Mouth of Duwamish Wet Weather Facilities
Learn about King County’s efforts to develop wet weather facilities to control combined sewer overflows in the area of the mouth of the Duwamish River.
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.
Previous updates
- Share your input on SODO facilities design by August 31! August 2025
- Duwamish River Festival and survey reminder, August 2025
- Survey work beginning in SODO and West Seattle as early as July 28, July 2025
- We want to hear from you! Help shape the look and feel of proposed clean water facilities in SODO. July 2025
- Survey work beginning in SODO and West Seattle as early as June 23, June 2025
- Learn more about planned Mouth of Duwamish Wet Weather Facilities at upcoming drop-in sessions! June 2025
- King County's plan for Mouth of Duwamish Wet Weather Facilities, May 2025
- Planning continues for Mouth of Duwamish Wet Weather Facilities, March 2025
- Thank you for learning about Mouth of Duwamish Wet Weather Facilities this year! December 2024
- Survey work beginning in SODO as early as November 11, 2024, November 2024
- Reporting back on summer 2024 community input! October 2024
- Join us: Building Your Future Jobs Fair October 23 at South Seattle College, Georgetown Campus, October 2024
- You shared, we listened, September 2024
- Geotechnical sampling work in SODO beginning as early as September 2, 2024, August 2024
- Don’t miss this opportunity to share your insights about the SODO community! June 2024
- Planning progresses for Mouth of Duwamish Wet Weather Facilities, June 2024
- Upcoming survey work in the SODO neighborhood, April 2024
- King County is starting work to address combined sewer overflows in SODO, March 2024
King County is working on an effort called the Mouth of Duwamish Wet Weather Facilities that will reduce overflows from five combined sewer outfalls in Seattle where the Duwamish River meets Elliott Bay. The CSOs were designed many decades ago to be overflow relief points in our sewer system during periods of heavy rain. Today, to better protect local water bodies, King County is investing in solutions to limit the number of overflows that occur and to ensure we are meeting regulatory requirements to protect water quality.
The Mouth of Duwamish Wet Weather Facilities is the largest CSO project to date and will complete our CSO work in the Duwamish River, preventing approximately 430 million gallons of polluted water from entering Elliott Bay and the Duwamish River per year on average. This major investment will support a healthier waterway for people, fish, and orcas.
The proposed solution, announced in May 2025, complies with federal and state regulations. Visit the projects page to learn more about the proposed solution and next steps.
What we’re doing
King County is working to control 5 CSO outfalls in the area of the mouth of the Duwamish River. The outfalls (called Chelan, Hanford #2, Lander, Kingdome, and King Street) are located in the east and west waterways of the Duwamish River on both sides of Harbor Island and along the Seattle shoreline of Elliott Bay in the neighborhoods of SODO and West Seattle (Industrial District West). This work fulfills legal requirements (called a consent decree) we have with the U.S. Environmental Protection Agency and the Washington State Department of Ecology.
King County has made great progress in controlling CSO outfalls in Puget Sound and other local water bodies. As of 2023, we've controlled 18 outfalls to reduce overflows to reduce pollution into local water bodies. But our work is not done yet. Controlling CSO outfalls at the mouth of the Duwamish River represents a large portion of the remaining CSO outfalls we strive to control.
The Mouth of Duwamish Wet Weather Facilities is one of the most complex efforts we will undertake. The effort will help King County be a good environmental steward by improving water quality. We must control CSO overflows and limit pollution to Duwamish River and Elliott Bay to do our part in creating healthy water bodies where communities and wildlife thrive.
About combined systems and CSOs
Much of Seattle was built using a combined sewer system. Combined sewer systems carry sewage and stormwater in the same pipe. They prevent sewer backups in homes, businesses, and streets during heavy rain. When heavy rains fill the pipes, CSO outfalls release stormwater and sewage into local water bodies.
Today, King County is modernizing a system that’s been in place for more than a century to ensure sustainable operations into the future by investing in more wet weather facilities. Since our CSO Control Program began in the 1980s, we’ve reduced the average CSO discharge volume from an estimated 2.3 billion gallons to about 1 billion gallons per year. Controlling CSO outfalls at the mouth of the Duwamish River represents a large portion of the remaining CSO outfalls we strive to control.
Key terms
Stormwater is the rain that falls on our roofs and streets and flows into storm drains. The water picks up what it touches, like oil from the road, yard chemicals, and pet waste. Then the polluted water runs off into local bodies of water — like the Duwamish River.
CSO outfalls are relief points designed in combined sewer systems that carry sewage and stormwater in the same pipe. They prevent sewer backups in homes, businesses, and streets during heavy rain. When heavy rain fills the pipes, CSO outfalls release sewage and stormwater into rivers, lakes, or Puget Sound.
Wet weather facilities are designed to treat a combination of stormwater and wastewater that fills pipes to overcapacity during periods of heavy rain.