Potomac River, a river of the eastern United States. It begins in the Appalachians at the junction of the North Branch and South Branch of the Potomac River, near Cumberland, Maryland. From here the river flows about 280 miles (450 km), generally southeastward, along the Maryland-West Virginia and Maryland-Virginia borders, past Washington, D.C., to Chesapeake Bay. The Potomac is joined by its chief tributary, the Shenandoah, at Harpers Ferry, West Virginia. Below Washington the Potomac is a tidal estuary and is navigable by oceangoing ships.

The upper and middle courses of the Potomac are interrupted by falls and rapids. The largest of these is the Great Falls, a short distance upstream from Washington. The Chesapeake and Ohio Canal, built in the 19th century and abandoned in 1924, parallels the river from Washington to Cumberland. The canal and adjoining areas now make up the Chesapeake and Ohio Canal National Historical Park.