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 Web site. 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.

Undercurrents, 2003 -- Plaza view at the Denny Way Regulator Outfall, Myrtle Edwards Park, Seattle, Washington. Artist: Laura Haddad

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.