River

A river is a natural waterway, which moves water across the landscape from higher to lower elevations, and is an important component of the water cycle. The water within a river is usually from precipitation through surface runoff and release of stored water in natural reservoirs, such as glaciers and groundwater.

Origins of river water

A river may have its source in a spring, lake, from damp, boggy landscapes where the soil is waterlogged, from glacial melt, or from surface runoff of precipitation. Almost all rivers are joined by other rivers and streams termed tributaries the highest of which are known as headwaters. Water may also come from groundwater 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 underlie the river and its floodplain (called the hyporheic zone). 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, through a confluence. In arid areas rivers sometimes end by losing water to evaporation. River water may also infiltrate into the soil or pervious rock, where it becomes groundwater. Excessive abstraction of water for use in industry, irrigation, etc., can also cause a river to dry before reaching its natural terminus.

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, or in some cases, continental divide.

Topography

The water in a river is usually 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 over-topping the channel. Flood plains may be very wide in relation to the size of the river channel. This distinction between river channel and flood-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 typically contains a single stream of water but some rivers flow as several interconnecting streams of water, producing a braided river. Extensive braided rivers are found in only a few regions worldwide, such as the South Island of New Zealand. They also occur on peneplains and 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 identified 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 or billabong. 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. (Examples: 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. (Examples: Mississippi River, Ohio River, River Thames)
• Old river – a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains. (Examples: Ganges River, Tigris River, Euphrates River, Indus River, Nile River)
• Rejuvenated river – a river with a gradient that is raised by tectonic uplift.

Other types of rivers

Most rivers flow on the surface, however other rivers may flow underground in caves or caverns. Such rivers can be found in karst regions with limestone geologic formations.

An intermittent river (or ephemeral river) flows occasionally and can be dry for several years at a time. These rivers are found in regions with limited and highly variable rainfall.

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.

For thousands of years rivers has been used for navigation (The earliest evidence of navigation is found in the Indus Valley Civilization, which existed in Northwestern India around 3300 BC). Riverine navigation provides the cheapest means of transport and is still used extensively in major rivers of the world like Ganges River, River Nile, Mississippi River, River Indus, etc.

In some places in the world like Scandinavia, Canada, the lumberjacks uses the river to float down the logs cut by them, down to the lumber camps (which are present downstream). This natural transportation saves them the entire effort to carry the huge heavy logs to the lumber camps.

River has been a source for food since the dawn of civilization. Apart from being a rich source of fish, rivers indirectly aids cultivation with its supply of water for the crops. Rivers sustains its own food chain. They are a major source of fresh water. Hence, no doubt we find most of the major cities of the world is situated on the banks of rivers. Rivers also provides a easy means of disposing of waste.

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). Fast flowing rivers and waterfalls are harnessed as sources of energy, via watermills and hydroelectric plants.

Rivers have been important historically in determining political boundaries and defenses of countries. 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 and Limpopo rivers in Southern Africa form the boundaries between various provinces and countries along their routes.

The noted Greek historian Megasthenes (350BC - 290BC) mentions about River Ganga several times in his work Indika: "India, again, possesses many rivers both large and navigable, which, having their sources in the mountains which stretch along the northern frontier, traverse the level country, and not a few of these, after uniting with each other, fall into the river called the Ganges. Now this river, which at its source is 30 stadia broad, flows from north to south, and empties its waters into the ocean forming the eastern boundary of the Gangaridai, a nation which possesses a vast force of the largest-sized elephants." (Diodorus II.37.)

Biology

Main article: Aquatic ecosystem

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 and Hilsa 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, ^[1][2] is that most, or even all, rivers flow from north to south. Some attribute this "fact" to such causes as the rotation of the Earth, centrifugal force, angular momentum, and the local geography effects. Rivers in fact flow downhill irrespective of direction, often in a complex meandering path involving all directions of the compass.^[3][4][5]

Indeed, few major rivers in the continental US flow north, as most of the country is located in the watershed of the Pacific or Atlantic oceans or the Gulf of Mexico, with very few rivers flowing northward toward the Arctic Ocean, Great Lakes, or Hudson Bay. However, thousands of north-flowing rivers exist elsewhere, including such major watercourses as the Nile, Mackenzie, Rhine, Yenisei, Nelson, Lena, etc. Four of the ten longest river systems of the world flow mainly north.

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

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.

1. Nile (6,690 km)
2. Amazon (6,400 km)
3. Mississippi-Missouri (6,270 km)^[7]
4. Yangtze (Chang Jiang) (6,245 km)^[8]
5. Yenisey-Angara (5,550 km)
6. Huang He (Yellow) (5,464 km)
7. Ob-Irtysh (5,410 km)
8. Amur (4,410 km)
9. Congo (4,380 km)
10. 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)

• Amazon – largest river in the world (in terms of volume and water cubic metres/second)
• American – site of Sutter's Mill, a place in the USA
• Amu Darya – longest river in central Asia
• Amur – principal river of eastern Siberia and the border between Russia and China
• Arkansas – a major tributary of the Mississippi River
• Arno – the river that runs through Florence
• Arvandrud (Shatt al-Arab) – the river that borders Iran and Iraq
• Brahmaputra – principal river in northeast India, Bangladesh and Tibet
• Chao Phraya – principal river of Thailand
• River Clyde – runs through Glasgow
• Colorado – in Argentina
• Colorado – principal river of the American Southwest
• Columbia – principal river of the Pacific Northwest
• Congo – principal river of central Africa
• Damodar - the principal river of Chotanagpur plateau, India and a primary tributary of Hooghly River
• Danube – principal river of central and southeastern Europe
• Río de la Plata – widest river in the world
• Dnieper – one of the main rivers of Russia, Belarus and Ukraine.
• Ebro – a river in northwestern Spain
• Elbe – a major German river, running through the city of Hamburg
• Euphrates – one of the twin principal rivers of Anatolia (Turkey) and Mesopotamia (Iraq)
• Ganges – principal river of India and Bangladesh
• Godavari - principal river of South India
• Han – river that runs through Seoul, Korea
• Helmand – principal river of Afghanistan
• Hooghly - the primary distributary of Ganges, which runs through Kolkata, India
• Hudson – principal river of New York, USA
• Indus – principal river of India and Pakistan
• James – principal river of Virginia in the USA, historically important.
• River Jordan – principal river of Israel, Jordan, and Palestine.
• Karun – principal navigable river of southern Iran
• Kaveri – principal river of South India
• Lena River – the principal river of northeastern Siberia
• Mackenzie – longest river in Canada
• Magdalena – the principal river of Colombia
• Main – a river in Germany which runs through Frankfurt am Main
• Mekong – a principal river of Southeast Asia
• River Mersey – near Liverpool
• Meuse River – principal river of the southern provinces of the Netherlands and eastern Belgium
• Mississippi – principal river of the central and southern United States
• Missouri – one of the principal rivers of the Great Plains
• Murray – principal river of southeastern Australia
• Niagara – between Lake Erie and Lake Ontario, and which flows over the Niagara Escarpment (better known as Niagara Falls)
• Niger – principal river of west Africa
• Nile – longest river in the world, principal to Egypt and northeastern Africa
• Ob – a large river of Siberia
• Oder – a major river in Central Europe
• Ohio – largest river between Mississippi River and Appalachian Mountains
• Orinoco – principal river of Venezuela
• Paraná – one of the longest and most important rivers in South America, running through Brazil, Paraguay and Argentina
• Rhine – one of the longest and most important rivers in Europe
• Rio Grande – forms part of the border between the United States and Mexico
• Sabarmati – flows through Ahmedabad, India
• Saint Lawrence – drains the Great Lakes
• São Francisco River – longest river wholly within Brazil
• Sava – 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
• Savannah – a major river in the southeastern United States, forming most of the border between Georgia and South Carolina
• Seine – river that runs through Paris, France
• Segura – a river in southeastern Spain
• Seti – river in Nepal
• River Severn – longest river in Great Britain
• Shinano-gawa – longest river in Japan
• Snake – largest tributary of the Columbia River in Washington
• Susquehanna River – principal river of Pennsylvania and the Chesapeake Bay
• Tagus – longest river in the Iberian Peninsula
• River Tay – largest river in Scotland
• Tennessee – an important tributary of the Mississippi that flows through Eastern/Western Tennessee, Northern Alabama, and Kentucky
• River Thames – the primary river of England, which runs through London
• Tiber – river that runs through Rome
• Tietê – river that runs through São Paulo towards the centre of the continent
• Tigris – one of the twin principal rivers of Anatolia (Turkey) and Mesopotamia (Iraq)
• Tone – one of the largest rivers in Japan
• Vistula – principal river of Poland
• Vltava – river that runs through Prague
• Volga – principal river of Russia and the longest river in Europe
• Volta – large river in Western Africa, runs through Burkina Faso
• Wabash – principal river of Indiana
• Yangtze – longest river in China and Asia
• Yellow – one of the principal rivers of China
• Yenisei – a large river in Siberia
• Yukon – principal river of Alaska
• Zambezi – principal river of southeastern Africa

Other lists

• List of waterways
  • List of rivers of Oceania
    • List of rivers of New Zealand
• List of river name etymologies

Rivers in myth and fiction

Real rivers

• River Thames

• In Edward Rutherfurd's London
• In Jerome K. Jerome's Three Men in a Boat
• In Joseph Conrad's Heart of Darkness

• Mississippi River in Mark Twain's Huckleberry Finn
• 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
• 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
• In Indian mythology, the Saraswati, in North India

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 Amazon River is twelfth order. Approximately 80 percent of the rivers and streams on Earth are of the first and second order.

Río Peralonso - El Zulia (Norte de Santander), Colombia	River Gambia flowing through Niokolokoba National Park	Bridges are a common way of crossing rivers, as seen here at the Hooghly River, Kolkata, India	|Nevėžis River in Lithuania showing ox-bows typical of the middle reaches of a mature river
Zambezi and Victoria Falls (Zambia/Zimbabwe, Africa)	|Hooghly River (Kolkata, India)

References

1. ^ Children's Misconceptions about Science. Operation Physics, American Institute of Physics (September 1998).
2. ^ William C. Philips (February 1991). Earth Science Misconceptions.
3. ^ Matt Rosenberg (2006-06-08). Do All Rivers Flow South?.
4. ^ Matt Rosenberg. Rivers Flowing North: Rivers Only Flow Downhill; Rivers Do Not Prefer to Flow South.
5. ^ Nezette Rydell (1997-03-16). Re: What determines the direction of river flow? Elevation, Topography,Gravity??. Earth Sciences.
6. ^ Cristi Cave. How a River Flows. Stream Biology and Ecology.
7. ^ Missouri.
8. ^ Chang.

Further reading

• Luna B. Leopold (1994). A View of the River. Harvard University Press. ISBN.  — a non-technical primer on the geomorphology and hydraulics of water
• Jeffrey W. Jacobs. "Rivers, Major World". Water Encyclopaedia.

See also

• Aqueduct
• Canal
• Drought
• List of river name etymologies
• Water dispute
• River cruise

Crossings

Rivers may be crossed by:

• Bridges
• Ferries
• Fords
• Tunnels.

Transport

• Barge
• Riverboat
• Sailing
• Towpath

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