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The Potomac River watershed covers counties in four states and the District of Columbia
The Potomac River (pronounced /pəˈtoʊmək/) flows into the Chesapeake Bay, located along the mid-Atlantic coast of the United States. The river is approximately 383 statute miles (616 km) long, with a drainage area of about 14,700 square miles (38,000 km²).1 In terms of area, this makes the Potomac River the fourth largest river along the Atlantic coast of the USA and the 21st largest in the USA. Over 5 million people live within the Potomac watershed, where precipitation provides the equivalent of over 8 m³ (more than 2,100 US gallons) of water per person per year.
The river forms part of the borders between Maryland and Washington, D.C. on the left descending bank and West Virginia and Virginia on the river’s right descending bank. The entire lower Potomac River is part of the State of Maryland, with the exception of a small tidal portion within the District of Columbia. Except for a small portion of its headwaters in West Virginia, the North Branch Potomac River is considered part of Maryland to the low water mark on the opposite bank. The South Branch Potomac River lies completely within the state of West Virginia except for its headwaters, which lie in Virginia.
The Potomac River runs 383 miles (616 km) from the Fairfax Stone in West Virginia to Point Lookout, Maryland and drains 14,679 square miles (38,020 km2). The average flow is 10,800 ft³/s (306 m³/s).1 The largest flow ever recorded on the Potomac at Washington, D.C. was in March 1936 when it reached 425,000 ft³/s (12,000 m³/s).1 The lowest flow ever recorded at the same location was 600 ft³/s (17 m³/s) in September 1966.1
An average of approximately 486 million gallons of water per day (21 m³/s) is withdrawn daily in the Washington area for water supply.1
The river has two sources. The source of the North Branch is at the Fairfax Stone located at the junction of Grant, Tucker, Preston counties in West Virginia. The source of the South Branch is located near Hightown in northern Highland County, Virginia. The river’s two branches converge just east of Green Spring in Hampshire County, West Virginia to form the Potomac.
Once the Potomac drops from the Piedmont to the Coastal Plain, tides further influence the river as it passes through Washington, D.C. and beyond. Salinity in the Potomac River Estuary increases thereafter with distance downstream. The estuary also widens, reaching 11 statute miles (17 km) wide at its mouth, between Point Lookout, Maryland and Smith Point, Virginia before flowing into the Chesapeake Bay.