Potomac (pətō´mək), river, 285 mi (459 km) long, formed SE of Cumberland, Md., by the confluence of its North and South branches and flowing generally SE to Chesapeake Bay. It forms part of the boundary between Maryland and West Virginia and then separates Virginia from both Maryland and the District of Columbia. The upper course of the Potomac has cut several gaps across the parallel ridges of the Appalachian Mts.; the water gap at Harpers Ferry, W.Va., is the largest. The river passes over the Great Falls above Washington, D.C., where it is crossed by Arlington Memorial Bridge and others, and enters a tidal estuary below the city. It is navigable for large ships to Washington, D.C., and formerly many smaller boats went to Cumberland, Md., via the Chesapeake and Ohio Canal. Its principal tributary is the Shenandoah River, which it receives at Harpers Ferry. The river is noted for both its beauty and its historical associations. Mt. Vernon is on the Virginia shore below Washington, D.C.