Skip to main content

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.

View current and past CSO plan updates.

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.

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

Comprehensive Sediment Quality Summary Report for CSO Discharge Locations

Nov 2018


Monitoring data

Effluent quality

Nov 2018


Sediment quality

Nov 2018


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

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:

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


Full report

Comprehensive Sediment Quality Summary Report for CSO Discharge Locations

Dec 2009 22.8MB

Monitoring data

Effluent quality

Dec 2009 10.1MB

Sediment quality

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

Appendix B1, Table 3-3

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

Appendix B3, Tables 3-6 to 3-10

Feb 1999 58KB

Appendix B3, Tables 3-11 to 3-15

Feb 1999 33KB

Appendix B3, Tables 5-1 to 5-4

Feb 1999 807KB

Appendix B3, Tables 5-5 to 5-8

Feb 1999 296KB
Appendix B4: Aquatic Life Risk Assessment
Feb 1999 2.6MB

Appendix B4, Tables 3-1 to 3-28

Feb 1999 245KB

Appendix B4, Tables 5-3 to 5-6

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)