The river forms part of the borders between Maryland and Washington, D.C. on the left bank and West Virginia and Virginia on the river's right 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 (38,018 km²). The average flow is 10,800 ft³/s (306 m³/s). 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). The lowest flow ever recorded at the same location was 600 ft³/s (17 m³/s) in September 1966.
An average of approximately 486 million gallons of water per day (21 m³/s) is withdrawn daily in the Washington area for water supply.
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.
"Potomac" is a European spelling of an Algonquian name for a tribe subject to the Powhatan confederacy, that inhabited the upper reaches of the Northern Neck in the vicinity of Fredericksburg. Some accounts say the name means "place where people trade" or "the place to which tribute is brought". The natives called the river above the falls Cohongarooton, translated as "river of geese", and that area was renowned in early years for an abundance of both geese and swans. The spelling of the name has been simplified over the years from "Patawomeke" (as on Captain John Smith's map) to "Patowmack" in the 18th century and now "Potomac". Some scholars have also suggested it is rooted in the ancient Greek for river, "potamos", blended with the Powhatan name "Patawomeke". The river's name was officially decided upon as Potomac by the Board on Geographic Names in 1931.
The Potomac River brings together a variety of cultures throughout the watershed from the coal miners of upstream West Virginia to the urban residents of the nation's capital and, along the lower Potomac, the watermen of Virginia's Northern Neck.
Being situated in an area rich in American history and American heritage has led to the Potomac being nicknamed "the Nation's River." George Washington, the first President of the United States, was born in, surveyed, and spent most of his life within the Potomac basin. All of Washington, D.C., the nation's capital city, also lies within the watershed. The 1859 siege of Harper's Ferry at the river's confluence with the Shenandoah was a precursor to numerous epic battles of the American Civil War in and around the Potomac and its tributaries. General Robert E. Lee crossed the river, thereby invading the North and threatening Washington, D.C. twice in campaigns climaxing in the battles of Antietam and Gettysburg.
The Patowmack Canal was intended by George Washington to connect the Tidewater near Georgetown with Cumberland, Maryland. Started in 1785, it was not completed until 1802. Financial troubles closed the canal in 1830. The Chesapeake and Ohio Canal operated along the banks of the Potomac in Maryland from 1850 to 1924 and also connected Cumberland to Washington, D.C. This allowed freight to be transported around the rapids known as the Great Falls of the Potomac River, as well as many other, smaller rapids.
With increasing mining and agriculture upstream and urban sewage and runoff downstream, the water quality of the Potomac River deteriorated. This created conditions of severe eutrophication. It is said that President Abraham Lincoln used to escape to the highlands on summer nights to escape the river's stench. In the 1960s, with dense green algal blooms covering the river's surface, President Lyndon Johnson declared the river "a national disgrace" and set in motion a long-term effort to reduce sewage pollution and restore the beauty and ecology of this historic river. By the end of the 20th century, there was notable success, as massive algal blooms vanished and recreational fishing and boating rebounded. Still, the aquatic habitat of the Potomac River and its tributaries remain vulnerable to eutrophication, heavy metals, pesticides and other toxic chemicals, over-fishing, alien species, and pathogens associated with fecal coliform bacteria and shellfish diseases. On November 13, 2007, environmental group the Potomac Conservancy issued the river a grade of "D-plus", citing high levels of pollution and reports of "intersex" fish.
The Potomac was designated as one of the American Heritage Rivers in 1997.
For 400 years Maryland and Virginia have disputed control of the Potomac and its North Branch, since both states' original colonial charters grant the entire river rather than half of it as is normally the case with boundary rivers. In its first state constitution adopted in 1776, Virginia ceded its claim to the entire river but reserved free use of it, an act disputed by Maryland. Both states acceded to the Compact of 1785 and the 1877 Black-Jenkins Award which grants Maryland the river bank-to-bank from the low water mark on the Virginia side, while permitting Virginia full riparian rights short of obstructing navigation.
From 1957 to 1996, the Maryland Department of the Environment (MDE) routinely issued permits applied for by Virginia entities concerning use of the Potomac, however, in 1996 the MDE denied a permit applied for by the Fairfax County Water Authority to build a water intake valve 725 feet (220 m) offshore, citing potential harm to Maryland's interests by an increase in Virginia sprawl caused by the project. After years of failed appeals within the Maryland government's appeal processes, in 2000 Virginia took the case to the Supreme Court of the United States, which exercises original jurisdiction in cases between two states. Maryland claimed Virginia lost its riparian rights by acquiescing to MDE's permit process for 63 years (MDE began its permit process in 1933). A Special Master appointed by the Supreme Court to investigate recommended the case be settled in favor of Virginia, citing the language in the 1785 Compact and the 1877 Award. On December 9, 2003, the Court agreed in a 7-2 decision. Virginia v. Maryland, 124 S.Ct. 598.
The original charters are silent as to which branch from the upper Potomac serves as the boundary, but this was settled by the 1785 Compact. When West Virginia seceded from Virginia in 1863, the question of West Virginia's succession in title to the lands between the branches of the river was raised, as well as title to the river itself. Claims by Maryland to West Virginia land north of the South Branch (all of Mineral and Grant Counties and parts of Hampshire, Hardy, Tucker and Pendleton Counties) and by West Virginia to the Potomac's high water mark were rejected by the Supreme Court in two separate decisions in 1910. State of Md. v. State of W.Va., 217 U.S. 1 State of Md. v. State of W.Va., 217 U.S. 577
As a result of the damaging floods of 1936 and 1937, the Army Corps of Engineers proposed a series of dams that were intended to regulate the river and to provide a more reliable water supply. One dam was to be built at Little Falls, backing its pool up to Great Falls. Just above Great Falls, a much larger dam was proposed whose reservoir would extend to Harpers Ferry. Several other dams were proposed on the Potomac and its tributaries. When detailed studies were issued by the Army Corps of Engineers in the 1950s, they met sustained opposition, led by US Supreme Court Chief Justice William O. Douglas, resulting in the plans' abandonment. The only project that did get built was Jennings Randolph Lake on the North Branch.
From the Fairfax Stone, the North Branch Potomac River flows 27 miles to the man-made Jennings Randolph Lake, an impoundment designed for flood control. Below the dam, the North Branch cuts a serpentine path through the eastern Allegheny Mountains. First, it flows northeast by the communities of Bloomington, Luke, and Westernport in Maryland and then on by Keyser, West Virginia to Cumberland, Maryland. At Cumberland, the river turns southeast. It is joined by the South Branch between Green Spring and South Branch Depot, West Virginia from whence it flows past Hancock, Maryland and turns southeast once more on its way toward Washington, D.C., and the Chesapeake Bay.
The following are tributaries of the North Branch Potomac River, listed in order from the source to its mouth.
The South Branch Potomac River has its headwaters in northwestern Highland County, Virginia near Hightown along the eastern edge of the Allegheny Front. The mouth of the South Branch lies east of Green Spring in Hampshire County, West Virginia where it meets the North Branch Potomac River to form the Potomac. A topographic map of the confluence of the North and South Branches can be viewed here
Places settled in the South Branch valley bearing variants of "Wappatomaka" include Wappacoma plantation built in 1773 and the unincorporated hamlet of Wappocomo (sometimes spelled Wapocomo) at Hanging Rocks, both north of Romney on West Virginia Route 28.
In its eastern course from Petersburg into Hardy County, the South Branch becomes more navigable allowing for canoes and smaller river vessels. The river splits and forms a series of large islands while it heads northeast to Moorefield. At Moorefield, the South Branch is joined by the South Fork South Branch Potomac River and runs north to Old Fields where it is fed by Anderson Run and Stony Run. At McNeill, the South Branch flows into the Trough where it is bound to its west by Mill Creek Mountain (2,119 ft) and to its east by Sawmill Ridge (1,644 ft). This area is the habitat to endangered bald eagles. The Trough passes into Hampshire County and ends at its confluence with Sawmill Run south of Glebe and Sector. The South Branch continues north parallel to South Branch River Road (County Route 8) toward Romney with a number of historic plantation farms adjoining it. En route to Romney, the river is fed by Buffalo Run, Mill Run, McDowell Run, and Mill Creek at Vanderlip. The South Branch is traversed by the Northwestern Turnpike (U.S. Route 50) and joined by Sulphur Spring Run where it forms Valley View Island to the west of town. Flowing north of Romney, the river still follows the eastern side of Mill Creek Mountain until it creates a horseshoe bend at Wappocomo's Hanging Rocks around the George W. Washington plantation, Ridgedale. To the west of Three Churches on the western side of South Branch Mountain, 3,028 feet (923 m), the South Branch creates a series of bends and flows to the northeast by Springfield through Blue's Ford. After another horseshoe bend, the South Branch flows under the old Baltimore and Ohio Railroad mainline between Green Spring and South Branch Depot, and joins the North Branch to form the Potomac.
The North Fork South Branch Potomac River forms just north of the Virginia/West Virginia border in Pendleton County at the confluence of the Laurel Fork and Straight Fork along Big Mountain (3,881 ft). From Circleville, the North Fork flows northeast through Pendleton County between the Fore Knobs (2,949 ft) to its west and the River Knobs, 2,490 feet (759 m) to its east. At Seneca Rocks, the North Fork is met by Seneca Creek. From Seneca Rocks, the North Fork continues to flow northeast along the western edge of North Fork Mountain 3,389 feet (1033 m) into Grant County. One of the most beautiful views in all of West Virginia is looking up at the cliffs of North Fork Mountain (about 100 yards south of Smoke Hole Road, south of Cabins, WV) called "Shelby's Cliffs" by the locals. Flowing east through North Fork Gap, the North Fork joins the South Branch Potomac at the town of Cabins, west of Petersburg.
The Tidal or Lower Potomac River lies below the fall line. This stretch encompasses the Potomac from about one mile (2 km) below the Washington, DC - Montgomery County, MD line, just below the Little Falls of the Potomac River where the tidal river begins, to the Chesapeake Bay.