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Show Chapter 2 The River and Its Tributaries From its headwaters on the eastern slope of the Appalachian Mountains, the Potomac flows in a general southeasterly direction some 400 miles to Chesapeake Bay. The main stem is formed approx- imately 20 miles below Cumberland, Md., by the junction of the north and south branches of the Potomac and flows southeast to the Fall Line at Great Falls, Va. About 10 miles below the Falls, at Chain Bridge, the lower river, or tidal section, is formed. Thus, the river can be divided into two sections identifiable by certain characteristics. The nontidal upper river-the portion above the Fall Line-266 miles long, runs its course through the mountainous terrain of the Alleghenies, the rolling hills of the Piedmont Plateau, and reaches tidal ele- vation just below Great Falls. It drains an area of approximately 11,580 square miles in Maryland, Virginia, West Virginia, Pennsylvania, and the Dis- trict of Columbia. It is a comparatively narrow, fast-flowing stream, flanked by steep banks and mountains and has many natural obstructions and mountain rapids. The Potomac tributaries possess essentially the same features as the nontidal part of the main stream. The Shenandoah is the largest tributary stream. It flows along the western base of the Blue Ridge Mountains and enters the Potomac at Har- pers Ferry. Other principal tributaries are the Cacapon, Monocacy, Conococheague, and the north and south branches of the Potomac. Below Harpers Ferry, tributaries bring in large quantities of sediment, causing high turbidity in the main stream. The river gradient is relatively flat in many places, but it is broken by many rapids and riffles. Con- tinuous navigation is not practical, although early pioneer craft were floated downstream during freshets from as far as Westernport, Md. The Chesapeake and Ohio Canal, completed in 1850, created the only commercial navigation of any con- sequence above Washington. Present navigation is limited to the tidal section. National defense estab- lishments in the Washington area require the main- tenance of deep navigable channels, including a main channel 24 feet deep, along the tidewater por- tion of the Potomac from the District of Columbia to Chesapeake Bay. Draining an area of 2,920 square miles, this 117-mile stretch of the river is wide, fairly deep, and has very little current. The flow is affected more by tides than by fresh-water flow. As the river approaches Chesapeake Bay, the water becomes brackish. During occasional pro- longed droughts, salt water may penetrate the estu- ary as far as Washington, adversely affecting fish and recreation, and increasing pollution problems.1 The longest period of sustained drought occurred during 1930-32; the average flow from July through De- cember 1930 was about 1,100 cubic feet per second at Point of Rocks, Md. (drainage area 9,651 square miles). During the period of record from 1930 to 1946, the average flow from the 11,560 square miles above Washington was 11,080 cubic feet per second, but extreme variations in flow occur. A maximum dis- charge at Point of Rocks, Md., of 480,000 cubic feet per second has been recorded; a minimum dis- charge of less than 800 cubic feet per second has been recorded at Great Falls, Va. Average annual runoff is about 35 percent of the average rainfall, or over 10 million acre-feet. The intense thunderstorms of summer frequently cause local floods, with widespread inconvenience 'U. S. Corps of Engineers. 581 |