POTOMAC  , a

river in the east central part of the United States, having its source in the Alleghany Mountains and flowing S.E. into Chesapeake Bay . It is formed by the union of its north and south branches, about 15 M . S.E. of Cumberland, Maryland . The main stream has a length of about 450 M. and is navigable for large vessels for 113 M. above its mouth . The north branch, about r 10 m. long, rises in the north-eastern part of West Virginia, pursues a north-easterly course, and forms part of the boundary between Maryland and West Virginia . The south branch has its sources in Highland county, Va., and in Pendleton county, W.Va., and flows north-east for about 140 M. until it joins the north branch . From the junction of these two streams 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 scenic 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 245 ft., and at Georgetown (Washington), 62 m. distant, it meets tidewater . Of this descent about 90 ft. occurs about 15 M. above Washing-ton, at the Great Falls, a series of rapids about a mile long and including a cataract about 35 ft. high . Three and a half miles above Washington are the Little Falls, which mark the head of navigation .

Large vessels, however, are prevented by a

bridge from proceeding above Georgetown . At Washington there are two channels, with respective depths at mean low water of iS and 21 ft . Large sums have been spent since 187o on improving these channels . A few miles below the city the river broadens into a deep tidal estuary from 21 to 7 M. wide; and channels 24 ft. deep and zoo ft. 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 2 M. and for small scows and lighters as far as Bladensburg, Md., 84 m. above its mouth; its natural channel was narrow and tortuous, and about 18 ft. deep; in 1909 improvements (begun in 1902) had procured a channel 20 ft. deep at mean low water and 380 ft. wide . The Chesapeake & Ohio Canal, from Georgetown to Cumberland, Md., follows the Potomac closely on the Maryland side . The shipments over the Potomac above Washington in 1907 were valued at $7,596,494, and those below Washington at $21,093,800, the principal commodities being sand and gravel, ice, oils, naval ordnance and supplies, and building and paving materials . The shipments on the Anacostia river were of much the same character, and in 1907 were valued at $4,312,687 .