Translate
Projects
Find information about current and planned future Mouth of Duwamish Wet Weather Facilities projects.
Project update
September 2025
What’s happening now
In July, King County launched a survey asking community members to share ideas about the look and feel of the proposed facilities in SODO. The survey closed on August 31, and we are now putting together a summary of the results. We’ll share that summary later this fall. Thank you to everyone who participated!
We will continue to keep the community updated about any future opportunities to share their thoughts and ideas.
Apply to join a Community Sounding Board
Community members are invited to apply to join a Community Sounding Board to help advise King County during design. Learn more by reviewing this flyer and complete this brief application form by Thursday, September 25, at 5 pm.
Ongoing survey work in SODO and West Seattle
We are studying the project area to better understand what’s underground. This work will help us check for places or objects with historical or cultural importance. This work began in August and will continue through September.
Read the July project update to see a map of survey locations and learn more about what to expect.
Build your Sustainable Future: Job and resource fair
When: Wednesday, October 15, 2025, from 3 to 7 pm
Where: Gene J. Colin Education Hall at South Seattle College – Georgetown Campus, 6737 Corson Avenue S
What: Jobseekers can explore diverse career and paid training opportunities in clean water, sustainable building, and construction! The event will feature hands-on “day in the life” job demonstrations, networking, resume assistance, and more. It’s a great chance to meet employers hiring now and in the near future. This free event is open to the public, welcoming people at all stages of careers and education.
This free event is open to the public, welcoming people at all stages of careers and education. Refreshments & Spanish interpretation available. Se proporciona interpretación en español. Registration is encouraged but not required.
Sponsored by: King County Wastewater Treatment Division and South Seattle College
Stay connected!
As we progress through design and construction, we will continue to provide opportunities for you to ask questions and share thoughts and concerns. For project updates, join our email list using the link in the “Stay informed” section of this webpage.
Contact Bibiana Ocheke-Ameh at dwetweafacilities@kingcounty.gov or 206-477-5604 with questions or to request a briefing for your organization.
Project description
King County has proposed a solution to control overflows from five combined sewer overflow (CSO) outfalls at the mouth of the Duwamish River. The proposed solution includes the facilities outlined in the drop-down menu below. Together, these facilities will control CSOs from the five project outfalls. The proposed solution will meet current and future needs. Read more about the proposed facilities by exploring our Fact Sheet and Frequently Asked Questions.
Facilities
The proposed wet weather treatment station will control overflows from four of the project outfalls (called Hanford #2, Lander, Kingdome, and King Street). The proposed location is on about eight acres of land near the intersection of South Spokane Street and First Avenue South.
During periods of heavy rain, stormwater and wastewater that currently overflow into the Duwamish River will be directed to the wet weather treatment station. There, the combined stormwater and wastewater will be cleaned before it is discharged into the Duwamish River.
King County recently built a similar facility in Georgetown. The Georgetown Wet Weather Treatment Station can treat up to 70 million gallons of combined stormwater and wastewater per day. To put the proposed wet weather treatment station in context, it will have the capacity to treat up to about 240 million gallons of combined stormwater and wastewater per day during heavy rains. That is nearly four times as much as the Georgetown Wet Weather Treatment Station. Learn more about that facility by visiting kingcounty.gov/GeorgetownWWTS.
The storage tank will control overflows from the fifth project outfall (called Chelan). It will be underground near the intersection of Southwest Dakota Street and West Marginal Way Southwest.
The storage tank will help control CSOs by holding combined stormwater and wastewater that would otherwise overflow into the Duwamish River during large rainstorms. Once heavy rain subsides, the combined stormwater and wastewater held in the storage tank will flow back into pipes and on to West Point Treatment Plant for cleaning. The proposed storage tank would be able to hold up to 7.5 million gallons of combined stormwater and wastewater. That is more than 10 Olympic-sized swimming pools!
King County is preparing to build a similar storage tank in South Park. The West Duwamish Wet Weather Storage project will be able to hold up to 1.25 million gallons of combined stormwater and wastewater. Learn more about that project by visiting kingcounty.gov/WestDuwamish-cso.
After leaving the new wet weather treatment station, clean water will be released into the Duwamish River from a new outfall between SODO and Harbor Island. The outfall will be underwater and will not be visible from the shore.
To connect the new wet weather treatment station and storage tank to the existing system serving homes and businesses, we will install new pipes along the following roads in SODO and West Seattle (Industrial District West):
- Utah Avenue South from South Lander Street to South Hinds Street
- South Hinds Street from Utah Avenue South to the Duwamish River
- West Marginal Way Southwest from Chelan Avenue Southwest to Southwest Dakota Street
Supporting facilities will connect the wet weather treatment station and storage tank to the existing system and direct flows on rainy days. These facilities will be above-ground buildings with a footprint between the size of a two-car garage and a basketball court. The proposed solution includes supporting facilities at the following intersections:
- Alaskan Way South and South King Street
- First Avenue South and South Royal Brougham Way
- Utah Avenue South and South Lander Street
- Utah Avenue South and South Hanford Street
- Chelan Ave Southwest and West Marginal Way Southwest
Timeline
King County anticipates submitting a final engineering report to the Washington State Department of Ecology in March 2026, after environmental review under the State Environmental Policy Act (SEPA) is complete. This step is needed for approval of the proposed solution. We will also start the design phase.
Community outreach
For information about community outreach, visit the Mouth of Duwamish Wet Weather Facilities public engagement webpage.