SR 503 follows the route of State Road 15, established in 1909 as the Lewis River Road, traveling east from Woodland to the Skamania county line in Columbia National Forest. The Lewis River Road was removed from the state highway system in 1919 and transferred to Cowlitz County for maintenance. Clark and Cowlitz counties contracted the Portland Bridge and Iron Company to build a bridge over the Lewis River in 1917 and the bridge was opened to traffic in September 1920 at a cost of $15,000. After the completion of the Merwin Dam downstream in 1931 caused water levels on the Lewis River to rise, 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.