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. As a river flows over relatively flat areas, the river will often meander: start to form loops and snake through the plain through erosion of the river banks. Loops of the river occasionally are cut off, forming a shorter riverbed and leaving an oxbow lake[?]. Erosion caused by rivers often cause them to develop deltas at their mouths, consisting of mazes of river arms and islands made up of the silt deposited as the river loses speed, eventually fading into the ocean or lake.
Where a river descends quickly, rapids with whitewater or even waterfalls may occur. Rapids are often used for recreational purposes, see Whitewater canoeing[?] and kayaking. Waterfalls are commonly used as sources of energy, via watermills and hydroelectric plants.
The flora and fauna of rivers are much different from those of the oceans because the water is sweet (non-salty). All living things in a river must be adapted to the current of the moving water.
Human pollution of rivers is very common, and very few rivers in the world today are clean of artificial substances. Most common is unfiltered sewage being 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.
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 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.
List of waterways
The world´s ten longest rivers:
Well-known rivers (in alphabetic order) include:
Chocolate river in Willy Wonka and the Chocolate Factory.