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River


River


A river is a natural stream of freshwater that is larger than a brook or creek. Rivers are normally the main channels or largest tributaries of drainage systems. Typical rivers begin with a flow from headwater areas made up of small tributaries, such as springs. They then travel in meandering paths at various speeds. Finally, they discharge or flow out into desert basins, into major lakes, or most likely, into oceans.

Sixteen of Earth's largest rivers account for close to one-half of the planet's river flow. The world's longest river is the Nile River in Africa, which runs 4,187 miles (6,739 kilometers) from its source in Burundi to the Mediterranean Sea. 

However, the world's largest river is the Amazon River in South America. It runs about 3,900 miles (6,275 kilometers) from its source in the Andes Mountains in Peru to the southern Atlantic Ocean. Discharging an average of 7,000,000 cubic feet (198,000 cubic meters) of water each second, the Amazon River alone accounts for 20 percent of the water discharged each year by Earth's rivers.

Important Terms
Brook: A significant, continuously flowing body of water formed by the convergence of a number of rills.

Catchment area: Also known as a drainage basin, the entire land area drained by a river.
Episodic rivers: Rarely occurring rivers formed from runoff channels in very dry regions.
Perennial rivers: Rivers that have a constant stream-flow throughout the year, usually located in more humid climates where rainfall exceeds evaporation rates.
Periodic rivers: Rivers that run dry on occasion, usually located in arid climates where evaporation is greater than precipitation.

Rill: A small brook that forms from surface runoff.
Runnels: Eroded shallow channels created when rills pass over fine soil.
Tributary: A stream or other body of water that flows into a larger one.
Watershed: A ridge of high land that separates the catchment area of one river system from that of another; also used synonymously with catchment area.


A natural stream of freshwater flowing downhill from its source in the mountains to meet an ocean or a lake is known as a river. The river water is confined to a channel or a stream bed.

The rivers are formed when group of springs and streams known as headwaters having their origin in the mountains flow down to form a large stream or springs. The stream bed of a river lies between the banks of a river. The large streams are called as a river while the smaller one’s are called as creeks, brooks, rivulets or tributaries.

The rivers form the major component of the water cycle. The water in a river is accumulated from precipitation of ground water and also through the release of stored water in natural reservoirs such as glaciers.


The size and importance of the world's rivers are measured in terms of discharge and length. A river's importance may also be measured in terms of local and regional water availability and population. A small river flowing through a densely populated, arid region may be extremely important, for example.
Rivers flow downhill from their sources to their mouths at the sea. The sources of the world's major rivers are often in highly elevated or mountainous areas, but there are exceptions. For example, the source of the Mississippi River is Lake Itasca, Minnesota, which lies only about 461 meters (1,515 feet) above sea level.
Although the world's major rivers flow through many different types of terrain, they provide similar physical and biological functions. Rivers transport sediments from their basins to the sea through the processes of erosion, transport, and deposition. In a river system's upland areas, erosion is the dominant process.  

Tectonic processes result in the uplift and formation of major mountain chains, while the world's major river systems help erode those mountains. For example, the Colorado River of the southwestern United States flows through the Colorado Plateau region. The Colorado River's world-famous canyons were created over millions of years as the river eroded its way downward through the Colorado Plateau, which was being driven upward by tectonic forces.
A river's flow is halted when it reaches the sea, where the river deposits its sediments and creates a delta. River deltas commonly assume a triangular pattern that resembles the Greek letter delta (Δ)—a letter based on the shape of the Nile River delta in northern Egypt. Fresh water and salt water meet in these deltas, which are some of the world's most biologically productive areas.

The world's major river systems are storehouses of biological productivity and diversity. Rivers and their floodplains provide habitat for aquatic and terrestrial species. Many of the world's large rivers experience an annual flooding cycle that is important for spreading water, nutrients, and sediment into floodplains as well as providing reproductive cues for fish.
Large dams for reducing floods and producing hydroelectricity have been constructed on many of the world's large rivers. Flood damage reduction works, such as dams and levees, encourage people to move into floodplains; however, the loss of this annual flood cycle often has had negative ecological consequences.

Water resources managers and scientists around the world are exploring ways to restore parts of these annual flood cycles in order to restore natural river processes and habitat. Controlling the releases of waters from reservoirs to mimic natural flood patterns is a notion under discussion in many areas of the world.

Formation of Rivers


Every river has a point of origin. Because gravity plays a key role in the direction that rivers take, rivers almost always follow a downhill slope. Thus, the point of origin for rivers tends to be the highest point in the watercourse.

Some rivers start from springs, especially in humid climates. Springs occur as groundwater rises to Earth's surface and flows away. Other rivers originate from lakes, marshes, or runoff from melting glaciers located high in the mountains. Often, rivers having their beginnings in huge glaciers are quite large by the time they emerge from openings in the ice.

Precipitation, such as rainwater or snow, on highlands is the source of the water for most rivers. When a heavy rain falls on ground that is steeply sloped or is already saturated with water, water runoff trickles down Earth's surface rather than being absorbed.

Initially, the water runs in an evenly distributed, paper-thin sheet, called surface runoff. After it travels a short distance, the water begins to run in small parallel rivulets called rills. At the same time, the water becomes turbulent. As these rills pass over fine soil or silt, they begin to dig shallow channels, called runnels. This is the first stage of erosion.

Another major source of river water is the rain. When it rains heavily in the hills, the water trickles down the steep slopes and flows onto a riverbed. Initially, the water from the hills flows in an evenly distributed fashion and is called as surface run-off. When this water flow travels a certain distance, it begins to flow in parallel rills and also gathers momentum. Soon these parallel rills unite to form a stream.

As the rills converge with the stream a brook is formed. This brook flows through a valley. The volume of the water in a brook becomes constant when it gains sufficient volume of groundwater. The brook becomes a river when the water level in the brook increases.


Types of Rivers


The rivers are classified on the basis of the sediments it carries. The sediment carried is controlled by factors such as climate, geology and the stream gradient. Here are a few classifications of the rivers.

Youthful River - A youthful river has a steep gradient and very few tributaries. A youthful river is bound to flow quickly and swiftly. A few examples of youthful river include Trinity River and Brazos in the USA, and Ebro River in Spain.

Mature River - A mature river is less steep and flows slowly compared to the youthful river. There are many tributaries that feed a mature river. The sediment deposit is also less. Examples of mature river include St.Lawrence River, Ohio River and River Thames.

Old River - An old river has a low gradient and is depended on flood plains is known as old river. Some of the world famous old rivers include the Ganges, Nile, and Euphrates.

Rivers have been one of the sources of food, water and transport since pre-historic times. The rivers aid the cultivation of crops by supplying water. Historians claim navigation of rivers date back to the Indus Valley Civilization. Rivers of the world are the major source of fresh water and they sustain their own food chain.


Rivers in Continents

Asia

The Yangtze ( Chang ) River of China is Asia's largest river in terms of discharge. Like many of Asia's major rivers, the Yangtze's headwaters are in central Asia's Himalaya Mountains. The Yangtze has a history of devastating


WORLD'S TEN LARGEST RIVERS BY DISCHARGE
River Country Average Discharge at Mouth (Thousands of cubicfeet per second)
Amazon Brazil 7,500
Congo Congo 1,400
Yangtze China 770
Brahmaputra Bangladesh 700
Ganges India 660
Yenisey Russia 614
Mississippi USA 611
Orinoco Venezuela 600
Lena Russia 547
Parana Argentina 526


Rivers, Major World


Around 3.7 million people were killed in floods in 1931, one of humanity's worst natural disasters. The Yangtze has gained attention as the site of the ongoing Three Gorges Dam project; when finished, it will be the world's largest hydropower dam. Potential negative effects of the dam include the resettlement of more than one million people and the submergence of large areas of scenic beauty and cultural significance.

The Brahmaputra River is the world's fourth-largest river, and the Ganges River the fifth largest. Both rivers have their headwaters in the Himalayas and, along with the Meghna River, merge in Bangladesh to form a river that has 2.5 times the discharge of the Mississippi River. Floods during the summer monsoon season in this river system are immense and often cause tens or hundreds of thousands of deaths. But the floods represent a paradox for Bangladesh, as the annual floods are essential to maintain soil fertility and fisheries.

Southeast Asia's Mekong River flows through six nations: China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. The Mekong has been noted for its relative lack of development, although this is changing, as China is constructing dams on the upper Mekong River. The Mekong also is noted for sustained international cooperation on river basin planning in the lower basin. Today's Mekong River Commission represents an international organization that has existed in many forms since its establishment in the late 1950s.

The Indus River and its river basin represent one of the world's highly developed and regulated river systems, with a history of massive inundation canals dating back four millennia. The Indus flows from the western flank of the Himalayas in Tibet and then primarily through Pakistan on its way to the Arabian Sea, but several of its large tributary streams are in India. Expansion of irrigated agriculture in the basin continues, with gains in agricultural production offset by salinity, waterlogging, and flood losses.

Some of the world's large rivers are in Russia. The Yenisey (sixth largest), Lena (ninth), and Ob (thirteenth) Rivers are among the world's twenty largest rivers. All these rivers flow northward to the Arctic Ocean.

Africa

As Africa is the world's most arid continent, its rivers are of great importance. Most of Africa's rivers are characterized by annual climate cycles that alternate seasonal rainfall with seasonal drought. Fifty-four of Africa's river basins cross international borders. There is thus a need for international collaboration if these large African rivers are to approach their full potential in balancing the demands of meeting human needs and sustaining ecosystems.

The basin of the world's second-largest river, the Congo River, covers eleven western African nations. The world's longest river, the Nile, is in northern Africa; its river basin is shared by twelve nations. The Nile is well known for the ancient civilizations that inhabited its valley. The Egyptian civilization depended greatly on the Nile and the irrigated agriculture in its floodplains. The Nile flooded annually, and the silt from these floods replenished agricultural soils, and the water helped flush salts that had accumulated in the irrigated fields. In the 1960s, the Aswan Dam was constructed on the Nile River. Although the dam eliminated this annual flooding cycle, it has brought several benefits to Egypt, especially hydroelectricity.

South America's Amazon River serves a range of human needs, including transportation. The contrast between the two local boys and their dugout canoes (foreground) and the cruise ship (background) exemplifies the changing and increasingly diverse demands on major world rivers.

Europe

Europe's largest river is the Danube, which flows eastward through more than a dozen nations on its way to the Black Sea. The Rhine River flows northward through several European nations before emptying into the North Sea. Both these rivers flow through many cities and are heavily used. Floods generally do not present problems in these rivers, but pollution is a concern in both of them. For example, a large chemical spill in 1986 in the upper Rhine killed fish far downstream, and prompted the formation of a pollution warning system along the river.

North America

The Mississippi River is North America's largest river. Joined by the Ohio River to the east and the Missouri River to the west, the Mississippi is the world's seventh-largest river when it reaches the Gulf of Mexico. The Mississippi is well known for its great floods. A great flood in 1927 initiated changes in federal water planning that have lasted for several decades. Even with improved planning and flood control, the 1993 Mississippi floods still caused widespread damage.

Canada and the United States share two of North America's great rivers: the Columbia and the St. Lawrence. The Columbia is in the Pacific Northwest, and its dozens of dams constitute the world's largest hydropower system. The Columbia also is famous for its legendary salmon fisheries and the conflicts between fisheries interests and operators of the hydropower dams. The St. Lawrence forms the boundary between eastern Canada and the United States and is the outflow of the Great Lakes to the Atlantic Ocean.

The Colorado River of the southwestern United States flows through an arid and highly populated region and is an important source of water for millions of people, especially in southern California and Arizona. The Colorado was well known for its wide variations in streamflow and the tremendous amounts of silt it carried to the Gulf of California. The Colorado has been so extensively dammed and diverted that it no longer reaches its mouth (terminus), instead drying up in the Mexican desert. Several environmental groups are currently investigating strategies for restoring these flows.

South America

South America combines the world's highest average annual rainfall values with the world's highest evaporation and is the site of the world's driest area (Atacama Desert) and the world's largest river, the Amazon.

The Amazon has its headwaters along the eastern flank of the Andes Mountains and drains the largest river basin on earth. The average flows of the Amazon are larger than the combined average flows of the world's next ten largest rivers.

Other large South American rivers include the Orinoco (eight largest), which flows northward through Venezuela, and the Parana (tenth largest), which flows through Brazil, Paraguay, Argentina, and Uruguay.


World's Top Fifty Longest Rivers



River Length (km) Length (miles) Drainage area (km²) Average discharge (m³/s) Outflow Countries in the drainage basin
1. Nile – Kagera 6,650
(6,853)
4,132
(4,258)
3,349,000 5,100 Mediterranean Sea Ethiopia, Eritrea, Sudan, Uganda, Tanzania, Kenya, Rwanda, Burundi, Egypt, Democratic Republic of the Congo
2. Amazon – Ucayali – Apurímac 6,400
(6,992)
3,976
(4,345)
6,915,000 219,000 Atlantic Ocean Brazil, Peru, Bolivia, Colombia, Ecuador, Venezuela, Guyana
3. Yangtze
(Chang Jiang)
6,300 3,917 1,800,000 31,900 East China Sea China
4. Mississippi – Missouri – Jefferson 6,275 3,902 2,980,000 16,200 Gulf of Mexico United States (98.5%), Canada (1.5%)
5. Yenisei – Angara – Selenga 5,539 3,445 2,580,000 19,600 Kara Sea Russia (97%), Mongolia (2.9%)
6. Yellow River
(Huang He)
5,464 3,395 745,000 2,110 Bohai Sea
(Balhae)
China
7. Ob – Irtysh 5,410 3,364 2,990,000 12,800 Gulf of Ob Russia, Kazakhstan, China, Mongolia
8. Paraná - Río de la Plata  4,880 3,030 2,582,672 18,000 Río de la Plata Brazil (46.7%), Argentina (27.7%), Paraguay (13.5%), Bolivia (8.3%), Uruguay (3.8%)
9. Congo – Chambeshi
(Zaïre)
4,700 2,922 3,680,000 41,800 Atlantic Ocean Democratic Republic of the Congo, Central African Republic, Angola, Republic of the Congo, Tanzania, Cameroon, Zambia, Burundi, Rwanda
10. Amur – Argun
(Heilong Jiang)
4,444 2,763 1,855,000 11,400 Sea of Okhotsk Russia, China, Mongolia
11. Lena 4,400 2,736 2,490,000 17,100 Laptev Sea Russia
12. Mekong
(Lancang Jiang)
4,350 2,705 810,000 16,000 South China Sea Laos, Thailand, China, Cambodia, Vietnam, Myanmar
13. Mackenzie – Peace – Finlay 4,241 2,637 1,790,000 10,300 Beaufort Sea Canada
14. Niger 4,200 2,611 2,090,000 9,570 Gulf of Guinea Nigeria (26.6%), Mali (25.6%), Niger (23.6%), Algeria (7.6%), Guinea (4.5%), Cameroon (4.2%), Burkina Faso (3.9%), Côte d'Ivoire, Benin, Chad
15. Murray – Darling 3,672 2,282 1,061,000 767 Southern Ocean Australia
16. Tocantins – Araguaia 3,650 2,270 950,000 13,598 Atlantic Ocean, Amazon Brazil
17. Volga 3,645 2,266 1,380,000 8,080 Caspian Sea Russia
18. Shatt al-Arab – Euphrates 3,596 2,236 884,000 856 Persian Gulf Iraq (60.5%), Turkey (24.8%), Syria (14.7%)
19. Madeira – Mamoré – Grande – Caine – Rocha 3,380 2,100 1,485,200 31,200 Amazon Brazil, Bolivia, Peru
20. Purus 3,211 1,995 63,166 8,400 Amazon Brazil, Peru
21. Yukon 3,185 1,980 850,000 6,210 Bering Sea United States (59.8%), Canada (40.2%)
22. Indus 3,180 1,976 960,000 7,160 Arabian Sea Pakistan (93%), India, China
23. São Francisco 3,180*
(2,900)
1,976*
(1,802)
610,000 3,300 Atlantic Ocean Brazil
24. Syr Darya – Naryn 3,078 1,913 219,000 703 Aral Sea Kazakhstan, Kyrgyzstan, Uzbekistan, Tajikistan
25. Salween
(Nu Jiang)
3,060 1,901 324,000 3,153 Andaman Sea China (52.4%), Myanmar (43.9%), Thailand (3.7%)
26. Saint Lawrence – Niagara – Detroit – Saint Clair – Saint Marys – Saint Louis 3,058 1,900 1,030,000 10,100 Gulf of Saint Lawrence Canada (52.1%), United States (47.9%)
27. Rio Grande 3,057
(2,896)
1,900
(1,799)
570,000 82 Gulf of Mexico United States (52.1%), Mexico (47.9%)
28. Lower Tunguska 2,989 1,857 473,000 3,600 Yenisei Russia
29. Brahmaputra – Tsangpo 2,948* 1,832* 1,730,000 19,200 Bay of Bengal India (58.0%), China (19.7%), Nepal (9.0%), Bangladesh (6.6%), Disputed India/China (4.2%), Bhutan (2.4%)
30. Danube – Breg
(Duna)
2,888* 1,795* 817,000 7,130 Black Sea Romania (28.9%), Hungary (11.7%), Austria (10.3%),  Serbia (10.3%),  Germany (7.5%), Slovakia (5.8%),
Bulgaria (5.2%),  Croatia (4.5%),
31. Zambezi
(Zambesi)
2,693* 1,673* 1,330,000 4,880 Mozambique Channel Zambia (41.6%),
Angola (18.4%),
Zimbabwe (15.6%), Mozambique(11.8%),
Malawi (8.0%),
Tanzania (2.0%),
Namibia, Botswana
32. Vilyuy 2,650 1,647 454,000 1,480 Lena Russia
33. Araguaia 2,627 1,632 358,125 5,510 Tocantins Brazil
34. Amu Darya 2,620 1,628 534,739 1,400 Aral Sea Uzbekistan, Turkmenistan, Tajikistan, Afghanistan
35. Japurá
(Rio Yapurá)
2,615* 1,625* 242,259 6,000 Amazon Brazil, Colombia
36. Nelson – Saskatchewan 2,570 1,597 1,093,000 2,575 Hudson Bay Canada, United States
37. Paraguay
(Rio Paraguay)
2,549 1,584 900,000 4,300 Paraná Brazil, Paraguay, Bolivia, Argentina
38. Kolyma 2,513 1,562 644,000 3,800 East Siberian Sea Russia
39. Ganges/Padma
(Ganga)
2,510 1,560 907,000 12,037 Brahmaputra, Bay of Bengal India, Bangladesh, Nepal
40. Pilcomayo 2,500 1,553 270,000
Paraguay Paraguay, Argentina, Bolivia
41. Upper Ob 2,490 1,547

Ob Russia
42. Ishim 2,450 1,522 177,000 56 Irtysh Kazakhstan, Russia
43. Juruá 2,410 1,498 200,000 6,000 Amazon Peru, Brazil
44. Ural 2,428 1,509 237,000 475 Caspian Sea Russia, Kazakhstan
45. Arkansas 2,348 1,459 505,000
(435,122)
1,066 Mississippi United States
47. Olenyok 2,292 1,424 219,000 1,210 Laptev Sea Russia
48. Dnieper 2,287 1,421 516,300 1,670 Black Sea Russia, Belarus, Ukraine
49. Aldan 2,273 1,412 729,000 5,060 Lena Russia
46. Ubangi – Uele 2,270 1,410 772,800 4,000 Congo Democratic Republic of the Congo, Central African Republic, Republic of Congo
50. Negro 2,250 1,450 720,114 26,700 Amazon Brazil, Venezuela, Colombia


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