POTOMAC, a river of the United States, which joins Chesapeake Bay by a considerable estuary after a course of about 400 miles. The northern branch of the upper river rises in the Alleghanies, "West Virginia, the southern in the Shenandoah Mountains. Affluents are received from Pennsylvania, Maryland, Virginia, and West Virginia, - the most important of all being the Shenandoah, which joins it at Harper's Ferry, below which the united stream breaks through the line of the Blue Ridge. Ships a.seend for a short distance above "Washington (the capital of the United States), or a total distance from the sea of 125 miles.