Potomac, a large river of the United States, constituting nearly the whole boundary between Maryland on the one hand and Virginia and West Virginia on the other, and formed by the junction of the North and South branches on the N. border of West Virginia. The North branch rises in the Alleghanies in the north of this state, and the South branch in the Shenandoah range on the border of Virginia and West Virginia. From the junction, which is about 20 m. S. E. of Cumberland, its course forms an irregular curve, first N. E. and then generally S. E., until it reaches the city of Washington. Thence flowing S. and S. W., it expands into an estuary from 6 to 8 m. broad, and after a course of 40 m. again changes to S. E. for about 70 m., and enters Chesapeake bay 75 m. from the Atlantic, the entire length being nearly 400 m. Its principal tributary is the Shenandoah, which enters it just W. of the Blue Ridge. Several other streams, but none of considerable size, also contribute to its volume. Of these the principal are the Savage and Monocacy rivers, and the Conegocheague, Patterson, Occoquan, and Aquia creeks, all of which are navigable for short distances.

The tide extends to Georgetown, 125 m. from its mouth, and it is navigable for ships of the line to the city of Washington. From its source to tide water the river has a great descent. There are a number of falls, including Houre's, the Shenandoah, Seneca, Great, and Little; and from Westport to "Washington, about 220 m., the difference in its altitude is 1,160 ft. It abounds in beautiful scenery, and its passage through the Blue Ridge at the junction of the Shenandoah at Harper's Ferry was pronounced by Jefferson one of the most stupendous scenes in nature. The navigation of this river was an early object of attention, and by the operations of the Potomac company, incorporated by Maryland and Virginia in 1784, many improvements were effected.