King County’s Wastewater Treatment Division (King County) prepares an annual report on its combined sewer overflow (CSO) control program and what happened at each of King County’s 38 CSOs over the past year. This report is submitted to the Washington State Department of Ecology (Ecology) and the U.S. Environmental Protection Agency (EPA).

Reports are available dating back to 1987.

View current and past CSO annual and consent decree reports.

Protecting Our Waters projects are planned and prioritized years in advance. The County reviews the program and updates the plan every 5 years.

View current and past CSO plan updates.

In 2013 King County signed an agreement with the the Environmental Protection Agency (EPA) and the Washington State Department of Ecology (Ecology). This agreement, called a “consent decree,” requires King County to complete its CSO control plan by 2030. In 2019, King County asked to begin negotiations to modify the agreement because conditions had changed since the consent decree was approved.

• King County is complying with CSO regulations and requirements.
• 2013 CSO Consent Decree
• 2019 Request to Modify Consent Decree

King County Combined Sewer Overflow (CSO) Locations

Current CSO outfall status

Glossary  - a list of definitions of terms about combined sewers and more generally about wastewater treatment.

Active CSO Control Projects

Georgetown Wet Weather Treatment Station

Ship Canal water quality (joint project with Seattle Public Utilities)

West Duwamish CSO control

Completed CSO Control Projects

Ballard Siphon Project library

Barton CSO Control Project library

Murray CSO Control Project library

North Beach CSO Control Project library

Rainier Valley Wet Weather Storage Project library

South Magnolia CSO Control Project library

This report documents the quality of the water released from each of King County’s CSOs and the quality of the underwater soils near them.

King County published a comprehensive look at water quality in Elliott Bay, Lake Union, the Lake Washington Ship Canal, and the Duwamish Estuary in 2017.

View a video about the key findings:

Understanding Our Waters -- summary of the findings , July 2017

The study’s 12 documents examine water quality past, present, and future:

Decades of data to understand long-term trends in area water bodies

• Duwamish Estuary Area Report
• Elliott Bay Area Report
• Lake Union/Ship Canal Report

Results from hundreds of water quality samples and other analysis to fill gaps in the data

• Bacteria Sources/Pathways Report
• Chemicals of Emerging Concern Report
• Sewage Tracer Report
• Description of how King County selected data gaps to fill

Estimates of major pathways for pollution and how planned programs will affect water quality in the future

• Present-Day Loadings Report
• Future Loadings Report

The synthesis of the findings from all the reports

• Synthesis Report
• Understanding Our Waters -- summary of the findings

The findings of the independent science and technical review team that reviewed the assessment to ensure quality work:

• Science and Technical Review Team Report

Using results to reduce combined sewer overflows

The results will:

• Make sure future pollution control projects are well-planned and timed to improve water quality.
• Inform King County’s Protecting Our Waters Program, including the 2018 CSO Long Term Control Plan Update. This program is working to reduce “combined sewer overflows,” or CSOs, where sewage mixed with stormwater overflows to water bodies on very rainy days.
• Establish baseline conditions King County can use to monitor how conditions change after we build CSO projects.

Sharing what we learn

Many people work hard to improve water quality. King County partners with others to achieve our region’s goals.

The City of Seattle has a similar program for Protecting Seattle’s Waterways . The two agencies continue to work together to get the most out of these water quality investments and coordinating with each other on studies like this one.

The Water Quality Assessment and Monitoring Study can support many more water quality efforts, including these:

• Stormwater management (King County) or Stormwater management (Seattle)
• Contaminated sediment remediation
• Land use and source control
• Boat paint controls
• Creosote-treated pilings removal
• Ballard Locks Upgrade
• Methods for lowering surface water temperatures
• Actions individuals can take to protect water quality

If you have questions or comments about the Water Quality Assessment and Monitoring Study, please contact Erika Peterson.

This report documents the quality of the water released from each of King County’s CSOs and the quality of the underwater soils near them.

This study was completed in 1998 with help from a large stakeholder group and the Water Environment Research Foundation and describes the effects of CSOs on the Duwamish River and Elliott Bay. A new Water Quality Assessment and Monitoring Study began in 2013.