SR 503 intersects its spur route and turns west to parallel the Lewis River downstream to Woodland, where the highway ends at an interchange with Interstate 5 (I-5).

A spur route, established in 1991, travels northeast into the Gifford Pinchot National Forest along Yale Lake, serving the community of Cougar.

[5] SR 503 continues heading northeast into a heavily forested region of the Cascades foothills, toward Fargher Lake and Amboy.

SR 503 enters Woodland and turns south onto Goerig Street before ending at a diamond interchange with I-5 near the confluence of the Lewis and Columbia rivers.

[16] After the completion of the Merwin Dam downstream in 1931 caused water levels on the Lewis River to rise,[17] the two counties completed a one-lane warren truss bridge named the Yale Bridge over the Lewis River the following year to serve the remote communities of Yale and Yacolt.

[19][20][21] The Yale Bridge was maintained by Clark and Cowlitz counties until the 1940s and had its timber approach spans replaced with steel in 1958 under maintenance of the Department of Highways.

[5][24] The spur route of SR 503 was added to the state highway system in 1991 as part of the original Lewis River Road between Yale and the Skamania county line in the Gifford Pinchot National Forest.

[2][13][25] WSDOT conducted a series of surveys to measure traffic volume in terms of AADT and calculated that 650 to 1,300 vehicles per day used the spur route in 2011.