river in the east central United States, rising in North and South branches in the Appalachian Mountains of West Virginia. The two branches (95 mi 【150 km】 and 130 mi long, respectively) flow generally northeast and unite southeast of Cumberland, Md., to continue southeast through the District of Columbia into Chesapeake Bay. The river drains an area of approximately 14,500 sq mi (37,600 sq km). Its course is 383 mi, of which 117 mi are tidal. With the North Branch it forms the boundary between Maryland and West Virginia from its source to Harpers Ferry, W.Va., and from there to its mouth it is the boundary between Maryland and Virginia. The Potomac's tributaries include the Shenandoah at Harpers Ferry, the Monocacy in the Piedmont region, and the Anacostia at Washington, D.C. The District of Columbia lies on the left (east) bank at the head of the tidewater. The river is navigable to Washington, D.C., above which it descends from the Piedmont in a series of rapids and falls, including Great Falls, a cataract about 35 ft (11 m) high.