Seattle Public Utilities

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 a 2.7-mile, 18'10"-diameter underground storage tunnel. During storms, this tunnel will hold polluted water from Ballard, Fremont, Wallingford, and north Queen Anne, preventing it from overflowing into Lake Union, the Ship Canal and Salmon Bay.

What’s happening now?

Learn more about work in your neighborhood:

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 over 60 million gallons of polluted water out of the Ship Canal, Salmon Bay, and Lake Union each year. 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.