River

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A river is a natural waterway that conveys water derived from precipitation from higher ground to lower ground. Most commonly rivers flow on the surface but there are many examples of undeground rivers where the flow is contained within chambers, caves or caverns. In some areas of highly variable rainfall, some rivers exist that carry water only occasionally and may be dry for several years at a time.

Origins of river water

A river may have its source in a spring, lake, from damp, boggy places where the soil is waterlogged, from glacial meltwater, or simply from rain flowing off impermeable rock or man-made surfaces. Almost all rivers are joined by other rivers and streams termed tributaries the highest of which are known as headwaters. Water may also be recruited to a river from ground-water sources. Throughout the course of the river, the total volume transported downstream will often be a combination of the free water flow together with a substantial contribution flowing through sub-surface rocks and gravels that underly the river and its floodplain. For many rivers in large valleys, this unseen component of flow may greatly exceed the visible flow.

From their source, all rivers flow downhill, typically terminating in the sea or in a lake. In arid areas rivers sometimes end by losing water to evaporation. River flow may also be lost by percolation into dry, porous material such as sand, soil, or into pervious rock. Excessive abstraction of water for use in industry, irrigation etc can also cause a river to dry before reaching a lake or the sea.

The mouth, or lower end, of a river is known by hydrologists as its base level.

The area drained by a river and its tributaries is called catchment, catchment basin, drainage basin or watershed. The term "watershed" is also used to mean a boundary between catchments, which is also called a water divide.

Topography

File:A21a.JPG

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

A river's water is generally confined to a channel, made up of a stream bed between banks. In larger rivers there is also a wider flood-plain shaped by flood-waters overtopping the channel. Flood plains may be very wide in relation to the size of the river channel. This distinction between river channel and floo-plain can be blurred especially in urban areas where the flood-plain of a river channel can become greatly developed by housing and industry.

The river channel itself may contain a single stream of water but many rivers, if left unconstrained by man's activities, will create several streams of water within a channel producing a braided river. Braided rivers can still be commonly seen in South Island New Zealand and on some of the larger river Deltas.

A river flowing in its channel is a source of considerable energy which acts on the river channel to change its shape and form. In mountainous torrential zones this can be seen as erosion channels through hard rocks and the creation of sands and gravels from the destruction of larger rocks. In U shaped glaciated valleys, the subsequent river valley can often easily be idenitified by the V shaped channel that it has carved. In the middle reaches where the river may flow over flatter land, loops (meanders) may form through eroding of the river banks and deposition on the inside of bends. Sometimes the river will cut off a loop, shortening the channel and forming an oxbow lake. Rivers that carry large amounts of sediment may develop conspicuous deltas at their mouths, if conditions permit. Rivers, whose mouths are in saline tidal waters, may form estuaries. River mouths may also be fjords or rias.

Although the following classes are a useful simplified way to visualize rivers, it is important to recognize there are other factors at work here. Gradient is controlled largely by tectonics, but discharge is controlled largely by climate and sediment load is controlled by various factors including climate, geology in the headwaters, and the stream gradient.

Youthful river - a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider. (Ex: Brazos River, Trinity River, Ebro River)
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. (Ex: Mississippi River, Ohio River, Thames River)
Old river - a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains. (Ex: Tigris River, Euphrates River, Indus River)
Rejuvenated river - a river with a gradient that is raised by tectonic uplift.

The beginning of a mountain river (Reichenbach in Grosse Scheidegg)

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

Use of rivers

Rivers have been used by man since the dawn of civilization as a source of water, for food, for transport as defences, as a source of power to drive machinery and as a means of disposing of waste. In addition the rocks and gravels generated and moved by rivers have been greatly used in construction. In more recent generations, the beauty of rivers and their wider habitats has contributed greatly to tourist income from areas well endowed with attractive riverine scenery.

In upland rivers 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.

Geopolitical boundaries

Rivers throughout history have been the source and support for civilization, and many major cities today are near a river of some sort. Rivers have been important historically in determining political boundaries. For example, the Danube was a longstanding 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. The Orange River forms the boundary between various provinces and countries along its route in Africa.

Biology

The flora and fauna of rivers have developed to utilise the very wide range of aquatic habitats available from torrential waterfalls through to lowland mires . Although many organisms are restricted to the fresh-water in rivers, some, such as Salmon have adapted to be able to survive both in rivers and in the sea..

Flooding

Flooding is a natural part of a river's cycles. The majority of the erosion of river channels and the erosion and deposition on the associated floodplains occur during flood stage. Human activity, however, has upset the natural way flooding occurs by walling off rivers, straightening their courses and by draining of natural wetlands.

Direction of flow

A misconception, particularly amongst schoolchildren and college students in North America^[1]^[2], is that most, or even all, rivers flow from north to south. Rivers in fact flow downhill irrespective of direction and is often a complex meandering path involving all directions of the compass.^[3]^[4]^[5]

Studying the flows of rivers is one aspect of hydrology.^[6]

Management

Rivers are often managed or controlled to make them more useful and less disruptive to human activity.

Dams (see above) or weirs may be built to control the flow, store water, or extract energy.
Levees may be built to prevent run-off of 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. The benefits sought through managing rivers may often be offset by the social and economic costs of mitigating the effects of such management. As an example, in many parts of the developed world, rivers have been confined within channels to free up flat flood-plain land for development. Subsequent extreme flood events can inundate such development at very high financial costs and often with loss of life.

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, the more precise the measurement, the longer the river will seem. Also, it is difficult to determine 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)
Mississippi-Missouri (6,270 km)[1]
Yangtze (Chang Jiang) (6,245 km)[2]
Yenisey-Angara (5,550 km)
Huang He (Yellow) (5,464 km)
Ob-Irtysh (5,410 km)
Amur (4,410 km)
Congo (4,380 km)
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, a place in the USA
Amu Darya, the longest river in central Asia
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
The Brahmaputra, the principal river in northeast 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
Río de la Plata, the widest river in the world
The Dnieper River, one of the main rivers of Russia, Belarus and Ukraine.
The Ebro, a river in northwestern Spain
The Elbe, a major German river, running through the city of Hamburg
The Euphrates, one of the twin principal rivers of Anatolia (Turkey) and Mesopotamia (Iraq)
The Ganga, 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, USA
The Indus River, the principal river of Pakistan
The James River. the principal river of Virginia in the USA, historically important.
The River Jordan, the principal river of Palestine, Jordan, and Israel
Karun, the principal navigable river of southern Iran
The River Kaveri, the principal river of South India
The Lena, the principal river of northeastern Siberia
The Mackenzie River, the longest river in Canada
The Magdalena, the principal river of Colombia
The Main, a river in Germany which runs through Frankfurt am Main
The Mekong, a principal river of Southeast Asia
The River Mersey, the river on which sits the English city of Liverpool
The Maas, the principal river of the southern provinces of the Netherlands and eastern Belgium
The Mississippi River, the principal river of the 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 the Mississippi and the Appalachian Mountains
The Orinoco, the principal river of Venezuela
The Rhine, one of the longest and most important rivers in Europe
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 Sava, which flows through four countries—Slovenia, Croatia, Bosnia and Herzegovina (making its northern border) and Serbia—and was therefore one of the symbols of former Yugoslavia
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 Anatolia (Turkey) and Mesopotamia (Iraq)
Tonegawa, one of the 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 and the longest river in Europe
The Wabash River, the principal river of Indiana
The Yangtze (Chang Jiang), the longest river in China and Asia
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 or Fictional rivers

see also river deity

in Greek mythology, the Acheron, Cocytus, Phlegethon, Lethe and Styx (the five rivers of Hades); and the Eridanus
Virginia's Nancy River alleged to have had mystical healing powers
the Alph, an underground river imagined by various mystics and mentioned in Coleridge's poem Kubla Khan
Zora's River in Ocarina of Time
the Tamanawis River in David James Duncan's "The River Why?"
the Anduin in J.R.R Tolkien's The Lord Of The Rings
the Lifestream in Final Fantasy VII
the Beartooth,edda,Gaena,Jiet,Toark,Ninor,Anora,and Ramr rivers in Christopher Paolini's Eragon and Eldest

Rating systems

International Scale of River Difficulty - The scale is used to rate the challenges of navigation—particularly those with rapids. 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.

References

^ "Children's Misconceptions about Science". Operation Physics, American Institute of Physics. September 1998.
^ William C. Philips (February 1991). "Earth Science Misconceptions".
^ Matt Rosenberg (2006-06-08). "Do All Rivers Flow South?". {{cite web}}: Check date values in: |date= (help)
^ Matt Rosenberg. "Rivers Flowing North: Rivers Only Flow Downhill; Rivers Do Not Prefer to Flow South".
^ Nezette Rydell (1997-03-16). "Re: What determines the direction of river flow? Elevation, Topography,Gravity??". Earth Sciences. {{cite web}}: Check date values in: |date= (help)
^ Cristi Cave. "How a River Flows". Stream Biology and Ecology.