Wednesday, October 28, 2009

Rivers and Lakes

A river is a large stream of fresh water flowing downhill within a channel to enter another river or a lake or sea. All rivers ultimately receive their water from precipitation, though this relation is not a very simple one. The flow of a river depends upon three main factors: the amount and intensity of rainfall, governed by climatic factors; the permeability of the surface on which it is flowing; and the morphological properties of the drainage basin. The nature of the vegetation cover also plays an important role. When water flows in a stream, it is subject to two basic forces: gravity, aimed downstream and causing the flow; and frictional resistance between the water and the river bed. The effect of friction ensures an uneven velocity of water flow in the river. Thus, water near the centre moves the fastest while it is slow near the banks.

PARTS OF RIVER

The place at which the river begins to flow is called its source. It may be in the melt waters of a glacier, e.g., the Rhone (France), or in a lake, e.g., the Nile (Africa), or in a spring, e.g., the Thames (England), or in a region of steady rainfall, e.g., the Zaire (Africa). The place where the river ends is called a river's mouth. It is usually in the sea, e.g., the Indus (Arabian Sea) and the Amazon (Atlantic), although it may be in a lake, e.g., the Volga (Caspian), or in a salt swamp, e.g., the Chari River (Lake Chad) and the Tarim River (Lap Nor).

A mature river has three sections: (i) the upper course representing the stage of youth of a river, called the torrent stage; (ii) the middle course, representing the state of maturing called the valley stage, and (Hi) the lower course, representing the stage of old age and called the plain stage. In the stage of youth, the river flows turbulently in a narrow steep-sided valley whose floor is broken by (a) 'potholes' or circular depressions in the river bed, caused by pebbles,
(b) waterfalls, occurring where the bed of the river becomes suddenly steepened. Some deep and narrow valleys having very steep sides are called gorges. These are formed where a waterfall retreats upstream. The head waters of the Ganga have cut deep gorges in the Himalayas. The Indus Gorge in Kashmir is 5180 metres. Canyons are large gorges, formed in dry regions where large rivers are actively eroding vertically. A canyon is a deep valley with steep, near vertical sides. River Colorado has cut a gorg~ 1.6 km (1 mile) deep and 480 km (300 miles) long into the Colorado plateau (USA).

As the valley widens, meanders are formed. A meander is a loop. in the course of a river channel. Water flows in corkscrew manner, causing erosion on the concave bank and deposition on the convex bank.
In the mature stage form the flood plains-low lying land composed of deposits of sediment (alluvium). Mean­ders migrating downstream cause widening of the valley. Ox-bow lakes are produced by cut-offs in a meander during floods.

Reaching old age, the river deposits its load into the sea or lake into which it flows. The deposited load some­times collects in the river mouth, where it builds up into a low-lying swampy plain called a delta. The river divide~ into several channels due to the depositions at its mouth, called distributaries. Arcuate deltas are formed by coarse sediments like gravel and sand and are triangular in shape, having many distributaries, e.g., in Rivers Ganga, Indus, Nile, Niger, Hwang-Ho. Bird's foot deltas are composed of very fine sediments, having few distributaries with clearly defined channels, like the Mississippi delta. An estuarine delta develops in the mouth of a submerged river, e.g., deltas of Elbe (Germany) and Ob (Russia).

A river does three types of work: it erodes, and most erosion takes place in the upper course; it transports: a process dominant in the middle course; and it deposits: depositional process is dominant in the lower course of the river. A river's ability to work, Le., to erode and transport material, depends upon its energy. Potential energy is provided by weight and the elevation of water. This is converted into downflow by gravity and thus into kinetic energy. However, about 95 per cent of this energy is lost due to frictional resistance.

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