Confluence of the Potomac and Shenandoah at Harpers Ferry - aer photo by NPS.jpg

In geography, a confluence (also: ''conflux'') occurs where two or more flowing bodies of water join to form a single channel. A confluence can occur in several configurations: at the point where a tributary joins a larger river (main stem); or where two streams meet to become the source of a river of a new name (such as the confluence of the Monongahela and Allegheny rivers at Pittsburgh, forming the Ohio); or where two separated channels of a river (forming a river island) rejoin at the downstream end.

Scientific study of confluences

Confluences are studied in a variety of sciences. Hydrology studies the characteristic flow patterns of confluences and how they give rise to patterns of erosion, bars, and scour pools. The water flows and their consequences are often studied with mathematical models. Confluences are relevant to the distribution of living organisms (i.e., ecology) as well; "the general pattern ownstream of confluencesof increasing stream flow and decreasing slopes drives a corresponding shift in habitat characteristics."

Another science relevant to the study of confluences is chemistry, because sometimes the mixing of the waters of two streams triggers a chemical reaction, particularly in a polluted stream. The United States Geological Survey gives an example: "chemical changes occur when a stream contaminated with acid mine drainage combines with a stream with near-neutral pH water; these reactions happen very rapidly and influence the subsequent transport of metals downstream of the mixing zone."

A natural phenomenon at confluences that is obvious even to casual observers is a difference in color between the two streams; see images in this article for several examples. According to Lynch, "the color of each river is determined by many things: type and amount of vegetation in the watershed, geological properties, dissolved chemicals, sediments and biologic content – usually algae." Lynch also notes that color differences can persist for miles downstream before they finally blend completely.

River confluence flow zones

Hydrodynamic behaviour of flow in a confluence can be divided into six distinct features which are commonly called confluence flow zones (CFZ). These include

# Stagnation zone
# Flow deflection zone
# Flow separation zone / recirculation zone
# Maximum velocity zone
# Flow recovery zone
# Shear layers

Confluences and mankind

Since rivers often serve as political boundaries, confluences sometimes demarcate three abutting political entities, such as nations, states, or provinces, forming a tripoint. Various examples are found in the list below.

A number of major cities, such as Chongqing, St. Louis, and Khartoum, arose at confluences; further examples appear in the list. Within a city, a confluence often forms a visually prominent point, so that confluences are sometimes chosen as the site of prominent public buildings or monuments, as in Koblenz, Lyon, and Winnipeg. Cities also often build parks at confluences, sometimes as projects of municipal improvement, as at Portland and Pittsburgh. In other cases, a confluence is an industrial site, as in Philadelphia or Mannheim. Often a confluence lies in the shared floodplain of the two rivers and nothing is built on it, for example at Manaus, described below.

One other way that confluences may be exploited by humans is as sacred places in religions. Rogers suggests that for the ancient peoples of the Iron Age in northwest Europe, watery locations were often sacred, especially sources and confluences. Pre-Christian Slavic peoples chose confluences as the sites for fortified triangular temples, where they practiced human sacrifice and other sacred rites. In Hinduism, the confluence of two sacred rivers often is a pilgrimage site for ritual bathing. In Pittsburgh, a number of adherents to Mayanism consider their city's confluence to be sacred.

Notable confluences

Africa

* At Lokoja, Nigeria, the Benue River flows into the Niger in Zambia, the Chobe River flows into the Zambezi. The confluence defines the tripoint of Zambia (north of the rivers), Botswana (south of the rivers) and Namibia (west of the rivers). The land border between Botswana and Zimbabwe to the east also reaches the Zambezi at this confluence, so there is a second tripoint (Zambia-Botswana-Zimbabwe) only 150 meters downstream from the first. See Kazungula and Quadripoint, and Gallery below for image.
* The Sudanese capital of Khartoum is located at the confluence of the White Nile and the Blue Nile, the beginning of the Nile.

Asia

* 82 km north of Basra in Iraq at the town of Al-Qurnah is the confluence of the rivers Tigris and Euphrates, forming the Shatt al-Arab.
*
* At Devprayag in India, the Ganges River originates at the confluence of the Bhagirathi and the Alaknanda; see images above.
* Near Allahabad, India, the Yamuna flows into the Ganges. In Hinduism, this is a pilgrimage site for ritual bathing; during a Kumbh Mela event tens of millions of people visit the site. In Hindu belief the site is held to be a ''triple'' confluence (Triveni Sangam), the third river being the metaphysical (not physically present) Sarasvati.
*Karad, in Maharashtra, India, is the site of the Pritisangam (meaning: Lovely Confluence), a T-shaped confluence of Krishna River and Koyna River, where Koyna River mergers into Krishna River forming a T-shape and then the merged rivers flow to the east as Krishna River.

* Kuala Lumpur, the capital of Malaysia, is where the Gombak River (previously known as Sungai Lumpur, which means "muddy river") flows into the Klang River at the site of the Jamek Mosque. Recently, the Kolam Biru (Blue Pool), a pool with elaborate fountains, has been installed at the apex of the confluence.
*Both Taipei and New Taipei are where the Dahan and Xindian meet and flow into the Tamsui River.
* The Nam Khan River flows into the Mekong at Luang Prabang in Laos.
* The Jialing flows into the Yangtze at Chongqing in China. The confluence forms a focal point in the city, marked by Chaotianmen Square, built in 1998.
* In the Far East, the Amur forms the international boundary between China and Russia. The Ussuri, which also demarcates the border, flows into the Amur at a point midway between Fuyuan in China and Khabarovsk in Russia. The apex of the confluence is located in a rural area, part of China, where a commemorative park, Dongji Square, has been built; it features an enormous sculpture representing the Chinese character for "East". The Amur-Ussuri border region was the location of the Sino-Soviet border conflict of 1969; the borderline near the confluence was settled peacefully by treaty in 2008.
* In Georgia, in the town of Pasanauri on the southern slopes of the Caucasus Mountains, the Tetri Aragvi ("White Aragvi") is joined by the Shavi Aragvi ("Black Aragvi"). Together, these two rivers continue as the Aragvi River. The conflux is known for its dramatic visual contrast of the two rivers.

Australia

* The two largest rivers in Australia, the Murray and its tributary the Darling, converge at Wentworth, New South Wales.

Europe

Seine

* The Seine divides in the historical center of Paris, flowing around two river islands, the Île Saint-Louis and the Île de la Cité. At the downstream confluence, where the river becomes a single channel again, the Île de la Cité is crossed by the famous Pont Neuf, adjacent to an equestrian statue of King Henri IV and the historically more recent Vert Galant park. The site has repeatedly been portrayed by artists including Monet, Renoir, and Pissarro.
* Further upstream, the Marne empties into the Seine at Charenton-le-Pont and Alfortville, just southeast of the Paris city limits. The site is dominated by the Huatian Chinagora, a four-star hotel under Chinese management.

Rhine

* The Rhine carries much river traffic, and major inland ports are found at its confluence with the Ruhr at Duisburg, and with the Neckar at Mannheim; see Mannheim Harbour.
* The Main flows into the Rhine just south of Mainz.
* The Mosel flows into the Rhine further north at Koblenz. The name "Koblenz" itself has its origin in the Latin name "Confluentes". In German, this confluence is known as the "Deutsches Eck" ("German corner") and is the site of an imposing monument to German unification featuring an equestrian statue of Kaiser Wilhelm I.
* Upstream in Switzerland, a small town also named Koblenz (for the same reason) is where the Aare joins the Rhine.

Danube basin

* Passau, Germany, sometimes called the (City of Three Rivers), is the site of a triple confluence, described thus in a guidebook: "from the north the little Ilz sluices brackish water down from the peat-rich Bavarian Forest, meeting the cloudy brown of the Danube as it flows from the west and the pale snow-melt jade of the Inn from the south .e.,_the_Alps.html"_;"title="Alps.html"_;"title=".e.,_the_Alps">.e.,_the_Alps">Alps.html"_;"title=".e.,_the_Alps">.e.,_the_Alpsto_create_a_murky_tricolour."
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