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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 U.S. Department of Justice and the Environmental Protection Agency (EPA). This agreement, called a “consent decree,” requires King County to complete its CSO control plan by 2030.

Learn more about how King County is complying with CSO regulations and requirements.

Download and read the consent decree agreement.

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 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

Faon O'Connor
CSO Control Program