I-90/SR 18 interchange improvements benefit cars, trucks – and fish

By David Rasbach and Kathy Mulady

If you've traveled through the Interstate 90/State Route 18 interchange near the city of Snoqualmie recently, you may have noticed construction on the east side of the interchange. Crews are diligently building walls beneath the I-90 bridge decks, some of which will stand 20 feet tall.

Most of the I-90/SR 18 interchange project is designed to improve traffic flow and safety for locals, commuters and freight. However, this part of the project has a different purpose. It's all about enhancing conditions for salmon, steelhead and other fish species.

Think of it as a sort of aquatic highway.

Once the new Diverging Diamond Interchange is complete later this year, a tributary to Lake Creek will cross beneath the I-90 off-ramp to SR 18, flow under both I-90 bridges alongside the new interchange, and head under the SR 18 ramp to eastbound I-90 without barriers to swimming fish.

Along SR 18, we are removing six barriers and installing two new bridges so that fish can swim further up Deep Creek and Lake Creek. These new stream crossings will restore access to 13 miles of vital stream habitat.

Channeling efforts under I-90

Beneath I-90, construction crews are reopening stream access that has been blocked for decades by culverts, making it difficult or impossible for fish to swim under the freeway near SR 18. Obstructions such as these have contributed to a decline in native salmon populations across the region. The walls being built on the east side of the interchange will help form a deep channel in the stream. This makes safe and easier passage for fish beneath the I-90 bridges.

Crews are building walls under I-90 bridges that will eventually form part of a stream channel under the freeway along SR 18. The channel will replace barriers to fish migration along a tributary to Lake Creek. (Photo courtesy Aecon)

Figuring out how to build these walls turned out to be a bit tricky, especially since parts of them needed to fit beneath the existing I-90 bridges. There isn’t room for standard 40-foot-tall supports, so crews opted for 51 “micro-piles,” each 25 feet tall. These smaller columns are tied together in groups of two or three to support the walls.

When finished, the concrete wall faces will be covered with a stone finish to give them a more natural look. Sand, gravel, boulders and other materials will be added to turn the three-sided stream channel into a welcoming environment for salmon returning to spawn.

The new stream channel will allow the Lake Creek tributary to flow along SR 18 under both directions of I-90 next to the new Diverging Diamond Interchange.

The reconnected waterway is expected to be ready for fish at the end of summer 2024. The channel has a secondary benefit of providing capacity for high stream flows during heavy rains. These new water crossing structures are built to last at least 75 years – helping reduce road closures during floods and providing more reliable transportation routes for the communities we serve.

This work is part of our ongoing effort to improve fish passage and reconnect waterways in Washington state that have been blocked by roads. We have worked for nearly three decades to improve fish passage and reconnect streams to help keep waterways healthy. A 2013 federal injunction also directed WSDOT to significantly speed up efforts to replace fish barriers.

The new channel for the Lake Creek tributary is also part of the $190 million I-90/SR 18 Interchange Improvement project being designed and built by Aecon. Once the project finishes in early 2025, we will have built the diverging diamond interchange and widened about two miles of SR 18 south of I-90 to improve safety and traffic flow in the area.

Crews build a bench to install columns that will eventually support a wall
beneath the eastbound lanes of I-90 over SR 18

Construction crews are in the early stages of building a wall along the east end of the I-90/SR interchange. The wall will help form and a stream channel under the I-90 bridges, helping to open fish access along a tributary to Lake Creek. (Photo courtesy Aecon)