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Project update

January 2023

Thank you for sharing what matters to you!

King County WTD is building a storage facility near the 1st Ave S Bridge that will reduce water pollution from the South Park and Highland Park drainage basins into the Duwamish River. Community members participated in a survey between July and October 2022 to share their priorities for the storage facility site design and community-driven initiatives. Look out for more information about how the project team is using your input in our next newsletter coming out in winter 2023.

Although our survey is now closed, it’s not too late to visit our online open house to learn about the project history and find out what’s happening now. To share your input, ask questions about this project, or for language assistance needs, reach out to Kristine Cramer at or Demmelash Adera at

Project overview

King County will build a 1.25-million-gallon underground storage tank near the First Avenue Bridge to hold stormwater and wastewater that currently overflows into the Duwamish River during large storms. The water in the tank will then flow to the West Point Treatment Plant for treatment before it is released into Puget Sound. This storage tank will reduce overflows and pollution into the Duwamish River.

The project has recently reached the 30% design milestone meaning the site layout and general features for the storage tank and associated equipment are set. Moving forward, the project team will work on design details. Community input can shape some of these details, including fencing, landscaping, and building architecture.

The project is in the design phase, and we expect to start construction in 2025.

Project timeline

Read the most recent project newsletter or visit our online open house to learn more.

Project need

Like many cities around the country, older parts of King County’s sewer system use the same set of pipes to carry both sewage and stormwater to a treatment plant. During storms, the pipes can fill with stormwater that runs off roofs, driveways and streets. When the system is overwhelmed, it is designed to overflow. These overflows are called Combined Sewer Overflows, or CSOs.

To keep the sewer system working and to prevent sewer backups, the excess water and sewage is released into our local water bodies through CSOs. However, CSOs pose a risk to public health and the environment. Over the past several years, King County has been planning a project to reduce CSOs from the South Park drainage basin into the Duwamish River. Our requirement is to reduce CSOs to no more than one overflow per year on average.

Our commitment to our local communities

Through this project, our team will seek opportunities to support community-driven initiatives in the nearby neighborhoods. After talking to community leaders and partners, we have identified some high priority neighborhood goals around water quality, river and community connections, and economic support. We will continue to engage community members to help us refine the opportunities for community benefits. 

Our commitment to sustainability

The storage tank and surrounding site will incorporate many sustainable elements. Some examples of design elements already added include new trees, solar panels, and rain gardens. Visit our online open house to learn more about other sustainability features this site will have.

Map displaying location of new storage facility near the 1st avenue bridge (State Route 509), on the west bank of the Duwamish Waterway. Two additional underground structures, next to West Marginal Way SW, are part of the project..


Contact us

Contact Kristine Cramer, community services lead at:


Demmelash Adera at:

If you’d like to receive email or text updates on the West Duwamish CSO Control project:

Project location

locator map

Back to the capital projects overview map .

Combined sewer systemWhy does sewage overflow on the rainiest days?

Person talking to a King County staff person at information boothKing County RainWise team at the 2021 Duwamish River Festival