Thursday, October 29, 2009

Landforms Made By River Action

Landforms Made By River Action Rivers are one
the greatest sculpturing agents at work in humid regior In its youthful stage the river flows turbulently in a narr01 steep-sided valley whose floors are broken by pot hol, and waterfalls. A youthful valley is 'V' -shaped, with steE gradient. The water of a fast-flowing river swirls if its bE is uneven. The pebbles carried by a swirling river C1 circular depressions in the river bed. These graduall deepen and are called pot holes. Much larger but simil; depressions form at the base of a waterfall; these are callE
plunge pools. Interlocking spurs are another feature of youthful valley.
Some valleys have very steep sides and are bot
. narrow and deep; these are called gorges. A gorge is ofte formed when a waterfall retreats upstream. One of the most famous gorges formed in this way lies below tl1 Victoria Falls. A gorge will also form when a riVE maintains its course across a belt of country which is bein uplifted. The Indus, the Brahmaputra and the headwateJ of the Ganga have cut deep gorges in the Himalayas. , huge gorge is called a canyon, and it usually occurs in dr regions where large rivers are actively eroding verticall and where weathering of the valley sides is minimum.
Valleys of the mature stage haye the shape of an ope 'V' in cross-section. The gradient is more gentle, river bend are more pronounced, spurs are removed by lateral erosio: and their remains form a line of bluffs on each side of th valley floor.

Active deposition starts taking place on the conve Danks of meanders during maturity. After the stage maturing is reached, the river begins to overflow its bank and it deposits fine silts and muds on the valley floor. Thi is the final stage in the growth of a flood plain. Meander are pronounced and cut-ofts develop and produce ox-bolA lakes. The river builds up its banks with alluvium (thi banks are called ievees). The river thus flows betweeJ pronounced banks and above the level of the flood plain In course of time, river erosion, transportation, and depo sition turn the original surface into an almost level plaiI which is called a pleneplain.

A delta is formed at the mouth of a river where it deposits more material than can be carried away, as the speed of the river is reduced by the time it enters a sea or lake. Also, fine clay particles carried in suspension in the river coagulate in the presence of salt water and get deposited. Deltas can form on the shores of tidal seas, e.g., River Colorado. Any river, irrespective of its stage of development, can form a delta. The river must have a large load, obviously having active erosion in the upper valley. There are three basic types of delta: (i) Arcuate: This delta is composed of coarse sediments such as gravel and sand and is triangular in shape; e.g., Nile, Ganga, Indus, Mekong, Hwang-ho, Niger; (ii) Bird's Foot or Digitate, composed of very fine sediments called silt, the river channel divides into a few distributaries only and these maintain clearly defined channels across the delta. The Mississippi Delta is an example. This type occurs in seas which have few currents and tides to disturb the sediments; (ii) Estuarine: This delta develops at the mouth of submerged rivers; e.g. the deltas of the Ob, Elbe and the Vistula.
In the first stage, deposition divides the river into several distributaries. Spits and bars rise and lagoons are formed. (Lagoons are shallow stretches of water separated from the sea by a barrier such as a spit.) In the next stage the lagoons begin to get filled in with sediments, and they become swampy. The delta begins to assume a more solid appearance. In the third stage, the old part of the delta becomes colonised by plants. As a delta grows larger, the old parts merge imperceptibly with the flood plain, and they no longer have the appearance of a delta.

A river and its tributaries drain an area, which is called a 'river basin'. Its boundary formed by the crest line of the surrounding highland is the watershed of the basin. A river system usually develops a patten which is related to the general structure of the basin. A dendritic pattern develops in a region made of rocks which offer the same resistance to erosion and which has a uniform structure. A trellis drainage pattern develops in a region made up of alternate belts of hard and soft rocks which all dip in the same direction and which lie at right angles to the general slope, down which the river flows. A radial pattern develops on a dome or volcanic cone. The river flows outward, forming a pattern like the spokes of a wheel.

A river' at any stage of its development from youth to old age may be rejuvenated, and a young valley may occur in an old landscape. Where the river crosses from the original flood plain to the new flood plain, there may be a waterfall or rapids: this point is called knickpoint.

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