The North Branch of the Potomac River rises on the Appalachian Plateau of West Virginia, within the boundaries of Fairfax Stone State Park. Flowing northeastward, it forms the border between Maryland and West Virginia; about halfway to Cumberland, it was dammed to create Jennings Randolph Lake in 1981. From Cumberland, the Potomac makes a broad dip to the south, where it is joined by its South Branch, and then continues northeastward to Hancock, Maryland, where it reaches its most northerly point.
From Hancock, the Potomac River flows southeastward and receives the waters of the Shenandoah at Harper's Ferry, West Virginia. There it becomes the border between Maryland and Virginia, passing through a double water gap in the Blue Ridge Mountains and then angling more sharply to the SSE. Fourteen miles north of Washington, D.C., the Potomac drops through its Great Falls, a point on the Fall Line of the American Southeast, marking the boundary between the resistant bedrock of the Piedmont and the softer sediments of the Coastal Plain. The spectacular falls drop 76 feet through Mather Gorge, the major impetus for constructing the Chesapeake & Ohio Canal between Georgetown, D.C., and Cumberland, Maryland; used from 1831 to 1924, the canal and its corridor along the Potomac are now protected as a National Historic Park.
Beyond Washington, the Potomac River broadens into an ever-widening estuary, reaching a maximum width of 11 miles as it enters the Chesapeake Bay, more than 400 miles from its headwaters. Before the end of the Pleistocene Ice Age, the Potomac was a tributary of the lower Susquehanna River, now flooded by rising seawater to form the broad, shallow Bay.
