Riley Slough Bridge 155

Riley Slough Bridge, Before & After

Description - Completed 

The new Riley Slough Bridge 155 opened to traffic on July 10, 2020. The bridge is located on Tualco Road southwest of Monroe and provides access to rural residences, active farmland and recreational areas in the lower Skykomish River valley. Tualco Road also serves as an alternative route for many commuters working south of Monroe.

Built in 1930, the original structure was 206-feet long and 18-feet wide. The bridge had load limits posted since 2011 due to structural deficiencies, with a current sufficiency rating of 19.25 out of 100 due to deteriorating shallow timber piles and fragmentation of the concrete deck.

The replacement bridge is 206-feet long and 32.5-feet wide to accommodate two 11-feet vehicle lanes and two 5-feet shoulders. The wider design provides safer access for people driving, biking and walking. The new structure features a three-span concrete bridge positioned on driven piles with larger openings to ease water flow during high river conditions.


Construction for this project began in April 2019 and completed on July 10, 2020. Crews stopped work for two months during the COVID-19 pandemic, but later resumed during Phase II of the Washington State Safe Start Plan with safeguards in place for all workers. Landscape preparation work and further planting was completed in October 2020.

Traffic Impacts

Tualco Road is open to traffic. Throughout construction of the replacement bridge, Tualco Road was closed and a detour route was signed.


Funding for the replacement bridge is 80% federal funding from the Bridge Replacement Program (BROS), and 20% from the Snohomish County Roads Fund. The total project cost $4.4 million.