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Thornton Creek Basin - sewer study and upgrade

The Thornton Creek Basin Sewer Study and Upgrade Project will ensure that the basin’s sewer pipe is able to carry wastewater flows to meet King County’s clean water goals into the future. 

Project description

The Thornton Creek sewer is an approximately 1.2-mile-long sewer pipe that collects wastewater from other pipes in our sewer system, serving 9.6 square miles of North Seattle and Shoreline. The pipe carries this wastewater to the Matthews Park Pump Station, where it is then conveyed and treated at the West Point Treatment Plant.

During heavy storms, the sewer pipe sometimes reaches its capacity. To avoid possible overflow, King County will need to either reduce the amount of stormwater and groundwater entering the system – known as infiltration and inflow (I/I) – or increase the size of the sewer pipe. Increasing the size of the sewer pipe may also require more costly upgrades to other parts of the regional wastewater system, including Matthews Park Pump Station.

Project update

Spring 2023

Phase one of the Thornton Creek Basin Sewer Study and Upgrade Project is complete.

Learn more about the study’s findings

Previous project updates

Project location