Skip to main content
Most King County offices will be closed on July 4, for Independence Day.  
King County logo

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.

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.

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 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

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:

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.

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

Janice Johnson
CSO Control Program

206-477-5624