POTOMAC, a river in the east central part of the United States, having its source in the Allegheny mountains and flow ing south-east into Chesapeake bay. It is formed by the union of its north and south branches, about 15m. S.E. of Cumberland (Md.). The main stream has a length of about 45om. and is navigable for large vessels for 113m. above its mouth. From the junction of its two branches until it reaches Harper's Ferry the Potomac river separates Maryland from West Virginia. At Harper's Ferry it receives the waters of the Shenandoah river and cuts through the Blue Ridge mountains in a gorge noted for its beauty. From this point to its mouth it forms the boundary between Virginia and Maryland. The stream crosses the Blue Ridge mountains at an elevation of about 245ft., and at George town (Washington, D.C.), 62m. distant, it meets tidewater. Of this descent about Soft. occurs about 15m. above Washington at the Great Falls, a series of rapids about a mile long and including a cataract about 35ft. high. Three and a half miles above Wash ington are the Little Falls, which mark the head of navigation.
At Washington there are two channels, with respective depths at mean low water of 18 and 2i ft. Large sums have been spent
since 1870 on improving these channels. A few miles below the city the river broadens into a deep tidal estuary from 21 to 7m. wide; and channels 24ft. deep and goof t. wide through all the shoals were secured by the project of 1899. The Anacostia river, or "East Branch," which flows into the Potomac just south of Washington, is navigable for large vessels for about 2m. and for small scows and lighters as far as Bladensburg (Md.), 84m. above its mouth. Improvements (begun in 1902) have produced a channel 2I f t. deep at mean low water and 38oft. wide. The Chesapeake and Ohio canal, from Georgetown to Cumberland (Md.), follows the Potomac closely on the Maryland side. The shipments over the Potomac below Washington in 1925 were 2,439,365 tons valued at $54,042,300, the principal commodities being naval ordnance and supplies, gasolene and kerosene, sand and gravel and other building materials. The shipments on the Anacostia river in 1925 were valued at $12,385,175.