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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.

September 2024

Thank you for sharing your insights with the Mouth of Duwamish Wet Weather Facilities team!

Over the last two months, our team provided opportunities for community members who live, work, and travel through the SODO neighborhood to learn about Mouth of Duwamish Wet Weather Facilities and share their insights with the King County team. Thank you to those who participated and shared your thoughts!

Geotechnical sampling work in SODO beginning as early as September 2, 2024

King County crews will conduct geotechnical testing to better understand underground conditions in different areas of the SODO neighborhood. Most work will occur off of the roadway. If temporary lane, shoulder, or sidewalk detours are necessary, flaggers will be on site to assist vehicles and pedestrians. Work will begin as early as September 2, 2024, and will take up to six weeks to complete. Read more and see a map of survey locations here.

Build Your Sustainable Future: Fall Job Fair and Workforce Training Symposium

You’re invited to learn about jobs and paid training opportunities in clean water, sustainable building, and construction! Participate in hands-on “day in the life” job demonstrations, career networking, resume help, and other job-seeking activities. Meet employers who are hiring now and in the near future. This event is free and open to the public, including people in all stages of careers and education.

When: Wednesday, October 23rd, 2024 from 4-7pm

Where: South Seattle College Georgetown Campus: 6737 Corson Avenue South, Seattle, WA 98108

Stay connected!

As we progress through planning, 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 or by scanning the QR code below.

Can’t make it to one of our virtual or in-person events? Contact Bibiana Ocheke-Ameh at bibiana.ocheke-ameh@kingcounty.gov or 206-477-5604 with questions or to request a briefing for your organization.

King County is working on an effort called the Mouth of Duwamish Wet Weather Facilities that will reduce overflows from 5 combined sewer outfalls where the Duwamish River meets Elliott Bay.

We are evaluating options that control combined sewer overflows (CSOs) to comply with environmental laws. 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.

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 SODO neighborhood. 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.

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