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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.
December 2024
Thank you for learning about Mouth of Duwamish Wet Weather Facilities this year!
King County kicked off the planning phase for Mouth of Duwamish Wet Weather Facilities in early 2024. Since then, we have made progress towards identifying a solution that meets regulatory requirements. We have also worked to raise awareness of this new effort.
We expect the planning phase to end in spring 2025. This phase will result in a proposal for facilities to control Mouth of Duwamish CSO outfalls and meet regulatory requirements. After the planning phase, King County will begin the work to advance the proposal with input from regulatory agencies, including the Washington State Department of Ecology. We will also begin environmental review under the State Environmental Policy Act (SEPA).
Read our year-end project update for more information about our progress in 2024 and what to expect in the new year
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.
Previous updates
- Survey work beginning in SODO as early as November 11, 2024, November 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 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 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.