SR 503 turns east onto Fargher Lake Highway at Rock Creek and enters Amboy by following Chelatchie Creek towards Yale Lake. The highway travels north and crosses the Lewis River into Cowlitz County on the Yale Bridge. The one-lane warren truss bridge, listed on the National Register of Historic Places, is located downstream from the Yale Dam on Yale Lake and upstream of Lake Merwin. SR 503 continues into Cowlitz County and turns west onto Lewis River Road at an intersection with its spur route that heads east to Skamania County. The highway travels southwestward in the Lewis River valley and passes through the communities of Yale and Ariel. 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.