What is River?

A river is a natural waterway that transits water through a landscape from higher to lower elevations. It is an integral component of the water cycle. The water within a river is generally collected from precipitation through surface runoff, groundwater recharge (as seen at baseflow conditions / during periods of lack of precipitation) and release of stored water in natural reservoirs, such as a glacier.

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 originate 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 a 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. Anastamosing rivers are similar to braided rivers. They have multiple sinuous channels carrying large volumes of sediment. Due to the dynamics of this type of system, they are also quite rare.

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, Euphrates River, Indus River, Nile River)
• Rejuvenated river – a river with a gradient that is raised by tectonic uplift.

It has been noted that on average, the air distance (euclidean distance) from the beginning to the end of most rivers is about one third their actual length. For rivers that flow in plain areas, this number is very close to Pi; Einstein wrote an explanation of why this is so.^[1][2]

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 a defensive barrier, as a source of power to drive machinery, and as a means of disposing of waste.

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

In some highly-forested countries like Scandinavia and Canada, lumberjacks use the river to float felled trees downstream to lumber camps for further processing, saving much effort and cost by transporting the huge heavy logs by natural means.

Rivers have been a source of food since pre-history. Apart from being a rich source of fish, rivers indirectly aid cultivation by supplying water for the crops. Rivers sustain their own food chain. They are a major source of fresh water, hence, it is no surprise to find most of the major cities of the world situated on the banks of rivers. Rivers also provide an easy (if unsanitary) means of disposing of waste.



The rocks and gravel 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 defending 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 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

Main article: Flood

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.

Flooding can be mitigated.

Direction of flow

A misconception, is that most, or even all, rivers flow from north to south.^[3][4] Rivers in fact flow down gradient irrespective of direction, often in a complex meandering path involving all directions of the compass.^[5][6][7]

As it happens, 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, and Lena. Four of the ten longest river systems of the world flow mainly north.

Studying the flows of rivers is one aspect of hydrology.^[8]

Rate of water flow

Volumetric flow rate, also called volume flow rate and rate of water flow, is the volume of water which passes through a given volume per unit time, measured in cubic meters per second ( 1 m³/s = 35.51ft³/s).

Management

Main article: River engineering

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 river water from flowing on floodplains or floodways.
• 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

Because rivers are approximately fractal in nature 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 River (6,695 km)
2. Amazon River (6,683 km)
3. Yangtze (Chang Jiang) (6,380 km)^[9]
4. Missouri River/ Mississippi River (5,970 km)^[10]
5. Ob River (5,410 km)
6. Huang He (Yellow) (4,830 km)
7. Congo (4,630 km)
8. Lena (4,400 km)
9. Amur (4,350 km)
10. Yenisei River (4,106 km)

Other lists disagree go to: [1]

Well-known rivers (in alphabetic order)

• Amazon – largest ^[11] river in the world (in terms of volume and water cubic metres/second) and possibly the longest
• 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
• River Boyne - principal river on the east coast of Ireland
• 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
• Detroit - forms part of the border of the USA and Canada, part of the Great Lakes Waterway
• Dnieper – one of the main rivers of Russia, Belarus and Ukraine.
• Dniester – one of principal rivers in Eastern Europe, runs through Ukraine and Moldova into the Black Sea.
• Elbe – a major German river, running through the city of Hamburg
• Euphrates – one of the twin principal rivers of Anatolia (Turkey) and Mesopotamia (Iraq)
• River Forth - runs between Stirling and South Queensferry passing Edinburgh
• 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 Pakistan
• James – principal river of Virginia in the USA, historically important.
• River Jordan – principal river of Israel, Jordan, and the West Bank.
• Karun – principal navigable river of southern Iran
• Kaveri – principal river of South India
• Lena River – the principal river of northeastern Siberia
• River Liffey - runs through Dublin, Ireland.
• Loire – the longest river of France, an important natural reserve for wildlife preservation.
• 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 – The river passing Liverpool
• Meuse – 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
• Monongahela - one of the three rivers connected in Pittsburgh, PA
• 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
• Nila – longest river of Kerala South India
• Nile – principal river of 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
• Orange – the longest river in South Africa
• Orinoco – principal river of Venezuela
• Pamba - the sacred river of Kerala , South of India
• Paraná – one of the longest and most important rivers in South America, running through Brazil, Paraguay and Argentina
• Po - the longest river of Italy
• Rhine – one of the longest and most important navigable rivers in western Europe, flowing from Switzerland to the Netherlands, and part of the borders of Liechtenstein, Austria, Germany, and France.
• Rhône – one of the most important navigable rivers of Western Europe, going from Switzerland to France
• Río de la Plata – widest river in the world
• Rio Grande – forms part of the border between the United States and Mexico
• Sabarmati – flows through Ahmedabad, India
• Saint Lawrence – drains the Great Lakes
• Saint Mary's - part of the border of the USA and Canada, connects Lake Superior to Lake Huron, and contains the world's busiest Lock the Soo Locks
• 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
• River Severn – longest river in Great Britain
• River Shannon - longest river in Ireland
• Shatt al-Arab – the river that borders Iran and Iraq
• 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, which 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
• Yukon – principal river of Alaska
• Zambezi – principal river of southeastern Africa

Other lists

• List of rivers of Africa
• List of rivers of the Americas
• List of rivers of Asia
• List of rivers of Europe
• List of rivers of Oceania
• List of waterways
• List of river name etymologies

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	Zambezi and Victoria Falls (Zambia/Zimbabwe, Africa)
|Hooghly River (Kolkata, India)	Heathcote National Park	This river flows from Woronora Dam, Sydney.

See also

• Aqueduct
• Canal
• Drought
• List of river name etymologies
• Mainstem (hydrology)
• River cruise
• Water dispute
• Baer's law
• Rock-cut basin

Crossings

Rivers may be crossed by:

• Bridges
• Ferries
• Fords
• Tunnels.

Transport

• Barge
• Riverboat
• Sailing
• Towpath

References

Further reading

• Beyond the Bridges Life on American Rivers told by Riverlorian, Jerry Hay. http://www.indianawaterways.comfor more information
• 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.