Project overview
This project will remove a fish passage barrier along Thornton Creek under Interstate 5 near State Route 523/NE 145th Avenue in Shoreline. Contractor crews working for WSDOT will replace the two existing 2,000-foot-long, 6-foot culverts with three fish-passable structures. The current culverts are not passable for fish. The new structures have the potential to open more than 1.8 miles of habitat for coho, steelhead, sea-run cutthroat and resident trout.
What to expect
On I-5 at 145th Street near Shoreline, contractor crews are scheduled to begin work in summer 2026 and end fall 2029. Travelers can expect one or two-lane closures and traffic shifts on northbound and southbound I-5 to build the new structures.
This project will build structures to allow fish to pass along Thornton Creek under the I-5/SR 523 interchange in Seattle/Shoreline.
Thornton Creek is a tributary to Lake Washington crossing under I-5 near the border of Seattle and Shoreline. Thornton Creek’s I-5 crossing begins on the west side of the highway, just north of the intersection with SR 523/Northeast 145th Street and ends on the east side of I-5 on 5th Avenue Northeast near Jackson Golf Course. As the creek crosses I-5, it is funneled through two, 6-foot concrete culverts for 2,000 feet under the highway. The creek is only 33 percent passable and fish need to navigate 2,000 feet in the dark.
When replacing fish barriers, we must build structures and improve the stream so that fish can use it year-round at all stages of their life. That means during summer months when streams are usually lower, the stream must be usable by juveniles and adults. During winter, when water tends to run higher and faster, the stream must also have areas where fish can rest.
New fish passable structure
To replace the existing culverts, WSDOT will rechannelize the creek using three new fish-passable structures. With the new structures, a good portion of the rechanneled stream will be open to the air in contrast to being buried under I-5 as it is now. We'll also add woody vegetation, logs and other wood to this section of Thornton Creek to provide channel complexity and diversity, such as pools which provide refuge for young fish. These habitat elements enhance rearing and spawning habitat for fish. The improvements mean fish will have access to 1.8 miles of habitat that was previously inaccessible.
This work is part of our fish passage program. Replacing the existing culverts and rebuilding the stream channels expands healthy fish habitat; an important step in restoring fish runs and increasing populations. This benefits commercial seafood operations and recreational fishers and provides more food for a declining orca population. Additionally, replacing these fish barriers helps us meet our obligation to remove barriers under the 2013 U.S. District Court injunction.
