Protecting Our Waters program library
King County CSO Control Program documents and resources
Protecting Our Waters is King County’s program to prevent pollution caused by excess stormwater in the sewer system on rainy days. Relief points called combined sewer overflows (CSOs) are built into sewer systems that carry sewage and stormwater in the same pipe.
CSOs can release polluted water into waterways during heavy rains. They are needed to prevent sewer backups and flooding. A controlled CSO overflows no more than one time each year, on long-term average.
Below are a number of documents, reports, studies, maps, and resources related to the Protecting Our Waters CSO Control Program.
Protecting Our Waters documents
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.
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.
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)
Completed CSO Control Projects
Ballard Siphon Project library
Barton CSO Control Project library
Murray CSO Control Project library
North Beach CSO Control Project library
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 | 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:
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
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) 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.
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 |
Feb 1999 | 2.7MB | |
Feb 1999 | 937KB | |
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 | |
Feb 1999 | 5.2MB | |
Feb 1999 | 32KB | |
Feb 1999 | 213KB | |
Feb 1999 | 58KB | |
Feb 1999 | 33KB | |
Feb 1999 | 807KB | |
Feb 1999 | 296KB | |
Feb 1999 | 2.6MB | |
Feb 1999 | 245KB | |
Feb 1999 | 46KB | |
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) |
Janice Johnson
CSO Control Program
206-477-5624