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Through 2030, King County will complete a total of 14 projects to control pollution in the Duwamish River, Lake Washington Ship Canal, and Puget Sound.

These are some of the most complex projects in the Protecting Our Waters program. And they’re big investments. When the program ends, a significant source of pollution into local waterways will be controlled.

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.

stormwater grate

CSO Control Program System Plan

Protecting Our Waters projects are planned and prioritized years – even decades – in advance. The plan is updated every five years. The current plan was approved by the Environmental Protection Agency in March 2013.

Capital Projects

After the CSO system plan is made and approved, individual CSO control projects are planned, built, and put into operation.  

scientist on the Duwamish

Water Quality Assessment and Monitoring Study

Before the next CSO Control Program update, King County is conducting a scientific assessment. King County will use the findings and input from the community to develop program recommendations in the future.

Janice Johnson
CSO Control Program