confluence

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. 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 [downstream of confluences] of 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.

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. 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.

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; 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 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.

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.

Australia

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

Europe

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.

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.

Passau, Germany, sometimes called the Dreiflüssestadt (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 [i.e., the Alps] to create a murky tricolour."

The Thaya flows into the Morava in a rural location near Hohenau an der March in Austria, forming the tripoint of Austria, Czechia, and Slovakia.

North America

Mississippi basin

The Greater Twin Cities area of Minneapolis and St. Paul, Minnesota features two important Mississippi confluences. Near historical Fort Snelling and the town of Mendota—about 9 miles downstream on the Mississippi from Minneapolis—the Minnesota River flows into the Mississippi at Pike Island. The area around this confluence is a location of spiritual, cultural, and historical significance to the Dakota people and is also the site of the earliest European settlements in the Twin Cities area. About 30 miles further downstream from the Minnesota-Mississippi confluence—and 25 miles downstream from St. Paul—the Mississippi joins with the St. Croix River near Hastings, Minnesota, and Prescott, Wisconsin.

Vicksburg, Mississippi lies atop bluffs overlooking the confluence of the Mississippi River with its tributary the Yazoo. Both rivers, as well as the bluffs, played an important role in the Vicksburg Campaign, a pivotal event of the American Civil War.

The Missouri River flows into the Mississippi River at Jones-Confluence Point State Park, just north of St. Louis, Missouri. Slightly further upstream, the Illinois River flows into the Mississippi.

The Madison, Jefferson and Gallatin Rivers in Three Forks, Montana form the confluence of the Missouri River.

At Keokuk, Iowa, the Des Moines River flows into the Mississippi. This forms the political tripoint between the U.S. states of Iowa, Missouri, and Illinois.

Just south of Cairo, Illinois, the Ohio River flows into the Mississippi, forming the tripoint between the states of Illinois, Missouri, and Kentucky.

The Ohio River is formed by the confluence of the Monongahela and Allegheny rivers, located in Pittsburgh, Pennsylvania. The site is of great historical significance; in the 1970s it was upgraded by the creation of Point State Park, highlighted by a large fountain.

Atlantic watersheds

South America

Manaus, Brazil is on the Rio Negro near its confluence with the Amazon (see Meeting of Waters). It is the chief port and a hub for the region's extensive river system.

The Iguazú flows into the Paraná at the "Triple Frontier" (Spanish: La Triple Frontera, Portuguese: Tríplice Fronteira), the tripoint for Paraguay, Argentina, and Brazil.

In Ciudad Guayana, Venezuela there is a confluence between Orinoco River and Caroní River.

In Guyaquil and Durán, Ecuador, the convergence of the Daule and Babahoyo rivers forms the Guayas River.

Confluences of non-rivers

Occasionally, "confluence" is used to describe the meeting of tidal or other non-riverine bodies of water, such as two canals or a canal and a lake. A one-mile (1.6 km) portion of the Industrial Canal in New Orleans accommodates the Gulf Intracoastal Waterway and the Mississippi River-Gulf Outlet Canal; therefore those three waterways are confluent there.

The term confluence can also apply to the process of merging or flowing together of other substance. For example, it may refer to the merger of the flow of two glaciers.