The river has significant historical and political significance, as the nation's capital of Washington, D.C. is located on its banks, as isMount Vernon, the home ofGeorge Washington. During the American Civil War, the river became the boundary between theUnion and theConfederacy, and the Union's largest army, theArmy of the Potomac, was named after the river.
Once the Potomac drops from thePiedmont to theCoastal Plain at theAtlantic Seaboard fall line atLittle Falls,tides further influence the river as it passes through Washington, D.C., and beyond.Salinity in thePotomac River Estuary increases thereafter with distance downstream. Theestuary also widens, reaching 11 statute miles (17 km) wide at its mouth, between Point Lookout, Maryland, andSmith Point, Virginia, before flowing into the Chesapeake Bay.
The source of the North Branch Potomac River is at theFairfax Stone located at the junction ofGrant,Tucker andPreston counties in West Virginia. From the Fairfax Stone, the North Branch Potomac River flows 27 mi (43 km) to the man-madeJennings Randolph Lake, animpoundment designed for flood control and emergency water supply. Below the dam, the North Branch cuts a serpentine path through the eastern Allegheny Mountains. First, it flows northeast by the communities ofBloomington,Luke, andWesternport in Maryland and then on byKeyser, West Virginia toCumberland, Maryland. At Cumberland, the river turns southeast. 103 miles (166 km) downstream from its source,[4] the North Branch is joined by the South Branch betweenGreen Spring andSouth Branch Depot, West Virginia from whence it flows pastHancock, Maryland and turns southeast once more on its way towardWashington, D.C., and theChesapeake Bay.
The South Branch's source is northwest of Hightown alongU.S. Route 250 on the eastern side of Lantz Mountain (3,934 ft) in Highland County, Virginia. From Hightown, the South Branch is a small meanderingstream that flows northeast along Blue Grass Valley Road through the communities ofNew Hampden andBlue Grass. AtForks of Waters, the South Branch joins with Strait Creek and flows north across the Virginia/West Virginia border intoPendleton County.
The river then travels on a northeastern course along the western side of Jack Mountain (4,045 ft), followed by Sandy Ridge (2,297 ft) alongU.S. Route 220. North of the confluence of the South Branch with Smith Creek, the river flows along Town Mountain (2,848 ft) aroundFranklin at the junction of U.S. Route 220 andU.S. Route 33. After Franklin, the South Branch continues north through theMonongahela National Forest toUpper Tract where it joins with three sizeable streams: Reeds Creek, Mill Run, and Deer Run.
Between Big Mountain (2,582 ft) and Cave Mountain (2,821 ft), the South Branch bends around the Eagle Rock (1,483 ft) outcrop and continues its flow northward intoGrant County. Into Grant, the South Branch follows the western side of Cave Mountain through the 20-mile (32 km) longSmoke Hole Canyon, until its confluence with theNorth Fork atCabins, where it flows east toPetersburg. At Petersburg, theSouth Branch Valley Railroad begins, which parallels the river until its mouth atGreen Spring.
Canoeists atHanging Rocks on the South Branch in the 1890s
In its eastern course from Petersburg intoHardy County, the South Branch becomes more navigable allowing forcanoes and smaller river vessels. The river splits and forms a series of large islands while it heads northeast toMoorefield. At Moorefield, the South Branch is joined by theSouth Fork South Branch Potomac River and runs north toOld Fields where it is fed by Anderson Run and Stony Run.
AtMcNeill, the South Branch flows intothe Trough where it is bound to its west byMill Creek Mountain (2,119 ft) and to its east by Sawmill Ridge (1,644 ft). This area is the habitat tobald eagles. The Trough passes intoHampshire County and ends at its confluence with Sawmill Run south ofGlebe andSector.
Flowing north of Romney, the river still follows the eastern side of Mill Creek Mountain until it creates a horseshoe bend atWappocomo'sHanging Rocks around the George W. Washington plantation,Ridgedale. To the west ofThree Churches on the western side ofSouth Branch Mountain, 3,028 feet (923 m), the South Branch creates a series of bends and flows to the northeast bySpringfield through Blue's Ford. After two additional horseshoe bends (meanders), the South Branch flows under the oldBaltimore and Ohio Railroad mainline betweenGreen Spring andSouth Branch Depot, and joins the North Branch to form the Potomac.
The river itself is at least 3.5 million years old,[9] likely extending back ten to twenty million years before the present when the Atlantic Ocean lowered and exposed coastal sediments along the fall line. This included the area at Great Falls, which eroded into its present form during recent glaciation periods.[12]
Thestream gradient of the entire river is 0.14%, a drop of 930 m over 652 km.
"Potomac" is a European spelling ofPatawomeck, theAlgonquian name of a Native American village on its southern bank.[13] Native Americans had different names for different parts of the river, calling the river aboveGreat FallsCohongarooton, meaning "honking geese"[14][15] and "Patawomke" below the Falls, meaning "river of swans".[16] In 1608, Captain John Smith explored the river now known as the Potomac and made drawings of his observations which were later compiled into a map and published in London in 1612. This detail from that map shows his rendition of the river that the local tribes had told him was called the "Patawomeck". The spelling of the name has taken many forms over the years from "Patawomeck" (as onCaptain John Smith's map) to "Patomake", "Patowmack", and numerous other variations in the 18th century and now "Potomac".[15] The river's name was officially decided upon as "Potomac" by theBoard on Geographic Names in 1931.[17]
The similarity of the name to the Ancient Greek word for river,potamos, has been noted for more than two centuries but it appears to bedue to chance.[18][19][20]
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.
Civil War Era
Confederate troops crossing the fords of the Potomac in early September 1862 for the invasion of Maryland, which would culminate in theBattle of Antietam. (Print of a wood carving based on a drawing byThomas Nast; first published in the September 27, 1862, edition ofHarper's Weekly.)
Union defenses along the Potomac near Washington, DC Top row: Chain Bridge (two views) and Pimmit Run Bridge; Bottom Row: Aqueduct Bridget {two views) and Georgetown Ferry
Union soldiers manning the Lower Battery at the north end of Chain Bridge in 1862
Being situated in an area rich inAmerican history and American heritage has led to the Potomac being nicknamed "the Nation's River".George Washington, the firstPresident of the United States, was born in, surveyed, and spent most of his life within, the Potomac basin. There is an apocryphal legend that Washington threw a silver dollar all the way across the river as a youth (even though the first silver dollar wasn't minted until five years before Washington's death).[21] All of Washington, D.C., the nation'scapital city, also lies within the watershed. The First United States Congress by act of July 16, 1790 stated that the nation's capital was to be located on the river.[22] The 1859 siege ofHarper's Ferry at the river'sconfluence with theShenandoah was a precursor to numerous epic battles of theAmerican Civil War in and around the Potomac and its tributaries, such as the 1861Battle of Ball's Bluff and the 1862Battle of Shepherdstown.
GeneralRobert E. Lee crossed the river, thereby invading the North and threatening Washington, D.C., twice in campaigns climaxing in the battles ofAntietam (September 17, 1862) andGettysburg (July 1–3, 1863). Confederate GeneralJubal Early crossed the river in July 1864 on his attempted raid on the nation's capital. The river not only divided the Union from the Confederacy, but also gave name to the Union's largest army, theArmy of the Potomac.[23]
ThePatowmack Canal was intended by George Washington to connect theTidewater region nearGeorgetown withCumberland, Maryland. Started in 1785 on the Virginia side of the river, it was not completed until 1802. Financial troubles led to the closure of thecanal in 1830. TheChesapeake and Ohio Canal operated along the banks of the Potomac in Maryland from 1831 to 1924 and also connected Cumberland to Washington, D.C.[24] This allowed freight to be transported around therapids known as theGreat Falls of the Potomac River, as well as many other, smaller rapids.
An average of approximately 486 million US gallons (1,840,000 m3) of water iswithdrawn daily from the Potomac in the Washington area forwater supply, providing about 78 percent of the region's total water usage, this amount includes approximately 80 percent of the drinking water consumed by the region's estimated 6.1 million residents.[5][27]
The Potomac River surges over the deck ofChain Bridge during thehistoric 1936 flood. The bridge was so severely damaged by the raging water, and the debris it carried, that its superstructure had to be re-built; the new bridge was opened to traffic in 1939. (This photograph was taken from a vantage point onGlebe Road in Arlington County, Virginia. The houses on the bluffs in the background are located on thePotomac Palisades of Washington, DC.)
As a result of damaging floods in 1936 and 1937,[28] theArmy Corps of Engineers proposed thePotomac River basin reservoir projects, 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, just north of Washington, backing its pool up to Great Falls. Just above Great Falls, the much largerSeneca Dam was proposed whose reservoir would extend to Harpers Ferry.[29] Several other dams were proposed for the Potomac and its tributaries.
C&O Feeder Dam No. 7 and Guard Lock No. 7 were proposed to be located near milepost 164, close to the mouth of the South Branch of the Potomac, but were never built due to financial considerations.[32]
When detailed studies were issued by the Corps in the 1950s, they met sustained opposition, led byU.S. Supreme Court JusticeWilliam O. Douglas, resulting in the plans' abandonment.[33] The only dam project that did get built wasJennings Randolph Lake on the North Branch.[34]The Corps built a supplementary water intake for the Washington Aqueduct at Little Falls in 1959.[35]
In 1940Congress passed a law authorizing the creation of aninterstate compact to coordinate water quality management among states in the Potomac basin. Maryland, West Virginia, Pennsylvania, Virginia, and the District of Columbia agreed to establish theInterstate Commission on the Potomac River Basin. The compact was amended in 1970 to include coordination of water supply issues and land use issues related to water quality.[36]
Eutrophication in the Potomac River is evident from this bright green water in Washington, D.C., caused by a dense bloom ofcyanobacteria, April 2012.
Beginning in the 19th century, with increasingmining andagriculture upstream and urbansewage andrunoff downstream, thewater quality of the Potomac River deteriorated. This created conditions of severeeutrophication. It is said that PresidentAbraham Lincoln used to escape to the highlands on summer nights to escape the river's stench. In the 1960s, with dense greenalgal blooms covering the river's surface, PresidentLyndon Johnson declared the river "a national disgrace" and set in motion a long-term effort to reducepollution fromsewage and restore the beauty and ecology of this historic river. One of the significant pollution control projects at the time was the expansion of theBlue Plains Advanced Wastewater Treatment Plant, which serves Washington and several surrounding communities.[37] Enactment of the 1972Clean Water Act led to construction or expansion of additionalsewage treatment plants in the Potomac watershed. Controls onphosphorus, one of the principal contributors to eutrophication, were implemented in the 1980s, through sewage plant upgrades and restrictions on phosphorus in detergents.[36]
By the end of the 20th century, notable success had been achieved, as massive algal blooms vanished and recreational fishing and boating rebounded. Still, the aquatichabitat of the Potomac River and its tributaries remain vulnerable to eutrophication,heavy metals,pesticides and other toxic chemicals, over-fishing,alien species, andpathogens associated withfecal coliformbacteria andshellfish diseases. In 2005 two federal agencies, theUS Geological Survey and theFish and Wildlife Service, began to identify fish in the Potomac and tributaries that exhibited "intersex" characteristics, as a result ofendocrine disruption caused by some form of pollution.[38]
On November 13, 2007, the Potomac Conservancy, an environmental group, issued the river a grade of "D-plus", citing high levels of pollution and the reports of "intersex" fish.[39] Since then, the river has improved with a reduction in nutrient runoff, return of fish populations, and land protection along the river. As a result, the group has issued a grade of "B" since 2018.[40][41] In March 2019, thePotomac Riverkeeper Network launched a laboratory boat dubbed the "Sea Dog", which will be monitoring water quality in the Potomac and providing reports to the public on a weekly basis;[42] in that same month, the catching nearFletcher's Boat House of aStriped Bass estimated to weigh 35 lb (16 kg) was seen as a further indicator of the continuing improvement in the health of the river.[43]
Top Ten Historic Crests of the Potomac River, 1877–2017
This chart displays the Annual Mean Discharge of the Potomac River measured at Little Falls, Maryland for Water Years 1931–2017 (in cubic feet per second). Source of data: USGS.[2]
The average daily flow during thewater years 1931–2018 was 11,498 cubic feet (325.6 m3) /s.[2] The highest average daily flow ever recorded on the Potomac atLittle Falls, Maryland (near Washington, D.C.), was in March 1936 when it reached 426,000 cubic feet (12,100 m3) /s.[2] The lowest average daily flow ever recorded at the same location was 601.0 cubic feet (17.02 m3) /s in September 1966[2] The highest crest of the Potomac ever registered at Little Falls was 28.10 ft, on March 19, 1936;[44][28]however, the most damaging flood to affect Washington, DC and its metropolitan area was that of October 1942.[45]
Boundary between Maryland, Virginia, and West Virginia at Harpers Ferry
Satellite view of the Potomac River passing through two water gaps downstream of Harpers Ferry
For 400 years Maryland and Virginia have disputed control of the Potomac and its North Branch since both states' originalcolonial charters grant the entire river rather than half of it as is normally the case with boundary rivers. In its first stateconstitution 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 1785Mount Vernon Compact and the 1877 Black-Jenkins Award which granted Maryland the river bank-to-bank from the low-water mark on the Virginia side while permitting Virginia fullriparian rights short of obstructing navigation.
From 1957 to 1996, theMaryland Department of the Environment (MDE) routinely issued permits applied for by Virginia entities concerning the use of the Potomac. However, in 1996 the MDE denied a permit submitted by theFairfax County Water Authority to build a water intake 725 feet (221 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 theSupreme 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). ASpecial 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.[46]
Map of land use in the watershed
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 Virginiaseceded 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 ofMineral andGrant Counties and parts ofHampshire,Hardy,Tucker andPendleton Counties) and by West Virginia to the Potomac's high-water mark were rejected by the Supreme Court in two separate decisions in 1910.[47][48]
After having been depressed for many decades, the river's population ofAmerican shad is currently re-bounding as a result of theICPRB's successful "American Shad Restoration Project" that was begun in 1995. In addition to stocking the river with more than 22 million shad fry, the Project supervised the construction of a fishway that was built to facilitate the passage of adults around the Little Falls Dam on the way to their traditional spawning grounds upstream.[53]
Several hundred bottle-nosed dolphins live six months of the year (from mid-April through mid-October) in the Potomac. Depicted here, a mother with her young.
Early European colonists who settled along the Potomac found a diversity of large and small mammals living in the dense forests nearby.Bison,elk, wolves (bothgray andred) andcougars were still present at that time, but had been hunted to extirpation by the middle of the 19th century. Among the denizens of the Potomac's banks, beavers and otters met a similar fate, while small populations of American mink and American martens survived into the 20th century in some secluded areas.
There is no record of early settlers having observed marine mammals in the Potomac, but several sightings ofAtlantic bottle-nosed dolphins (Tursiops truncatus) were reported during the 19th century. In July 1844, a pod of 14 adults and young was followed up the river by men in boats as high as theAqueduct Bridge (approximately the same location occupied byKey Bridge today).[54]
Since 2015, perhaps as a result of warmer temperatures, rising water levels in the Chesapeake Bay and improving water quality in the Potomac, unprecedented numbers of Atlantic bottle-nosed dolphins have been observed in the river. According to Dr. Janet Mann of Georgetown University'sPotomac-Chesapeake Dolphin Project, more than 500 individual members of the species have been identified in the Potomac during this period.[55]
Sources: http://dnr.maryland.gov/wildlife/Documents/herpchecklist.pdf A Guide to the Snakes of Virginia (Virginia Department of Game and Inland Fisheries, Wildlife Diversity Division, Special Publication No. 2.1) 2002; by Michael J Pinder (Author)
2025 Potomac River mid-air collision, an American Eagle Bombardier CRJ701ER collided with a Sikorsky VH-60M Black Hawk, both crashed into Potomac River on January 29, 2025.
^AQU: The diversion dam at Great Falls, often called the"Aqueduct Dam", was built in the 1850s by the US Army Corps of Engineers as part of the project assigned to them by Congress to supply clean water from above Great Falls to Washington, DC. Water diverted by the dam flows 12 miles through a 9-foot diameter pipeline to Dalecarlia Reservoir on the outskirts of the city where it is first allowed to settle and then filtered and purified before being distributed to consumers. Since 1927, potable water from Dalecarlia has also been provided to Arlington County and some other sections of nearby northern Virginia through three 20-inch diameter pipelines that cross the Potomac under the deck of Chain Bridge. In addition, there is nearby a 4-foot diameter conduit constructed in 1967 that traverses the Potomac beneath the riverbed which is used primarily for backup purposes.[56][57]
^GHL: "Evidence of the ancient Potomac River bed can be seen in well-rounded boulders, smoothed surfaces and grooves, and beautifully formed potholes. Look for sandstone boulders along the trail, which were deposited by massive floods. The sandy soils along the river trail, with shells mixed in, are a result of sediment deposits from floods. Some of the oldest sediment deposits in the area can be found onGlade Hill, between the Matildaville and Carriage Road trails.Glade Hill was once an island in the Potomac River, and the deposits found there were left before Mather Gorge formed."[58]
^PIF: "In the Late Pennsylvanian, the rocks of the Stubblefield Falls domain of the Mather Gorge Formation moved up relative to the Sykesville Formation on the steep, west-dipping Plummers Island fault and mylonite zones (Schoenborn, 2001) within an existing Plummers Island shear zone (figs. 5, 6). Shearing formed S2 cleavage with below-closure muscovite growth and more pervasive S2 cleavage in the Sykesville Formation. By the earliest Permian, all of the rocks in the Potomac terrane had cooled through 235°C (figs. 3, 5). Apatite fission-track data indicate cooling through ≈90°C to 100°C in Early Jurassic to Early Cretaceous time, with increasing ages to the east, suggesting kilometer-scale rotation of the Potomac terrane in the Cretaceous and (or) Tertiary, with the west side up."[59]
^BLK: "Two samples collected from the terrace dissected by Great Falls indicate that the Falls were established in their current location by 30 ky. A series of 6 samples taken from a vertical transect just below the falls, indicates that vertical incision continued a rate of 0.5 m/ky between 27 and 12 ky, increasing to nearly 1.0 m/ky during the Holocene. These data suggest that the drop over Great Falls is growing with time. A dramatic increase in outcrop weathering and soil depth 3.5 km downstream of the Falls, suggests that prior to establishment of the Great Falls knickzone, a similar feature was likely present near Black Pond. 10-Be data are not yet available for this paleo knick zone; however, a 10-Be model age >200 ky from the top of Plummers island 5 km down stream of Black Pond suggests a much older period of retreat led to the formation of the Black Pond paleo knick zone."[60]
^PES: "The Potomac Estuary: From the Chain Bridge in Washington, DC, to Point Lookout at the confluence with the Chesapeake Bay, the Potomac Estuary is a long and narrow estuary—approximately 189 km. With its many tributaries and bays, however, the Potomac Estuary has a shoreline of 1,800 km. The Estuary meanders in a south, southeasterly direction, except for a sharp bend about halfway downriver. The Estuary has three well-defined and distinct zones. The upper zone, from Chain Bridge to Indian Head, is the tidal freshwater reach, with salinities of less than 0.5 parts per thousand (ppt). The middle reach, between Indian Head and the Route 301 Bridge at Morgantown, is the transition zone. The salinity of this zone varies from 0.5 to 7.0 ppt and is often referred to as the zone of maximum turbidity. The lower zone, from the 301 Bridge to Point Lookout, has salinities ranging from 7 to 16 ppt."[61]
^TRI: The rocky western (upriver) and central portions of the island are part of the Piedmont Plateau, while the southeastern part is within the Atlantic Coastal Plain. At one point opposite Georgetown, the Atlantic Seaboard fall line between the Piedmont and the Coastal Plain can be seen as a natural phenomenon. The island has about 2.5-mile (4.0 km) of shoreline, and the highest area of the island (where the Mason mansion stood) is about 44 feet (13 m) above sea level.
^abcU.S. Geological Survey. National Hydrography Dataset high-resolution flowline data.The National MapArchived March 29, 2012, at theWayback Machine. Retrieved August 15, 2011
^ab"Facts & FAQs". Interstate Commission on the Potomac River Basin (ICPRB), Rockville, MD. September 16, 2009. Archived fromthe original on January 15, 2010. RetrievedFebruary 5, 2010.
^"POTOMAC BASIN FACTS".Interstate Commission on the Potomac River Basin. RetrievedAugust 16, 2024.
^"Potomac Riverkeeper Network".www.potomacriverkeepernetwork.org. Potomac Riverkeeper Network. 2019.Archived from the original on November 19, 2020. RetrievedMarch 25, 2019.
^"Potomac River Basin Fact Sheet"(PDF).www.potomacriver.org. Interstate Commission on the Potomac River Basin (ICPRB). October 2015.Archived(PDF) from the original on December 16, 2017. RetrievedMarch 28, 2019.
^Bugbee, Mary F. "The Early Planning of Sites for Federal and Local Use in Washington, D. C."Records of the Columbia Historical Society, Washington, D.C., vol. 51/52, 1951, p. 19. JSTOR,http://www.jstor.org/stable/40067294. Retrieved 19 Feb. 2024.
^Peck, Garrett (2012).The Potomac River: A History and Guide. Charleston, SC: The History Press. p. 18.ISBN978-1-60949-600-5.
^Hahn, Thomas (1984).The Chesapeake & Ohio Canal: Pathway to the Nation's Capital. Metuchen, NJ: Scarecrow Press.ISBN0-8108-1732-2.
^Ways, Harry C. (1996).The Washington Aqueduct: 1852-1992. (Baltimore, MD: U.S. Army Corps of Engineers, Baltimore District)
^Holdsworth, Bill (April 2013)."Level 51 (Dam #6)".candocanal.org. C&O Canal Association.Archived from the original on June 23, 2016. RetrievedMarch 16, 2019.
^Joel Achenbach (May 5, 2002)."America's River".The Washington Post. pp. W12. Archived fromthe original on September 16, 2002.
^"Jennings Randolph Lake, MD & WV"(PDF).www.nab.usace.army.mil. USACE (United States Corps of Engineers). February 2015.Archived(PDF) from the original on September 28, 2020. RetrievedApril 5, 2019.
^Fahrenthold, David A. (November 13, 2007)."Potomac Recovery Deemed At Risk".The Washington Post.Archived from the original on July 1, 2011. RetrievedNovember 13, 2007.
^"Potomac Report Card". Potomac Conservancy. March 28, 2018.Archived from the original on November 12, 2020. RetrievedMarch 26, 2019.
^Paul Bierman, et al.,Great Falls is 30,000 Years OldArchived September 7, 2008, at theWayback Machine, Paper No. 35-5, Session No. 35, Geomorphic Process Rates on the Passive Margin, March 26, 2004. Geological Society of AmericaAbstracts with Programs, Vol. 36, No. 2, p. 94
Rice, James D.,Nature and History in the Potomac Country: From Hunter-Gatherers to the Age of Jefferson. (2009), Baltimore: Johns Hopkins University Press;ISBN0-8018-9032-2;ISBN978-0-8018-9032-1
Smith, J. Lawrence,The Potomac Naturalist: The Natural History of the Headwaters of the Historic Potomac (1968), Parsons, WV: McClain Printing Co.;ISBN0-87012-023-9;ISBN978-0-87012-023-7
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