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King County Washington home
Banner for the 2025 CSO Annual Snapshot featuring a Seattle shoreline photo and the King County Wastewater Treatment Combined Sewer Overflow Control Program logo.

We protect public health and the environment by collecting and cleaning wastewater while recovering valuable resources for a healthy and resilient Puget Sound.

Combined Sewer Overflows

Most of King County has a network of pipes that send sewage directly to treatment plants to clean the water. However, in older sewer systems, rainwater flows into the same pipes as sewage. Heavy rainstorms can sometimes fill these pipes, causing combined sewer overflows (CSOs) at specific relief points. These relief points—or outfalls—are designed to prevent backups into homes and businesses by releasing some of the combined stormwater and sewage to nearby bodies of water.

King County is committed to reducing CSOs in accordance withstate and federal law. We accomplish this by building betterinfrastructure to manage wastewater, by using green stormwaterinfrastructure (GSI) to keep rainwater out of our system, and bycoordinating with Seattle Public Utilities to reduce CSOs. Weare committed to controlling the frequency of overflows at eachof our CSO outfalls to no more than one per year over a longtermaverage. Learn more about our projects at kingcounty.gov/cso-projects.

Map of Seattle showing the locations of six wet weather treatment stations, wastewater facilities, waterways, highways, and monitoring sites across the city.

Construction workers observe water entering a large concrete shaft during a breakthrough at the Ship Canal Water Quality Project site.

The Ship Canal Water Quality Project is a joint effort between King County and Seattle Public Utilities. The project will capture and store 29 million gallons of polluted water during heavy rains. This project is currently under construction, on track for completion in 2027. Visit https://www.seattle.gov/utilities/ShipCanalProject to learn the latest updates about the project.

Architectural rendering of a landscaped wet weather treatment facility surrounded by trees, sidewalks, roads, fencing, and educational signs.

King County is starting construction in 2026 on the West Duwamish Wet Weather Storage Project, a 1.25-million-gallon underground storage tank that will control the Terminal 115 CSO and the West Michigan Regulator Station CSO. Visit kingcounty.gov/westduwamish-csoto learn the latest updates about the project.

King County staff speak with community members beside educational displays about stormwater and wastewater systems at an outdoor Duwamish Alive event.

The Mouth of the Duwamish Wet Weather Facilities will address pollution from five King County outfalls at the mouth of the Duwamish River, by treating an average of 430 million gallons of polluted water each year. Visit kingcounty.gov/MDWetWeatherFacilities to learn the latest updates about the project.

Monitoring Progress

King County monitors all outfalls, including controlled CSO outfalls, to make sure they remain controlled.If we find a problem, we outline the additional steps we will take in a Supplemental Compliance Plan.We keep monitoring as we make changes.

2025 CSO Outfall Statistics

Infographic showing 38 King County combined sewer overflow outfalls: 21 controlled, 9 uncontrolled with projects underway, 2 uncontrolled with projects planned, and 6 uncontrolled with additional measures underway.

Pie chart showing that Seattle treated 30.3 billion gallons, or 98.6%, of combined-system flow in 2025, while 0.403 billion gallons, or 1.4%, was discharged untreated.

2025 Rainwise Program Accomplishments

The RainWise program provides rebates to private property owners living in specific combined sewer areasfor installing rain gardens and cisterns on their property. King County has administered this program inpartnership with Seattle Public Utilities since 2013. We continued to make progress in 2025, working withproperty owners to minimize overflows and prevent polluted runoff from entering our waterways.

Infographic showing that 66 RainWise projects in King County CSO basins captured 682,166 square feet of roof area in 2025—about 10,495 bathtubs of stormwater—with an average home rebate of $6,190.