Program library
This page lists documents, reports, studies, maps, and resources related to the Protecting Our Waters CSO Control Program.
Protecting Our Waters documents
King County prepares an annual report on its CSO control program and what happened at each of our 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. King County reviews the program and updates the plan as part of the application for renewal of the county’s West Point Treatment Plant NPDES Permit.
In 2013 King County signed an agreement with the EPA and Ecology. This agreement, called a “consent decree,” requires us to complete our CSO control plan by 2030. In 2019, we 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
Glossary - a list of definitions of terms about combined sewers and more generally about wastewater treatment.
Additional studies and reports
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.
Full report |
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Comprehensive Sediment Quality Summary Report for CSO Discharge Locations | Nov 2018 | 53MB |
Monitoring data | ||
Effluent quality | Nov 2018 | 1.3MB |
Sediment quality | Nov 2018 | 0.5MB |
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:
The study’s 12 documents examine water quality past, present, and future:
Decades of data to understand long-term trends in area water bodies
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
The synthesis of the findings from all the reports
The findings of the independent science and technical review team that reviewed the assessment to ensure quality work:
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)
- 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
Full report |
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Comprehensive Sediment Quality Summary Report for CSO Discharge Locations |
Dec 2009 | 22.8MB |
Monitoring data |
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Dec 2009 | 10.1MB | |
Dec 2009 | 3.8MB | |
Effluent and sediment quality | Dec 2009 | 1.5MB |
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.
Volume 1: Overview and Interpretation | Feb 1999 | 8.4MB |
Appendix A1: Problem Formulation |
Feb 1999 | 2.7MB |
Appendix A2: Analysis Plan |
Feb 1999 | 937KB |
Appendix A3: Field Sampling Work Plan |
Feb 1999 | 1.2MB |
Appendix B1: Hydrodynamic Fate and Transport Numerical Model for the Duwamish River and Elliott Bay |
Feb 1999 | 4.7MB |
Feb 1999 | 32KB | |
Appendix B1, subappendices G through K are available upon request |
Feb 1999 | |
Appendix B2: Human Health Risk Assessment |
Feb 1999 | 5.2MB |
Appendix B2, Tables 3-12 and 3-13 |
Feb 1999 | 32KB |
Appendix B3: Wildlife Risk Assessment |
Feb 1999 | 213KB |
Feb 1999 | 58KB | |
Feb 1999 | 33KB | |
Feb 1999 | 807KB | |
Feb 1999 | 296KB | |
Appendix B4: Aquatic Life Risk Assessment |
Feb 1999 | 2.6MB |
Feb 1999 | 245KB | |
Feb 1999 | 46KB | |
Appendix C: Issue Papers |
Feb 1999 | 513KB |
Volume 2: Public Information Document | Aug 1999 | 4.2MB |
Volume 3: Stakeholder Committee Report | Jan 1999 | 224KB |
Volume 4: WERF Peer Review Committee Report (not available online) |