforts with external partners in support of LDW Superfund sediment cleanup.

The plan offers information on the County’s continued commitment in the following areas:

  • Regulating and monitoring Duwamish area businesses that send wastewater to the County sewer system
  • Protecting Our Waters - King County’s long-term plan to control combined sewer overflows (CSOs)
  • Managing and implementing the County’s Phase I Municipal Stormwater permit, and compliance with water and air quality permits and regulations at County facilities
  • Scientific assessments of pollution sources, including sampling, source tracing and system mapping

The plan was developed following a series of studies to help identify source contributions throughout the entire Duwamish-Green watershed. The studies included:

  • Chemical characterization of the County’s CSOs
  • Chemical characterization of atmospheric deposition, water, sediment and suspended sediments in the Green watershed 
  • Specific studies targeting certain basins or problem chemicals

View source control implementation plan, reports and studies on the library page.

Reducing CSO volumes

In older parts of Seattle, pipes designed to carry both sewage and stormwater fill to capacity during heavy rain and must be released into a local waterbody to prevent them from backing up into streets, homes and businesses. While CSOs are about 90 percent stormwater, uncontrolled CSOs can contribute to degraded water quality and public health risks. Since 1990, King County has reduced CSO volumes in the Lower Duwamish Waterway by more than 90 percent. With five of 10 Lower Duwamish CSOs controlled as of 2007, King County's CSO program is now focusing on the remaining Duwamish CSOs, with planning targeting the beginning of major work in 2012. King County's current CSO Control Plan will invest $500 million to control remaining Lower Duwamish CSOs by 2030.

Selected King County studies and research

King County monitors existing conditions of the Duwamish River and Elliott Bay both with CSOs and the conditions of those water bodies if CSOs were eliminated. The original study was conducted in 1999. An updated assessment was completed in 2017. These studies help identify the most significant issues and focus source control efforts.


Other source control efforts include: