Landforms by Running Water and its Three Stages

Landforms by Running Water and its Three Stages

Denudation is general lowering of earth’s surface by agents of erosion such as Wind, Water, Ice, Waves etc. Unlike glaciers & snow, which are confined to cold & temperate latitudes;  waves, which acts only on coastlines;  winds, are only efficient in deserts; the effect of running water is felt all over the world, thus, making it the most important agent of denudation.

The source of river is generally found in an upland region with a slope down for the run offs

Hence, the uplands form the catchment areas of the rivers & the crest of mountains become the divide or watershed from which the streams flows down the slope

The initial stream that exists as a consequence of the slope is called the consequent stream

As the consequent stream wears down the surface, it is joined by several tributaries from either side

Course of a River

Upper Course

• Gorge
• Rapids
• Waterfall
• River Capture Middle Course
• Interlocking spurs
• River Cliffs
• Meanders

Upper or Mountain Course (Youth stage)

• Begins at the source of the river near the watershed, generally at the crest of mountain range
• Flow is very swift as it descends the steep slopes & predominant action of the river is vertical erosion
• Valley developed is thus deep, narrow & distinctively V shaped which sometimes results in formation of gorges & canyons

Some of the features associated with the upper course of the river –River Capture

• Also known as river piracy or river beheading
• Its development depends upon different rate of back cutting (headword erosion) into a divide mainly due to difference in precipitation received by streams.

• If one side of the divide cut more rapidly than the other then its greater erosive power will succeed in enlarging its basin at the expense of weaker stream.
• For example in given figure, Stream A may eventually break through the divide & capture & pirate stream B.
• The bend at which the piracy occurs is termed as Elbow of the capture & the beheaded stream is called as misfit.
• The valley below the elbow is wind gap which may be useful for road & rail route

Rapids, Cataracts & Waterfalls

• Liable to occur in any part of river course but most numerous in mountains course where changes in gradient are more abrupt & frequent.
• Due to unequal resistance of hard & soft rocks transverse by a river, the outcrop of hard rock may cause a river to jump & fall, known as
• Similar falls of greater dimensions are referred as
• When river plunges down in a sudden fall via. some height, they are called
• Their greater force usually wears out a plunge pool beneath.

Middle or Valley Course (Maturity stage)

• In the middle course, lateral corrosion tends to replace vertical corrosion; active corrosion of the bank thus widens V shaped valleys.
• Volume of water increases with the confluence of many tributaries which increases river’s load.
• The work of the river is predominantly transportation with some deposition in clearer manner, although velocity does not decrease.

Some of the features associated with the Middle course of the river 

  Interlocking spurs

• Downstream, interlocking spurs that project from both side of the valley are cut back into a line of bluffs.
• Rain-wash, soil creeps, landslides & gullying gradually widens the valley, cutting back the sides
• As the stream flows on, the meanders migrate progressively outward with the interlocking spurs alternating with the undercut slopes.
• Meanders in the middle course are only the beginning of the downward swing as bends are restricted by the interlocking spurs.
• In the lower course, the loops are enlarged across the level plain & meanders are fully developed

River cliffs & Slip off slopes

• When the flow of water PQ enters the bend of the river, it dashes straight into Q, eroding the outer bank into a steep river cliff at Q.

• The water piles up on the outside of the bend due to centrifugal force.
• A bottom current RS is setup in a cork screw motion & is hurled back into midstream & inner bank. Shingle is thus deposited here at S, where the slip off is gentle.
• The outer bank is therefore the bank of continuous erosion & the inner bank is the bank of continuous deposition.

Meanders

• As water flowing under gravity seldom flows straight for long distance, a winding course soon develops

• The irregularities of the ground forces the river to swing in loops forming Meanders

Lower or Plain Course (Old Stage)

• The river moving downstream across a broad, level plain is heavy with debris brought down from the upper course.
• Vertical corrosion has almost ceased though lateral corrosion still goes on to erode its banks further.
• Volume of water is greatly swelled with work of the river is mainly depositional, building up its bed & forming flood plains.

BEST OF LUCK

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