Seattle Public Utilities Mami Hara, General Manager/CEO

Ship Canal Water Quality


Seattle Public Utilities and King County are building underground stormwater and sewer storage.

What & Why?

Seattle Public Utilities and King County are working together to build an underground storage tunnel. During storms, this tunnel will hold polluted water from Ballard, Fremont, Wallingford, and north Queen Anne, preventing it from overflowing into the Lake Washington Ship Canal. Read the Joint Project Agreement (pdf) to learn about how Seattle Public Utilities and King County work together on this project.

What’s happening now?

Learn more about work in your neighborhood:

SPU has prepared the Final Supplemental Environmental Impact Statement (EIS) to understand how the Ship Canal Water Quality Project will affect the environment and community.

Why is this project needed?

Sewage and stormwater from many older parts of the city – including Ballard, Fremont, Wallingford, and north Queen Anne – all funnel into one set of pipes. In dry weather conditions, all sewage flows to King County’s treatment plant in Magnolia. During wet weather conditions, polluted runoff can exceed the pipes capacity and send a mixture of stormwater and sewage to the nearest body of water. These combined sewer overflows or “CSOs” contain contaminants that could make people sick and harm fish, wildlife, and the environment.

Project goals and benefits

This project will keep nearly 60 million gallons of polluted water out of the Ship Canal, Salmon Bay, and Lake Union each year. Combined sewer overflows in Ballard, Fremont, Queen Anne, and Wallingford currently send sewage and stormwater into the Ship Canal an average of more than 130 times per year at seven outfall locations. The project will limit those overflows to no more than one overflow per outfall per year on average, in compliance with state and federal laws.