Riley Slough Bridge 155

Update - March 2020 

Construction on this project has been temporarily suspended (March 27 to April 6) during the COVID-19 pandemic response. We are reassessing the next steps to determine best how to proceed for the safety of our employees, contractors and the public.


Riley Slough Bridge 155 is located on Tualco Road southwest of Monroe; it provides vehicular access to rural residences and agricultural areas in the lower Skykomish River valley. The current structure was built in 1930 and is 206-feet long and 18-feet wide. The bridge has had load limits posted on it 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 new replacement bridge will be 206-feet long and 32.5-feet wide to accommodate two 11-feet vehicle lanes and two 5-feet shoulders. The new structure will be a three-span concrete bridge positioned on driven piles.


Construction for this project is expected to be completed in May 2020. Minor work may be performed after that to wrap up the project site.

Traffic Impacts

During construction there will be a 1.5 mile detour via Tualco Loop Rd.


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.

Environmental Review