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Combined Sewer Overflow Control

By reducing combined sewer overflows into local water bodies, King County is protecting public health and the environment.
For questions about the King County CSO Control Program, please contact Karen Huber.

Wastewater Treatment Division
King Street Center
201 S. Jackson St., Suite 505
Seattle, WA 98104-3855

Get Directions to our office location in Seattle, Washington.

Real Time CSO Notification

King County's Combined Sewer Overflow Control Program

In 2013, King County entered 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.

Learn more about the RainWise Rebate program.
The RainWise program provides rebates that cover most or all of the cost of installing cisterns and rain gardens on your property. To receive a rebate, you must live in an eligible combined sewer overflow basin. Learn more.

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.

Real-time CSO notification (Seattle area)
Real-time CSO notification (Seattle area)
Real-time CSO notification (Seattle area)

Quick facts

The King County CSO Control Program:


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.