The Potomac River (// ( listen)) is located within the Mid-Atlantic region of the United States and flows from the Potomac Highlands into the Chesapeake Bay. The river (main stem and North Branch) is approximately 405 miles (652 km) long, with a drainage area of about 14,700 square miles (38,000 km2). In terms of area, this makes the Potomac River the fourth largest river along the Atlantic coast of the United States and the 21st largest in the United States. Over 5 million people live within the Potomac watershed.
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 majority of the lower Potomac River is part of Maryland. Exceptions include a small tidal portion within the District of Columbia, and the border with Virginia being delineated from "point to point" (thus various bays and shoreline indentations lie in Virginia). 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 405 miles (652 km) from Fairfax Stone Historical Monument State Park in West Virginia on the Allegheny Plateau to Point Lookout, Maryland, and drains 14,679 square miles (38,020 km2). The length of the river from the junction of its North and South Branches to Point Lookout is 302 miles (486 km). The average daily flow during the water years 1930-2017 was 11,416 cubic feet (323.3 m3) /s. The highest average daily flow ever recorded on the Potomac at Washington, D.C., was in March 1936 when it reached 426,000 cubic feet (12,100 m3) /s. The lowest average daily flow ever recorded at the same location was 601.0 cubic feet (17.02 m3) /s in September 1966
The river has two sources. The source of the North Branch is at the Fairfax Stone located at the junction of Grant, Tucker, and 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 at Little Falls, 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.
"Potomac" is a European spelling of Patowmeck, the Algonquian name of a Native American village on its southern bank. Native Americans had different names for different parts of the river, calling the river above Great Falls Cohongarooton, meaning "honking geese" and "Patawomke" below the Falls, meaning "river of swans". The spelling of the name has taken many forms over the years from "Patawomeck" (as on Captain John Smith's map) to "Patomake", "Patowmack", and numerous other spellings in the 18th century and now "Potomac". The river's name was officially decided upon as "Potomac" by the Board on Geographic Names in 1931.