King County's Combined Sewer Overflow Control Program

Thank you for visiting the web site for King County's Combined Sewer Overflow (CSO) Control Program. Here, we share information on what a CSO is, why they occur, and where they are found in the King County system. And we explain what we're doing to control them.

February 2013 Update
 
In 2013, King County expects to enter a consent decree, which is a legal settlement with the U.S. Department of Justice and EPA that ensures its CSO control plan is completed by 2030. King County had already committed to limiting combined sewer overflows to one per year at each outfall by 2030 through its adopted policies and a 2011 Agreement with the Department of Ecology.

Learn more: view frequently asked questions that address many questions associated with King County’s commitment to water quality and the terms of the consent decree.

January 30, 2013 news release -- Agreement to formalize King County’s long-term pollution control plan. 

Real-time CSO notification (Seattle area)

Quick facts

The King County CSO Control Program:

• began in the 1970s
• includes the combined sewer areas within Seattle
• manages 38 CSOs; City of Seattle manages about 90 CSOs
• expects to complete CSO control by 2030.

Background

In the 1950s, more than 20 billion gallons of untreated or poorly treated wastewater flowed from combined sewers into major Seattle lakes, the Duwamish River and Puget Sound. By the 1980s, efforts by King County and Seattle had reduced the CSO baseline to an average of 2.3 billion gallons per year. With construction of CSO control projects since then, King County by 2005 has reduced CSO volume to an average of less than 1 billion gallons per year. Learn more about the relationship between King County's and City of Seattle's management of CSOs.