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A river is a large natural waterway. It is a specific term in
the vernacular for large streams, stream being the umbrella term used in the scientific
community for all flowing natural waterways. In the vernacular, stream may be used to refer to smaller streams, as may creek,
run, fork, etc.
Passage via a river or stream is the usual way rainfall on land finds its way to the ocean or other large body of water
such as a lake. A river consists of several basic parts, originating from headwaters or a spring at the source, that flow into the main stream. Smaller side streams that join the river are tributaries. Water flow is normally confined to a channel, with a bottom or bed between banks. The lower end of a river is its mouth.
Topography
A river conducts water by constantly flowing perpendicular to the elevation curve of its bed, thereby converting the
positional 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. Loops that are formed are sometimes cut
off, forming a shorter river channel and leaving a remnant, oxbow lake. Rivers
that carry large amounts of sediment develop conspicuous deltas at their mouths. Rivers whose mouths are in saline tidal waters may form
estuaries.
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.
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.
See also
Biology
The flora and fauna of rivers are much different from those of the ocean because the water is sweet (non-salty). Living things in a river must be adapted to the current of the moving water.
Pollution
Human pollution of rivers is common, and very few rivers in the world today
are clean of man-made substances. The most common pollutant is sewage piped into
rivers, but chemical pollution is also common, and industrial accidents (and/or negligence) account for much of the destructon of
riparian biomes. Heat dumped into rivers by power plants and factories also affects river life.
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 and waterflow is no longer bounded by seasonal changes. 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, floods are disasters, causing untold
property loss each year.
Crossings
Rivers may be crossed by fords, bridges, ferries or tunnels.
Transport
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
The world's ten longest rivers:
- Nile (6,690 km).
- Amazon (6,280 km?).
- Chang Jiang (Yangtze) (6,380 km).
- Mississippi-Missouri (6,270 km).
- Ob-Irtysh (5,570
km).
- Amur (4,410 km).
- Congo (4,380 km or 4,670 km). (The source of the river is disputed.)
- Huang He (Yellow) (4,350 km).
- Lena (4,260 km).
- Mackenzie (4,240 km).
Well-known rivers (in alphabetic order):
See also
Fiction
Fictional rivers
Other fiction
- The Thames in Edward
Rutherfurd's London.
- The Thames in Jerome K. Jerome's Three Men in a
Boat.
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