Potomac River

The Potomac River (/pəˈtmək/ (listen)) is found 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,[4] with a drainage area of about 14,700 square miles (38,000 km2).[5] In terms of area, this makes the Potomac River the fourth largest river along the East Coast of the United States and the 21st largest in the United States. Over 5 million people live within the Potomac watershed.

Potomac River
The Potomac River watershed covers the District of Columbia and parts of four states
Native namePatawomeck
CountryUnited States
StateWest Virginia, Maryland, Virginia, District of Columbia
CitiesCumberland, MD, Harpers Ferry, WV, Washington, D.C., Alexandria, VA
Physical characteristics
SourceFairfax Stone
  locationPreston County, West Virginia, United States
  coordinates39°11′43″N 79°29′28″W
  elevation3,060 ft (930 m)
MouthChesapeake Bay
St. Mary's County, Maryland/Northumberland County, Virginia, United States
37°59′57″N 76°14′59″W
0 ft (0 m)
Length405 mi (652 km)
Basin size14,700 sq mi (38,000 km2)
  locationLittle Falls, near Washington, D.C. (non-tidal; water years: 1931–2018)[2]
  average11,498 cu ft/s (325.6 m3/s) (1931–2018)
  minimum4,017 cu ft/s (113.7 m3/s) (2002)
  maximum23,760 cu ft/s (673 m3/s) (1996)
  locationPoint of Rocks, Maryland
  average9,504 cu ft/s (269.1 m3/s)
  locationHancock, Maryland
  average4,168 cu ft/s (118.0 m3/s)
  locationPaw Paw, West Virginia
  average3,376 cu ft/s (95.6 m3/s)
Basin features
  leftConococheague Creek, Antietam Creek, Monocacy River, Rock Creek, Anacostia River
  rightCacapon River, Shenandoah River, Goose Creek, Occoquan River, Wicomico River
Note: Since 1996, the Potomac has been the 'sister river' of the Ara River of Tokyo, Japan[3]

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 in Washington, D.C., with Arlington Memorial Bridge in the foreground and Rosslyn, Arlington, Virginia in the background

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).[4] The average daily flow during the water years 1931–2018 was 11,498 cubic feet (325.6 m3) /s.[2] The highest average daily flow ever recorded on the Potomac at Little 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;[6][7] however, the most damaging flood to affect Washington, DC and its metropolitan area was that of October 1942.[8]

Map showing the five geological provinces through which the Potomac River flows [9]
Major sub-basins and cities of the Potomac River basin[10]

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. As it flows from its headwaters down to the Chesapeake Bay, the Potomac traverses five geological provinces: the Appalachian Plateau, the Ridge and Valley, the Blue Ridge, the Piedmont Plateau, and the Atlantic coastal plain.

Once the Potomac drops from the Piedmont to the Coastal Plain at the Atlantic Seaboard fall line 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.



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".

"Potomac" is a European spelling of Patawomeck, the Algonquian name of a Native American village on its southern bank.[11] Native Americans had different names for different parts of the river, calling the river above Great Falls Cohongarooton, meaning "honking geese"[12][13] and "Patawomke" below the Falls, meaning "river of swans".[14] 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 variations in the 18th century and now "Potomac".[13] The river's name was officially decided upon as "Potomac" by the Board on Geographic Names in 1931.[15]

Tundra swans were the predominant species of swan on the Potomac River when the Algonquian tribes dwelled along its shores, and continue to be the most populous variety today.[16]
The similarity of the name to the Ancient Greek word for river, potamos, has been noted for more than two centuries but it appears due to chance.[17][18][19]

The river itself is at least 3.5 million years old,[9] likely extending back ten to twenty million years before 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.[20]

Major Tributaries of the Potomac River and Area Drained
Name of Tributary Drainage Area (km2; mi2)
Upper Basin
North Branch2,266 km (1,408 mi)
South Branch3,756 km (2,334 mi)
Cacapon River677 km (421 mi)
Conococheague Creek469 km (291 mi)
Antietam Creek283 km (176 mi)
Opequon Creek309 km (192 mi)
Shenandoah River3,012 km (1,872 mi)
Monocacy River813 km (505 mi)
Lower Basin
Rock Creek210 km (130 mi)
Anacostia River418 km (260 mi)
Piscataway Creek163 km (101 mi)
Occoquan River1,614 km (1,003 mi)
Mattawoman Creek203 km (126 mi)
Wicomico River467 km (290 mi)
Based on data provided by the ICPRB

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.

View of the Potomac River from George Washington's birthplace in Westmoreland County, Virginia
Sunset over the Potomac near Mount Vernon
Civil War Era
Confederate troops crossing the fords of the Potomac in early September 1862 for the invasion of Maryland, which would culminate in the Battle of Antietam. (Print of a wood carving based on a drawing by Thomas Nast; first published in the September 27, 1862 edition of Harper'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 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, such as the 1861 Battle of Ball's Bluff and the 1862 Battle of Shepherdstown.
Map of the Potomac River and its environs circa 1862 by Robert Knox Sneden.
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 (September 17, 1862) and Gettysburg (July 1–3, 1863). Confederate General Jubal 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, the Army of the Potomac.[21] The Patowmack Canal was intended by George Washington to connect the Tidewater region near Georgetown with Cumberland, Maryland. Started in 1785 on the Virginia side of the river, it was not completed until 1802. Financial troubles led to the closure of the canal in 1830. The Chesapeake and Ohio Canal operated along the banks of the Potomac in Maryland from 1831 to 1924 and also connected Cumberland to Washington, D.C.[22] 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.

Washington, D.C. began using the Potomac as its principal source of drinking water with the opening of the Washington Aqueduct in 1864, using a water intake constructed at Great Falls.[23][24]

Water supply and water quality

An average of approximately 486 million US gallons (1,840,000 m3) of water is withdrawn daily from the Potomac in the Washington area for water 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][25]

The Potomac River surges over the deck of Chain Bridge during the historic 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 on Glebe Road in Arlington County, Virginia. The houses on the bluffs in the background are located on the Potomac Palisades of Washington, DC.)

As a result of damaging floods in 1936 and 1937,[7] the Army Corps of Engineers proposed the Potomac 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 larger Seneca Dam was proposed whose reservoir would extend to Harpers Ferry.[26] Several other dams were proposed for the Potomac and its tributaries.

When detailed studies were issued by the Corps in the 1950s, they met sustained opposition, led by U.S. Supreme Court Justice William O. Douglas, resulting in the plans' abandonment.[30] The only dam project that did get built was Jennings Randolph Lake on the North Branch.[31] The Corps built a supplementary water intake for the Washington Aqueduct at Little Falls in 1959.[32]

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]
Source of data: USGS[33]

In 1940 Congress passed a law authorizing creation of an interstate 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 the Interstate 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.[34]

Eutrophication in the Potomac River is evident from this bright green water in Washington, D.C., caused by a dense bloom of cyanobacteria, April 2012

Beginning in the 19th century, 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 pollution from sewage and restore the beauty and ecology of this historic river. One of significant pollution control projects at the time was the expansion of the Blue Plains Advanced Wastewater Treatment Plant, which serves Washington and several surrounding communities.[35] Enactment of the 1972 Clean Water Act led to construction or expansion of additional sewage treatment plants in the Potomac watershed. Controls on phosphorus, one of the principal contributors to eutrophication, were implemented in the 1980s, through sewage plant upgrades and restrictions on phosphorus in detergents.[34]

Monthly Mean Data for Water Years 1930 - 2018
Source of data: USGS[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 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. In 2005 two federal agencies, the US Geological Survey and the Fish and Wildlife Service, began to identify fish in the Potomac and tributaries that exhibited "intersex" characteristics, as a result of endocrine disruption caused by some form of pollution.[37]

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.[38] 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 same group issued a grade of "B" for 2017 and 2018.[39] In March 2019, the Potomac 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;[40] in that same month, the catching near Fletcher's Boat House of a Striped Bass estimated to weigh 35 lbs was seen as a further indicator of the continuing improvement in the health of the river.[41]

Top Ten Historic Crests of the Potomac River, 1877–2017
Kitzmiller Hancock Williamsport Shepherdstown
Harpers Ferry Point of Rocks Little Falls Georgetown
Source: National Weather Service
This westward-looking aerial photograph shows the Shenandoah River (left) flowing into the Potomac River (right) at Harpers Ferry, WV. The Potomac then continues eastward toward the Chesapeake Bay. (Visible in the foreground are the ruins of the famed B&O Bridge which was destroyed nine times during the Civil War -- four times by military action and five times by floods.)
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' 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 1765 Mount 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 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 submitted by the Fairfax County Water Authority to build a water intake 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.[42]

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 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.[43][44]

Flora of the Potomac River Basin

Fauna of the Potomac River and its Basin


After an absence lasting many decades, the American Shad has recently returned to the Potomac.

A variety of fish inhabit the Potomac, including bass, muskellunge, pike, walleye. The northern snakehead, an invasive species resembling the native bowfin, lamprey, and American eel, was first seen in 2004.[45][46] Many types of sunfish are also present in the Potomac and its headwaters.[47] Although rare, bull sharks can be found.[48]

After having been depressed for many decades, the river's population of American Shad is currently re-bounding as a result of the ICPRB'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 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.[49]


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 ( gray and red) and panthers (cougars) 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 minks and 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 of Atlantic 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 the Aqueduct Bridge (approximately the same location occupied by Key Bridge today).[50]

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's Potomac-Chesapeake Dolphin Project, more than 500 individual members of the species have been identified in the Potomac during this period.[51]


A Great Blue Heron
Birds of the Potomac River Basin


Eastern Box Turtles are frequently spotted along the towpath of the C&O Canal.
Five-lined skink, juvenile


Eastern Tiger Salamanders (Ambystoma tigrinum) can be found sheltering under rocks along the banks of the Potomac.

The Potomac River System

North Branch Potomac River

The North Branch between Cumberland, Maryland, and Ridgeley, West Virginia, in 2007

The source of the North Branch Potomac River is at the Fairfax Stone located at the junction of Grant, Tucker and Preston counties in West Virginia.

Confluence of the North and South Branches of the Potomac River near Potomac Forks Campsite (southeast of Cumberland), Allegany County, Maryland

From the Fairfax Stone, the North Branch Potomac River flows 27 miles (43 km) to the man-made Jennings Randolph Lake, an impoundment 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 of Bloomington, Luke, and Westernport in Maryland and then on by Keyser, West Virginia to Cumberland, 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 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 table shows the major tributaries of the North Branch Potomac River, listed in order from the source to its mouth. Numerous other tributary creeks exist.

South Branch Potomac River

The South Branch Potomac River has its headwaters in northwestern Highland County, Virginia near Hightown along the eastern edge of the Allegheny Front. After a river distance of 139 miles (224 km),[4] 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.[52]

The North Fork South Branch Potomac River, 43.6 miles (70.2 km) long,[4] 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 feet (1,183 m). From Circleville, the North Fork flows northeast through Pendleton County between the Fore Knobs 2,949 feet (899 m) 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. Flowing east through North Fork Gap, the North Fork joins the South Branch Potomac at the town of Cabins, west of Petersburg.

The South Fork South Branch Potomac River forms just north of U.S. Route 250 in Highland County, Virginia near Monterey, and flows 68.4 miles (110.1 km)[4] north-northeastward to the South Branch Potomac River at Moorefield in Hardy County, West Virginia. From 1896 to 1929, it was named the Moorefield River by the Board on Geographic Names to avoid confusion with the South Branch.

Upper Potomac River

This stretch encompasses the section of the Potomac River from the confluence of its North and South Branches through Opequon Creek near Shepherdstown, West Virginia.[53]

Lower Potomac River

Sunrise view from Jefferson Rock at Harpers Ferry, WV. (In the distance is the Sandy Hook Bridge over the Potomac River that connects Maryland (left bank) with Virginia along U.S. Highway 340.)

This section covers the Potomac from just above Harpers Ferry in West Virginia down to Little Falls, Maryland on the border between Maryland and Washington, DC.

Tidal Potomac River

View southwest across the tidal Potomac River from the south end of Cobb Island Road on Cobb Island, Charles County, Maryland

The Tidal Potomac River lies below the Fall Line. This 108-mile (174-km) stretch encompasses the Potomac from a short distance below the Washington, DC - Montgomery County line, just downstream of the Little Falls of the Potomac River, to the Chesapeake Bay.[54]


Additional images

See also

Notes and references


  • ^ 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.[55][56]
  • ^ 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 on Glade 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."[57]
  • ^ 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."[58]
  • ^ 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."[59]
  • ^ 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."[60]
  • ^ 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.


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  3. "(Arakawa - Potomac sister rivers)". Interstate Commission on the Potomac River Basin. January 27, 2012. Archived from the original on December 27, 2013. Retrieved September 23, 2016.
  4. U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National Map Archived March 29, 2012, at the Wayback Machine. Retrieved August 15, 2011
  5. "Facts & FAQs". Interstate Commission on the Potomac River Basin (ICPRB), Rockville, MD. September 16, 2009. Archived from the original on January 15, 2010. Retrieved February 5, 2010.
  6. "Historic Crests for Potomac near Washington, DC (Little Falls)". water.weather.gov/ahps/. National Weather Service (NOAA). 2019. Retrieved March 23, 2019.
  7. "1936 Flood Retrospective: The Flood of March 17-19 1936". weather.gov. NWS. March 16, 2016. Retrieved March 26, 2019.
  8. Little, Becky (September 14, 2018). "World War II-Era Flood Was the Worst in D.C.'s History". www.history.com. A&E Television Networks, LLC. Retrieved March 23, 2019.
  9. "Geology of Potomac Heritage National Scenic Trail". Potomac Heritage. NPS. 2019. Retrieved March 25, 2019.
  10. Jaworski, Norbert A. (July 20, 2015). "The Potomac River Basin and its Estuary: Landscape Loadings and Water Quality Trends, 1895 – 2005" (PDF). www.potomacriver.org. Interstate Commission on the Potomac River Basin(ICPRB). Retrieved March 30, 2019.
  11. Bright, William (2004). Native American Placenames of the United States. University of Oklahoma Press. p. 396. ISBN 978-0-8061-3598-4. Archived from the original on May 11, 2016.
  12. Legends of Loudoun: An account of the history and homes of a border county of Virginia's Northern Neck, Harrison Williams, p. 26.
  13. Achenbach, Joel (2004). The Grand Idea: George Washington's Potomac and the Race to the West. Simon and Schuster. pp. 35–36. ISBN 978-0-684-84857-0.
  14. Hagemann, James A. (1988). The Heritage of Virginia. The Donning Company, 2nd edition, 297 p. ISBN 0-89865-255-3.
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  60. Chapter One: Introduction

Works cited

  • Rice, James D., Nature and History in the Potomac Country: From Hunter-Gatherers to the Age of Jefferson. (2009), Baltimore: Johns Hopkins University Press; ISBN 0-8018-9032-2; ISBN 978-0-8018-9032-1
  • Smith, J. Lawrence, The Potomac Naturalist: The Natural History of the Headwaters of the Historic Potomac (1968), Parsons, WV: McClain Printing Co.; ISBN 0-87012-023-9; ISBN 978-0-87012-023-7
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