Gilliam Creek Fish Barrier Removal Project

Project Overview

Gilliam Creek is located at Green River mile 12.5, just south of the Interstate 405 bridge over the Green River and is the northernmost extent of the roughly 5-mile-long Tukwila 205 levee.  The final 207 feet of Gilliam Creek pass through a 1960s-era culvert with a 108”-diameter flapgate at the outlet.  The flapgate reduces flood risk by preventing flood waters from the Green River entering Gilliam Creek and flooding adjacent developed areas.  However, the flapgate impacts habitat and makes the creek inaccessible to aquatic species, including threatened Puget Sound Chinook salmon. The Washington Department of Fish and Wildlife indicates the flapgate has 0% passability. Preliminary modelling suggests that velocities within the culvert are low and conducive to most adult migration but exceed the <1 fps threshold for juvenile salmon. The culvert’s diameter (9-ft) is a limiting factor for passing debris through the culvert. Substantial maintenance is required to clear the upstream debris rack of debris to prevent upstream flooding and to prevent scour and erosion associated with impounded creek water rising behind the obstructed culvert inlet, which negates some of the flood protection benefit of the flapgate.

This project will have multiple benefits including:

• Improve flood control by replacing the aged, static and high maintenance system with a flood management system that responds to the full range of water levels.
• Restore fish access into Gilliam Creek for the first time in more than 60 years, providing critically needed rearing and refuge habitat for juvenile Chinook salmon on their out-migration to the estuary.
• Provide a public amenity, allowing people to experience an essentially ‘lost urban creek’ by setting back the 205-levee and creating confluence habitat downstream of the replacement tide gate.

Project Status

The project has received funding from the King County Flood Control District, WRIA 9, the State Recreation and Conservation Office and the Salmon Recovery Funding Board.  The project is in the planning and design phase and relies on stakeholder input.

Contact Information

For specific questions, contact Mike Perfetti at 206-433-7192 or email at mike.perfetti@tukwilawa.gov

Project Photos