The East River, a New River tributary, begins at the rail yards in Bluefield. Despite its name, the East is more of a creek than a river, with a moderate rate of flow, falling 1,040 feet from its head to its mouth. East River follows a direct course 23 miles northeastward, from Bluefield to its juncture with the New River at Glen Lyn, Virginia. Originating in the valley between Stony Ridge to the northwest and East River Mountain to the southeast, the river is contained entirely in Mercer County, except for its last half-mile in Giles County, Virginia. A gristmill, no longer in use but still standing near Ingleside, was powered by the stream.

The East River Valley is an important transportation corridor, as traffic which has come through the mountain gap which the New River makes at Narrows, Virginia, finds in the narrow valley of the East an unimpeded route westward. The former Virginian and Norfolk & Western railroads both followed the East River, as does Norfolk Southern, their modern successor. State Route 112 follows the river, as do County Route 30 and busy U.S. 460.