River

For other uses, see River (disambiguation).

A river is a large natural waterway. The source of a river may be a lake, a spring, or a collection of small streams, known as headwaters. From their source, all rivers flow downhill, typically terminating in the ocean. The mouth, or lower end, of a river is known as its base level.

A river's water is normally confined to a channel, made up of a stream bed between banks. Most rainfall on land passes through a river on its way to the ocean. Smaller side streams that join a river are tributaries. The scientific term for any flowing natural waterway is a stream; so in technical language, the term river is just a shorthand way to refer to a large stream.

Topography

File:A21a.JPG

Bridges are a popular way of crossing rivers, as seen here at the Buller River, West Coast, New Zealand.

A river conducts water by constantly flowing perpendicular to the elevation curve of its bed, thereby converting the potential energy of the water into kinetic energy. Where a river flows over relatively flat areas, the river will meander: start to form loops and snake through the plain by eroding the river banks. Sometimes the river will cut off a loop, shortening the channel and forming an oxbow lake from the cut off section. Rivers that carry large amounts of sediment develop conspicuous deltas at their mouths. Rivers whose mouths are in saline tidal waters may form estuaries.

Over time, rivers cut away at their beds, eventually forming a more gentle gradient.

Youthful river - a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider.
Mature river - a river with a gradient that is less steep than those of youthful rivers and flows more slowly than youthful rivers. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper.
Old river - a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains.
Rejuvenated river - a river with a gradient that is raised by the earth's movement.

Where a river descends quickly over sloped topography, rapids with whitewater or even waterfalls occur. Rapids are often used for recreational purposes (see Whitewater kayaking). Waterfalls are sometimes used as sources of energy, via watermills and hydroelectric plants.

The beginning of the mountain river (Reichenbach in Grosse Scheidegg.

The "mouth", where this river comes out of the melting ice (also visible in the previous photo)

Rivers begin at their source in higher ground, either rising from a spring, forming from glacial meltwater, flowing from a body of water such as a lake, or simply from damp, boggy places where the soil is waterlogged. They end at their base level where they flow into a larger body of water, the sea, a lake, or as a tributary to another (usually larger) river. In arid areas rivers sometimes end by losing water to evaporation and percolation into dry, porous material such as sand, soil, or pervious rock. The area drained by a river and its tributaries is called its watershed, catchment basin or drainage basin. (Watershed is also used however to mean a boundary between drainage basins.)

Starting at the mouth of the river and following it upstream as it branches again and again, the resulting river network forms a dendritic (tree-like) structure that is an example of a natural random fractal.

Geopolitical boundaries

Rivers have been important historically in determining political boundaries.

The Orange River discharges into the Atlantic ocean and forms the International boundary between various provinces and countries along its route. The source of the Orange River is in the Lesotho mountains. The Lesotho Highlands Water Project was conceived to divert water from Lesotho into the Vaal River system, itself a tributary of the Orange River, to supply the water needs of the Gauteng province in South Africa. Water from the Delivery tunnel is discharged into the Ash River. The spot has become a popular Kayaking venue.

The Danube was a long standing border of the Roman Empire, and today forms most of the border between Bulgaria and Romania.

The Mississippi in North America, and the Rhine in Europe, are major east-west boundaries in those continents.

Rating systems

International Scale of River Difficulty - The scale is used to rate rivers -- particularly those with rapids -- for navigation. Class I is the easiest and Class VI is the hardest.
Strahler Stream Order - The Strahler Stream Order is a method to rank rivers based on the connectivity and hierarchy of contributing tributaries. Headwaters are first order while the Amazon River is twelfth order. Approximately 80 percent of the rivers and streams on Earth are of the first and second order.

Biology

The flora and fauna of rivers are much different from those of the ocean because the water is fresh (non-salty). Living things in a river must be adapted to the current of the moving water. For a detailed look at these processes, see lotic system ecology.

Water quality

Human pollution of rivers is common, and few rivers in the world are free of man-made substances. The most common point source of pollutant is probably sewage piped into rivers, but chemical pollution is also common, and industrial accidents (and/or negligence) account for significant destruction of riparian biomes. Heated water discharged into rivers by power plants and factories also affects river biota. Non-point source pollutants such as diffuse pollution from agriculture, urbanisation, logging, construction activity and slash-and-burn techniques are all recognised as sources of sediment and other pollutants.

Techniques have been developed to analyze dispersal of water pollutants in river systems. For example the U.S. Environmental Protection Agency funded research to develop the DSSAM Model to assess the transport of soluble nitrogen, reactive phosphorus, total dissolved solids and dissolved oxygen in the Truckee River, which flows through both California and Nevada.

Dams

In places where the elevation changes of a river are great, dams for hydroelectric plants and other purposes are often built. This disrupts the natural flow of the river, and creates a lake behind the dam. Often the building of dams affects the whole of the river, even the part above the dam, as migrating fish are hindered (see fish ladder), waterflow is no longer bounded by seasonal changes and sediment flow is blocked. Dams are useful in many ways, such as: providing hydro-electric power; acting as regulator of river flow so as to regulate the occurrence of flooding, which is especially important to wet-rice agriculture; and also to improve navigation and transport on the river. Often, dams such as Hoover Dam along Colorado River become famous tourist attractions. Critics of dams, especially 'green' advocates, argue that dams remove upper-river biodiversity such as through deforestation and forced migration of rural villages and indigenous tribes. Furthermore, trapping of river sediments behind the dams lead to salination and loss of nutrients for down-water fish. It also raises concern of earthquakes due to instablity of incompetent dams which have to support thousands of tonnes of sediments behind them. One very famous and problematic dam is the Aswan High Dam in the Nile.

Flooding

Flooding is a natural part of a river's cycles. Human activity, however, has upset the natural way flooding occurs by walling off rivers and straightening their courses. Removal of bogs, swamps and other wetlands in order to produce farmland has reduced the absorption zones for excess water and made floods into sudden disasters rather than gradual increases in water flow. In ancient Egypt, life was made possible through the floods of the Nile and the accompanying silt and sediment which enriched the fields with fresh nutrients. Nowadays, since people have built on these floodplains, floods are disasters, causing untold property loss each year.

Human interference in the form of deforestation can also worsen conditions. The removal of vegetation leads to a reduction in interception (vegetation stopping precipitation) and the 'weakening' of soil since plant roots no longer hold it together. As a result there is a reduced Infiltration capacity (how much water the soil can hold) and greater infiltration (precipitation going into the ground). This leads to faster soil saturation and therefore greater overland flow (also known as surface runoff) and therefore, there are flash floods as the lag time decrease.

Logjams

Small river in rural Indiana, United States. Rivers of this size are often referred to as a "creek."

Logjams are barriers within rivers, created by dead and uprooted trees. Over time, the obstruction prevents further logs to bypass, resulting in the creation of new network channels. According to author David R. Montgomery in his book, King of Fish, a logjam also causes water to buildup within a small space, forming peaceful pools within the main channel which provide shelter for young salmon. The existence of these deep pools along with the complex web of channels creates an ideal salmon habitat. Today, many believe that the rebuilding of salmon runs is contingent upon reproducing the same environment shaped by logjams. As a result, many scientists have attempted to recreate artificial logjams. Marc Duboiski and Mike Ramsey of the Salmon Recovery Funding board staff, George Pess of the National Marine Fisheries Service, and Kevin Bauersfeld of Washington Department of Fish and Wildlife have prepared the Report to the Salmon Recovery Funding Board On the Engineered Log Jam (ELJ) Workshop ([1]), with the hope of mimicking natural logjams.

In wintertime, rivers can be subject to ice jams or dams. An ice dam can create flooding upstream. When the jam melts or breaks apart, it can lead to flash flooding downstream. Ice jams caused flooding in the Northeastern United States in 1996.

Management

In its natural state a river may be inconvenient to man in a variety of ways. Rivers in inhabited areas have therefore been managed or controlled to make them more useful and less disruptive to human activity.

The river channel may be dredged to make it deeper for navigation or to prevent flooding.
Dams (see above) or weirs may be built to control the flow, store water, or extract energy.
Levees may be built to prevent flooding.
Sluice gates provide a means of controlling flow and adjusting river levels.
floodways may be added to draw off excess river water in times of flood.
Canals connect rivers to one another for water transfer or navigation.
River courses may be modified to improve navigation, or straightened to increase the flow rate.

River management is an ongoing activity as rivers tend to 'undo' the modifications made by man. Dredged channels silt up, sluice mechanisms deteriorate with age, levees and dams may suffer seepage or catastrophic failure.

River lists

File:Amazon near Manaus.jpg

The Amazon River near Manaus in Brazil.

The world's ten longest rivers

It is difficult to measure the length of a river, mainly because rivers have a fractal property, which means that the more precise the measure, the longer the river will seem. Also, it is difficult to determine exactly where a river begins or ends, as very often, upstream rivers are formed by seasonal streams, swamps, or changing lakes.

These are average measurements.

Nile (6,690 km)
Amazon (6,452 km)
Yangtze (Chang Jiang) (6,380 km)
Mississippi-Missouri (6,270 km)
Yenisey-Angara (5,550 km)
Ob-Irtysh (5,410 km)
Huang He (Yellow) (5,464 km)
Amur (4,410 km)
Congo (4,380 km or 4,670 km). (The source of this river is disputed.)
Lena (4,260 km)

For a longer list see Longest rivers. This also gives more information on measuring river lengths.

Well-known rivers (in alphabetic order)

The Amazon River, the largest river in the world (in terms of volume and water cubic metres/second)
The American River, the site of Sutter's Mill
Amu Darya
The Amur, the principal river of eastern Siberia and the border between Russia and China
The Arkansas River, a major tributary of the Mississippi River
The Arno, the river that runs through Florence
The Arvandrud (Shatt al-Arab), the river that borders Iran and Iraq
Brahmaputra, the principal river in North East India and Tibet
The Chao Phraya, the principal river of Thailand
The River Clyde, which runs through Glasgow
The Colorado River (in Argentina)
The Colorado River (in the United States), the principal river of the American Southwest
The Columbia River, the principal river of the Pacific Northwest
The Congo, the principal river of central Africa
The Danube, the principal river of central and southeastern Europe
De la Plata, the widest river in the world
The Ebro, a river in northwestern Spain
The Elbe, a major German river, Hamburg is situated on it
The Euphrates, one of the twin principal rivers of Mesopotamia (Iraq)
The Ganges, the principal river of India
Han-gang, the river that runs through Seoul
The Helmand River, the principal river of Afghanistan
The Huang He (or Yellow River), one of the principal rivers of China
The Hudson River, the principal river of New York
The Indus River, the principal river of Pakistan
The River Jordan, the principal river of Israel
Karun, the principal navigable river of southern Iran
The River Kaveri, the principal river of South India
Lena, the principal river of northeastern Siberia
The Mackenzie River, the longest river in Canada
Magdalena, the principal river of Colombia
The Main, a river in Germany
The Mekong, a principal river of Southeast Asia
The River Mersey, the river on which sits the English city of Liverpool
Maas, the principal river of the southern provinces of the Netherlands and eastern Belgium
The Mississippi River, the principal river of central and southern United States
The Missouri River, one of the principal rivers of the Great Plains
The Murray River, the principal river of southeastern Australia
The Niagara River, the river which flows between Lake Erie and Lake Ontario, and which flows over the Niagara Escarpment (better known as Niagara Falls)
The Niger, the principal river of west Africa
The Nile, The longest river in the world, principal to Egypt and northeastern Africa
The Ob, a large river of Siberia
The Oder, a major river in Central Europe
The Ohio River, the largest river between Mississippi and the Appalachian Mountains
The Orinoco, the principal river of Venezuela
The Paraná, a major river in South America
The Paraguay, the principal tributary of the Parana river and an important river in Brazil, Bolivia, Paraguay and Argentina
The Po, one of the principal rivers of Italy
The Potomac River, the principal river of the Washington, D.C. and the souther border of Maryland in the United States
The Rhine, the principal river of northwestern Europe
The Rhône, the principal river of southern France
The Rio Grande, the river that forms part of the border between the United States and Mexico
The Saint Lawrence River, which drains the Great Lakes
The Savannah River, a major river in the southeastern United States, forming most of the border between the states of Georgia and South Carolina
The Seine, the river that runs through Paris
The Segura, a river in southeastern Spain
The River Severn, the longest river in Great Britain
Shinano-gawa, the longest river in Japan
The Snake River, the largest tributary of the Columbia river in Washington
The Susquehanna River, the principal river of Pennsylvania and the Chesapeake Bay
Tajo, the largest river in the Iberian Peninsula
The River Tay, the largest river in Scotland
The Tennessee River, an important tributary of the Mississippi that flows through Eastern/Western Tennessee, Northern Alabama, and Kentucky
The Thames, the river that runs through London
The Tiber, the river that runs through Rome
The Tigris, one of the twin principal rivers of Mesopotamia
Tonegawa, one of largest rivers in Japan
The Vistula, the principal river of Poland
The Vltava, the river that runs through Prague
The Volga River, the principal river of Russia
The Yangtze (Chang Jiang), the longest river in China
The Yenisei, a large river in Siberia
The Yukon, the principal river of Alaska and the Yukon Territory
The Zambezi, the principal river of southeastern Africa

Other lists

Rivers in myth and fiction

Real rivers

The Thames in Edward Rutherfurd's London.
The Thames in Jerome K. Jerome's Three Men in a Boat.
The Thames and the Congo in Joseph Conrad's Heart of Darkness.
The Mississippi in Mark Twain's Huckleberry Finn.
The River Liffey through Dublin in James Joyce's Finnegans Wake.

Mythological rivers

See also river deity.

In Greek mythology, the Acheron, Cocytus, Phlegethon, Lethe and Styx (the five rivers of Hades); and the Eridanus.
The Alph, an underground river imagined by various mystics and mentioned in Coleridge's poem Kubla Khan.
The Sambation river stops flowing every Saturday.
The rivers of the Rigveda (notably the Sapta Sindhu) are partly identifiable with actual rivers (Sindhu, Ganga, Kubha), and partly underwent transformation into mystical or cosmological rivers (Rasā). Notably, the name of the Sarasvati River over time came to refer to a goddess of wisdom, a mystical invisible river, and an actual river of Rajasthan.

Fictional rivers

River Ankh traversing the city of Ankh-Morpork in Terry Pratchett's Discworld series.
Chocolate river in Willy Wonka and the Chocolate Factory.
River Djel in the country of Djelibeybi in Terry Pratchett's Discworld series.
The River in the Riverworld novels of Philip José Farmer.
Rivers of Middle-earth in various works of J. R. R. Tolkien.
The Zora river in Ocarina of Time

External links

Management: River Basin Commissions.
[2] > for river discharge / runoff data
[3] > for worldwide realized and projected dam projects