a large river of North America, which flows through its whole course the boundary between the states of Maryland and Virginia. It rises in the Alleghany Mountains, and flowing in a north-easterly direction towards Pennsylvania, it forms a junction with the Shenandoah at Harper's Ferry, and then makes its celebrated passage through South Mountain. At its embouchure it is seven miles and a half in breadth; and at Alexandria, 290 miles from the ocean, it is one mile and a quarter in breadth. Tide-water reaches about 300 miles from the Atlantic; and for nearly that distance the Potomac is navigable for the largest vessels. The river is seven fathoms deep at its mouth, five at St George's Island, three at Swan's Point, and thence to Alexandria of the same depth. The length of its course, the curvatures included, is considerably above 300 miles.