Fluvial Erosional Landforms

Fluvial Erosional Landforms are landforms created by the erosional activity of rivers.Various aspects of fluvial erosive action include:

• Hydration:the force of running water wearing down rocks.
• Corrosion:chemical action that leads to weathering.
• Attrition:river load particles striking, colliding against each other and breaking down in the process.
• Corrasion or abrasion:solid river load striking against rocks and wearing them down.
• Downcutting (vertical erosion):the erosion of the base of a stream (downcutting leads to valley deepening).
• Lateral erosion:the erosion of the walls of a stream (leads to valley widening).
• Headward erosion:erosion at the origin of a stream channel, which causes the origin to move back away from the direction of the stream flow, and so causes the stream channel to lengthen.

The following are some of the major landforms formed as a result of fluvial erosion:

River Valley

• • The extended depression on the ground through which a stream flows is called a river valley.
  • At different stages of the erosional cycle, the valley acquires different profiles.
  • At a young stage, the valley is deep, narrow with steep wall-like sides and a convex slope.The erosional action here is characterized by predominantly vertical downcuttingThe profile of valley here is typically ‘V’ shaped.
  • A deep and narrow ‘V’ shaped valley is also referred to as gorgeand may result due to downcutting erosion or because of the recession of a waterfall (the position of the waterfall receding due to erosive action).
  • Most Himalayan rivers pass through deep gorges (at times more than 500 metres deep) before they descend to the plains.
  • An extended form of the gorge is called a The Grand Canyon of the Colorado River in Arizona (USA) runs for 483 km and has a depth of 2.88 km.
  • A tributary valley lies above the main valley and is separated from it by a steep slope down which the stream may flow as a waterfall or a series of rapids.
  • As the cycle attains maturity, the lateral erosion (erosion of the walls of a stream) becomes prominent and the valley floor flattens out (attains a ‘V’ to ‘U’ shape).
  • The valley profile now becomes typically ‘U’ shaped with a broad base and a concave slope.

Waterfalls:

• • A waterfall is simply the fall of an enormous volume of water from a great height.
  • They are mostly seen inthe youth stage of the river.
  • Relative resistance of rocks, the relative difference in topographic reliefs, fall in the sea level and related rejuvenation, earth movements are responsible for the formation of waterfalls.
  • Angel Fallsin Venezuela is the world’s highest waterfall, with a height of 979 metres and a plunge of 807 metres.
  • Tugela Falls(948 m) in the Drakensberg mountains, South Africa is the world’s second highest waterfall.

Potholes

• • The kettle-like small depressions in the rocky beds of the river valleys are called potholes which are usually cylindrical in shape.
  • Potholes are generally formed in coarse-grained rocks such as sandstones and granites.
  • Potholing or pothole drilling is the mecha­nism through which the grinding tools (fragments of rocks e.g., boulders and angular rock fragments) when caught in the water eddies or whirling water start dancing in circular manner and grind and drill the rock beds of the velleys like drilling machine and thus form small holes which are gradually enlarged by the repeti­tion of the said mechanism.
  • The potholes go on in­creasing in both diameter (and perimeter) and depth.
  • The diameter of pot holes ranges from a few centime­tres to several metres. The depth of potholes is far more than their diameters.
  • Potholes of much bigger size are called plunge pools. In fact, plunge pools are generally formed at the base of waterfalls due to pounding of rocks by gushing water of the falls (waterfalls).
  • Many of the river valleys are studded with numerous pot­holes. For example, in Chotanagpur highlands where the rivers have been rejuvenated due to upliftment effected during Tertiary period.
  • The basaltic bed of the Gaur river near Bhadbhada (east of Jabalpur, M.P.) presents a magnifi­cent view of numerous potholes of various dimension. Pothole drilling is the effective mechanism of valley deepening.

Terraces

• • The narrow flat surfaces on either side of the valley floor are called river terraces which represent the level of former valley floors and the remnants of former (older) flood plains.
  • Sometimes, the river valleys are frequented by several terraces on either side wherein they are arranged in step-like forms.
  • River terraces are generally formed due to dissection of fluvial sediments of flood plains deposited along a valley floor.
  • There are much variations in terraces as regards their morphology, structure and mode of ori­gin. River terraces are classified in various ways.
  • For example, terraces are divided into: Paired terraces and Unpaired terraces on the basis of nature of erosion.
  • Paired terraces are formed due to rapid rate of vertical erosion resulting into the occurrence of ter­races on both the sides of the river valley almost at the same level.
  • It may be pointed out that paired terraces mean occurrence of terraces on both the sides of valley at the same height.
  • Unpaired terraces are formed due to concamitant vertical erosion (valley deepening) and lateral movement of the channel.

Structural Benches:

• • The step-like flat surfaces on either side of the present lowest valley floors are called terraces.
  • The benches or terraces formed due to differential erosion of alternate bands of hard and soft rock beds are called structural benches or terraces because of lithological control in the rate of erosion and consequent develop­ment of benches.

Gullys and rills

• • Gulley is a water-worn channel, which is particularly common in semi-arid areas.
  • It is formed when water from overland-flows down a slope, especially following heavy rainfall, is concentrated into rills, which merge and enlarge into a gulley.
  • The ravines of Chambal Valleyin Central India and the Chos of Hoshiarpur in Punjab are examples of gulleys.

Meanders

• • A meander is defined as a pronounced curve or loop in the course of a river channel.
  • The outer bend of the loop in a meander is characterized by intensive erosion and vertical cliffs and is called the cliff-slope side. This side has a concave slope.
  • The inner side of the loop is characterized by deposition, a gentle convex slope, and is called the slip-off side.
  • The meanders developed during first cycle of erosion by a stream are called simple meanders. These are formed by lateral erosion.
  • These meanders may be wavy, horse-shoe type or oxbow type.
  • Incised meanders are the representative fea­tures of rejuvenation and are developed through verti­cal erosion leading to valley incision or deepening.
  • The narrow and deep meanders formed due to acceler­ated rate of valley incision caused by rejuvenation (either due to upliftment of land area or fall in sea level) inside simple meanders (having wide and shallow valleys) developed by lateral erosion during first stage of cycle of erosion are called incised meanders.
  • Simple meanders develop over loose geomaterials (such as alluvium) as well as over resistant bedrocks but incised meanders are always dug out in bedrocks.
  • Five terms are in use to indicate incised meanders which are developed due to vertical erosion (downcutting or valley incision) of bedrock viz.: Incised meanders, Entrenched me­anders, Intrenched meanders, Inclosed mean­ders and Ingrown meanders.
  • Inclosed and incised meanders represent those meanders of deep and narrow valleys which are inclosed by rocky walls.
  • In fact, incised meanders mean the formation of meanders in older meanders through downcutting of valley floors.

Oxbow lakes:

• Sometimes, because of intensive erosion action, the outer curve of a meander gets accentuated to such an extent that the inner ends of the loop come close enough to get disconnected from the main channel and exist as independent water bodies called as oxbow lakes.
• These water bodies are converted into swamps in due course of time.
• In the Indo-Gangetic plains, southwards shifting of Ganga has left many oxbow lakes to the north of the present course of the Ganga.

Peneplains:

• • Peneplains represent low featureless plain hav­ing undulating surface and remnants of convexo-con- cave residual hills. These are, in fact, the end products of normal cycle of erosion.
  • This refers to an undulating featureless plain punctuated with low-lying residual hills of resistant rocks. It is considered to be an end product of an erosional cycle.
  • Fluvial erosion, in the course of geologic time, reduces the land almost to base level (sea level), leaving so little gradient that essentially no more erosion could occur.
  • These are frequented with low residual hills known as monadnocks (named by W.M. Davis after Monadnock hills of New England region, USA) which are left out due to less erosion of relatively resistant rocks.
  • The end product of normal or fluvial cycle of erosion has been variously named by different geomorphologists. e.g., peneplain (W.M. Davis), endrumpf ((W. Penck), panplain (C.H. Crickmay), pediplain (L.C. King), etchplain (Pugh and Thomas), panfan (A.C. Lawson) etc.