In geography, a confluence (also conflux) occurs where two or more watercourses 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, forming the Ohio River); or where two separated channels of a river (forming a river island) rejoin downstream from their point of separation.
Scientific study
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.A widely cited work is James L. Best (1986) The morphology of river channel confluences. Progress in Physical Geography 10:157–174. For work citing Best, see . The water flows and their consequences are often studied with mathematical models.A recent contribution with review of earlier work is Laurent Schindfessel, Stéphan Creëlle and Tom De Mulder (2015) "Flow patterns in an open channel confluence with increasingly dominant tributary inflow," Water 7: 4724–4751; available on line. Confluences are relevant to the distribution of living organisms (i.e., ecology) as well; "the general pattern [downstream of confluences] of increasing stream flow and decreasing slopes drives a corresponding shift in habitat characteristics."Quoted from Beechie et al. (2012), who cite earlier work. Tim Beechie, John S. Richardson, Angela M. Gurnell, and Junjiro Negishi (2012) "Watershed processes, human impacts, and process-based restoration." In Philip Roni and Tim Beechie (eds.) (2012) Stream and Watershed Restoration: A Guide to Restoring Riverine Processes and Habitats, John Wiley & Sons. Excerpts available on line at Google Books.
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."U.S. Geological Survey, "How do contaminants mix at the confluence of two streams?", on line at .
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.David Lynch (2014) "The Confluence of Rivers"; Earth Science Picture of the Day, at .
Flow zones
The 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
Engineering
The broader field of engineering encompasses a vast assortment of subjects which concern confluences.
In hydraulic civil engineering, where two or more underground culverted / artificially buried watercourses intersect, great attention should be paid to the hydrodynamic aspects of the system to ensure the longevity and efficiency of the structure.
Engineers must design these systems with consideration of factors that ensure the discharge point is structurally stable as the entrance of the lateral culvert into the main structure may compromise the stability of the structure due to the lack of support at the discharge; this often constitutes additional supports in the form of structural bracing. The velocities and hydraulic efficiencies should be meticulously calculated and can be altered by integrating different combinations of geometries, components such a gradients, cascades and an adequate junction angle which is sympathetic to the direction of the watercourse's flow to minimise turbulent flow, maximise evacuation velocity and to ultimately maximise hydraulic efficiency.
Cultural and societal significance
Since rivers often serve as political boundaries, confluences sometimes demarcate three abutting polities, such as nations, states, or provinces, forming a tripoint. Various examples are listed below.
Many 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.Rogers, Adam (2011) Late Roman Towns in Britain: Rethinking Change and Decline. Cambridge: Cambridge University Press, p. 42. Excerpts available on line at Google Books. Pre-Christian Slavic peoples chose confluences as the sites for fortified triangular temples, where they practiced human sacrifice and other sacred rites.Gasparini, Evel (n.d.) "Slavic religion", in Encyclopedia Britannica, on line edition: In Hinduism, the confluence of two sacred rivers often is a pilgrimage site for ritual bathing.Source: Letizia (2017), who writes, "as rivers are considered holy entities, at the meeting of two streams the 'sacredness' of the first river add to that of the second one. The confluence seems to have a sort of 'additive fame' ... because it gives pilgrims the chance to bathe in two rivers at the same time." In Pittsburgh, a number of adherents to Mayanism consider their city's confluence to be sacred.Ann Rodgers, "So how did the Point get on a Mayan calendar?", Pittsburgh Post-Gazette, June 22, 2008. On line at .
Notable confluences
Africa At Lokoja, Nigeria, the Benue River flows into the Niger. At Kazungula 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;See reporting in the New York Times () and The Atlantic (). 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.The incorporation of invisible rivers into confluences elsewhere in the subcontinent is documented by Letizia (2017). 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 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.See New Straits Times, August 28, 2017, 'Najib launches River of Life, Blue Pool projects", at . 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. Pak Nam Pho, the downtown of Nakhon Sawan in Thailand, is the confluence of the rivers Ping and Nan, forming the Chao Phraya the main artery of central Thailand. Pak Phraek, the old town zone of Kanchanaburi in Thailand, is the confluence of the rivers Khwae Yai and Khwae Noi, forming the Mae Klong the main artery of western Thailand. Sam Sop, also known as Sam Prasop, is the confluence where the Ran Ti, Bikhli, and Songkalia Rivers meet near Uttamanusorn Bridge, commonly known as Mon Bridge, the longest wooden bridge in Thailand, located in Sangkhla, Kanchanaburi. The Pa Sak combines the Chao Phraya at Ayutthaya in Thailand. The confluence is a location of the historic monastery Wat Phanan Choeng, built around 26 years before the founding of the Ayutthaya Kingdom. 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".See Bruno Maçães, "Signs and Symbols on the Sino-Russian Border", published in The Diplomat. On line at . 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 , 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.