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King County Washington home

Mouth of Duwamish Wet Weather Facilities

King County is building a wastewater treatment facility and a storage tank. These projects will keep more than 400 million gallons of sewage and stormwater out of the Duwamish River and Elliott Bay during heavy rain each year. This will help water quality.

Program update

July 2026

Looking for information about a specific project?

We now have dedicated websites for the Chelan Wet Weather Storage project and the SODO Wet Weather Treatment Station project. Visit each site for the latest project updates, schedules, construction information, and ways to stay involved.

Upcoming community events

Come meet our team this summer and learn more about Mouth of Duwamish Wet Weather Facilities.

SODO Business Improvement District National Night Out

When: Tuesday, August 4, 2026, from 4 – 6 pm 

Where: Canine Companions Puget Sound Field Office  - 2454 Occidental Avenue South 

What: Connect with community members, business owners, and local law enforcement over cold drinks and tasty bites. Register here. 

SOPASUPA x Duwamish River Fest x River City Skatepark

When: August 8, 2026, from 12 pm – 9 pm 

Where: 14th Avenue South and Cloverdale Street South / Duwamish River People’s Park – 8700 Dallas Street South 

What: Join your neighbors for an epic day of free music, kid’s activities, skateboarding, beer garden, cultural traditions, and more. Learn more about the event here. 

Previous updates

Program description

King County plans to build new wet weather facilities in SODO and West Seattle. These facilities will help keep pollution out of the Duwamish River and Elliott Bay.

During heavy rain, combined sewer pipes in older parts of Seattle can fill up fast with stormwater. When this happens, pipes release extra water into nearby rivers, lakes, and Puget Sound. This release is called a combined sewer overflow, or CSO.

These facilities will reduce pollution from five locations where CSOs happen. It will help King County meet state water quality laws and protect public health. This means cleaner water for people, fish, and wildlife.

The program supports King County’s Equity and Social Justice goals. It improves water quality in communities that have faced more pollution in the past. It also supports King County’s Strategic Climate Action Plan. It prepares the system for heavier rain and stronger storms.

King County will build:

  • A wet weather treatment station in SODO. It will treat stormwater and sewage during heavy storms.
  • An underground storage tank in North Delridge (West Seattle) on West Marginal Way Southwest. During heavy storms, the tank will hold extra water in our sewer system. After storms, the water will be sent to West Point Treatment Plant for treatment and safe discharge into Puget Sound.
  • A new outfall pipe to the Duwamish River. This pipe will safely release treated water from the wet weather treatment station. It will be underwater in the Duwamish River between SODO and Harbor Island.
  • New pipes and support buildings. These will connect the treatment station and storage tank to the  sewer system. They will help everything work together.

Visit the projects page to learn more about the projects included in this program.

About our combined sewer system

In Seattle, most of the sewer pipes carry both stormwater and sewage.

Stormwater is rain that runs off streets and rooftops. Sewage comes from the toilets and sinks in homes and businesses.

When one pipe carries both, it is called a combined sewer system.

During heavy rain, combined sewer pipes can fill up fast with up to 90 percent more rainwater. When this happens, pipes release extra water into nearby rivers, lakes, and Puget Sound. This is called a combined sewer overflow, or CSO.

CSOs were designed and built decades ago to prevent sewage from backing up into homes and businesses.

CSOs release water that has not been treated. This water can carry harmful germs and chemicals. CSOs pollute local waterways, and harm people and wildlife.

Learn how King County is protecting our waters.

The five King County outfalls included in this planning process are located in the east and west waterways of the Duwamish River on both sides of Harbor Island and along the shoreline of Elliott Bay generally in SODO.
This program will reduce pollution from five combined sewer overflow outfall pipes.

What the new facilities will do

  • The SODO treatment station will treat combined sewer overflows during heavy rain.
  • The North Delridge storage tank will hold combined sewer overflows until it can be treated.
  • Together, they will keep about 400 million gallons of pollution out of the Duwamish River and Elliott Bay each year.
  • They will make the river and the bay cleaner and safer for people, fish, and wildlife.
  • They will help the system handle bigger storms.

Key terms

Stormwater is rain that runs off roofs, streets, and sidewalks. As it flows, it can pick up oil, yard chemicals, and pet waste.

Outfalls are pipes in our combined sewer system that act like safety valves. They release extra water into nearby rivers, lakes, and Puget Sound.

Wet weather facilities treat, store, or move extra water during heavy storms. They help keep untreated water out of local waterways.

Meeting water quality laws that protect rivers and streams

This work is required by a legal agreement called a consent decree.

King County has an agreement with the U.S. Environmental Protection Agency and Washington State Department of Ecology. It requires King County to build projects to reduce CSOs by 2037.

The agreement sets:

  • What work must be done
  • How often combined sewer overflows can happen
  • Deadlines for completing the work

If you have questions or comments about the consent decree, contact Faon O’Connor. You can reach her at Faon.OConnor@kingcounty.gov or 206-477-4499.