Rivers of the World
A river is a large natural waterway.
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 canoeing and 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 sink 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.
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(6,380 km).
- Mississippi-Missouri (6,270 km).
- Ob-Irtysh (5,570 km).
- Zaire (4,670 km).
- Amur (4,410 km).
- Huang He (Yellow) (4,350 km).
- Lena (4,260 km).
- Mackenzie (4,040 km).
Well-known rivers (in alphabetic order):
See also
Fiction
Fictional rivers
Other fiction
Referenced By
Alphabetical list of rivers in Canada | List of British Columbia rivers
|
