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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.

Project update

May 2025

What’s happening now

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 to improve water quality in the Duwamish River and Elliott Bay includes a new wet weather treatment station in SODO and a new storage tank in West Seattle (Industrial District West). It also includes a new outfall for cleaned water, supporting facilities, and new pipes to connect flows along the way.

Now that we have a proposed solution, we will prepare an engineering report for review by the Washington State Department of Ecology. We will also start the design phase and environmental review. Explore our project documents or read our latest update (also available as a PDF) to learn more about the proposed solution, how we got here, what is coming next, and how to share your questions.

We want to hear your questions and concerns. Attend one of our upcoming drop-in sessions to connect with our team.

  • Home Depot Drop-In: Tuesday, June 10, 10 a.m. to noon at 2701 Utah Ave. S.
  • Delridge Community Center Drop-In: Thursday, June 12, 10 a.m. to noon at 4501 Delridge Way S.W.

Can’t make it? Connect with the project team by emailing DWetWeafacilities@kingcounty.gov or calling the Community Services Lead, Bibiana Ocheke-Ameh, at 206-477-5604.

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 or by scanning the QR code below.

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Contact Bibiana Ocheke-Ameh at DWetWeafacilities@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 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.

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