POTOMAC, po-ta'mak, a river in the east ern part of the United States, formed by the junction of two rivers on the boundary between Maryland and West Virginia. The river called North Branch rises in the western Alleghanies and the South Branch in the central Alleghanies, and flowing northeast they unite about 15 miles southeast of Cumberland. The North Branch flows southeast from Cumberland for over 100 miles. From the .junction of the two head streams, the river flows generally east-southeast, forming the irregular boundary between Mary land and West Virginia and between Maryland and Virginia, and after a course of over 400 miles enters Chesapeake Bay about 73 miles from the Atlantic Ocean. The chief branches are the Shenandoah from Virginia, and the Monocacy from Maryland. At Harper's Ferry the river breaks through the Blue Ridge, form ing a picturesque gorge. There are a number of falls, especially in that part of the course through the mountains. About 15 miles above Washington begins a rapid descent which in two miles is 80 feet and which ends with tit( Great Falls, a cataract 35 feet high. At Wash
ington the river meets the tide, 125 miles from its mouth; and about 25 miles below it becomes an estuary from two to eight miles wide. It is navigable for large vessels to Washington. The Chesapeake and Ohio Canal is parallel to and near the river the whole length from Georgetown to Cumberland. The Potomac is the most important river in this section of the United States. It was the route whereby ex plorers journeyed to the interior in the early days, and it has been closely associated with many of the important events in the history of the country. The nation's capitol is on its bank, the home of the first President, and his grave, is a few miles below, and Arlington (q.v.), with its white tombs, may be seen from the Capitol. The drainage area is about 15,000 miles. SCC BOUNDARIES OF THE UNITED STATES.