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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

June 2026

Field survey work continues

We are doing on-the-ground field surveys in the project area. This work helps us plan our design and prepare for future construction. The surveys show us what is underground near our planned facilities. They also help us identify places that may be culturally or historically important.

This survey work began in summer 2025. We expect it to continue through spring 2026.

If you see survey activity in SODO or North Delridge, here’s what to expect:

  • Crews typically use one truck and one survey vehicle 
  • You may hear engine noise or hammering while crews are working. You may also feel slight vibrations.
  • Standard work hours are 7:30 a.m. to 5 p.m. We will reach out to nearby businesses in advance if we plan work outside these hours.
  • Crews may need to set up detours. If they do, flaggers will assist drivers and people walking or biking. 
  • Crews will remove equipment and restore disturbed surfaces once work is complete. 

Thank you for your continued coordination as our crews complete this work.

Stay connected!

We always welcome your questions, thoughts, and concerns. Contact the project team:

Join our email list using the link in the “Stay informed” section of this webpage.

Previous updates

Program description

King County will 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 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 cleaned 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 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.

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.