US 340 continues north as East Side Highway, which parallels the South River and the rail line along the flank of the Blue Ridge Mountains along the eastern edge of the Shenandoah Valley. The highway crosses Sawmill Run and reduces to two lanes in the community of Dooms and crosses Mine Branch in the village of Crimora. US 340 crosses Meadows Run, Pains Run, and Still Run and enters Rockingham County at the south end of the town of Grottoes. The highway follows Augusta Avenue through the east side of the town and meets the eastern end of SR 256 (Third Street). US 340 curves northeast and meets the eastern end of SR 253 (Port Republic Road) south of Port Republic, where the South River and North River merge to form the South Fork Shenandoah River. The highway passes to the south of the historic estate Bogota and by a monument to the Battle of Port Republic at Lynnwood. US 340 closely parallels the railroad through Berrytown to the town of Elkton, where the route follows Stuart Avenue. The highway expands to a four-lane divided highway for its partial cloverleaf interchange with US 33 (Spotswood Trail) and briefly runs concurrently with US 33 Bus. from the interchange to Old Spotswood Trail, onto which the business route continues west.