The significant landforms resulting from fluvial erosion by streams include river valleys, waterfalls, pot holes, structural benches, river terraces, river meanders, ox-bow lakes and peneplians etc.
Erosional Landform # 1. River Valleys:
The valleys carved out by the rivers are significant erosional landforms. The shape and dimension of fluvially originated valleys change with the advancement of the stages of fluvial cycle of erosion. The valley formed in the youthful stage of fluvial cycle of erosion and in the initial stage of valley development is V-shaped having steep valley side slope of convex element.
The valley is very deep and narrow, both the valley sides meet together at the valley floor and thus water always touches the valley sides. Such type of V- shaped valleys are the result of accelerated rate of down-cutting (vertical erosion or valley deepening).
The valleys are gradually widened due to lateral erosion with the advancement of the stage of cycle of erosion and they become quite broad with flat valley floor and uniform or rectilinear valley side slopes during mature stage or valley development and fluvial cycle of erosion.
They are further transformed into very broad and shallow valleys having concave valley side slope of very gentle gradient during old stage.
V- shaped valleys are divided into two types viz.:
(1) Gorges, and
(1) Gorges and canyons represent very deep and narrow valleys having very steep valley side slopes say wall-like steep valley sides. It is difficult to draw a line of distinction between these two types of valleys. Normally, a very deep and narrow valley is called a gorge and the extended form of a gorge is called a canyon (fig. 18.1).
Gorges are formed due to active downcutting of the valleys through the mechanism of pothole drilling during juvenile (youth) stage of the fluvial cycle of erosion. Gorges are also formed due to recession of water falls. Most of the Himalayan Rivers have carved out deep and narrow gorges.
The significant gorges formed due to recession of waterfalls include Hundrughagh gorge on the Subarnarekha river (near Ranchi), gorge of the Raru river below Johna or Gautamdhara falls (east of Ranchi), Dassamghagh gorge below Dassamghagh falls on the Kanchi river (east of Ranchi), Pheruaghagh gorge on the South Koel river (south of Ranchi), Chachai gorge on the Bihar river (Rewa, M.P.), Kevti gorge on the Mahana river (Rewa, M.P.), gorge of the Odda river (Rewa, M.P.) etc.
(2) Canyons are extended form of gorges. Canyons represent very deep, narrow but long valleys. The steepness of the valley sides depends on the nature of the rocks. Relatively resistant rocks support steep valley sides whereas resistant rocks alternated by soft rocks give birth to undulating valley sides. The Grand Canyon of the Colorado River in the state of Arizona (USA) having a length of 482.8 kilometres and depth of 2088.3 m is one of the most important canyons of the world. The Indus River has cut across the Himalayan ranges and flows through 17,000-foot deep gorge and canyon.
Erosional Landform # 2. Waterfalls:
Waterfalls or simply falls are caused because of sudden descents or abrupt breaks in the longitudinal course of the rivers due to a host of factors e.g., variation in the relative resistance of rocks, relative difference in topographic reliefs, fall in the sea level and related rejuvenation, earth movements etc. A waterfall may be defined as a vertical drop of water of enormous volume from a great height in the long profiles of the rivers. Rapids are of much smaller dimension than waterfalls.
Generally, they are found upstream from the main falls but they are also found independently. There is a chain of waterfalls along the junction of the Piedmont and Atlantic coastal plain from New England Region in the north-east to central Alabama in the south-west (USA) wherein all the Atlantic bound streams while descending through the Piedmont make numerous waterfalls.
This chain of water falls is called fall line in the USA. There is also a well-marked fall line in India. This Indian fall line extends between the Purwa or Tons falls on the Tons river (in the north-west part of the Rewa district of Madhya Pradesh) in the west and Sasaram (Bihar) in the east along the junction of the northern foreland of Peninsular India and the Ganga plains.
Hundreds of waterfalls ranging in height between 15m and 180m are found along this fall line as all the major streams emerging from the Kaimur ranges and draining due northward make stupendous water falls while descending through the rim of the northern foreland of the Indian Peninsula.
Significant waterfalls of this fall line are Purwa or Tons falls (70m) on the Tons river (in Rewa district, M.P.), a tributary of the Ganga river, Chachai falls (127m) on the Bihar river (Rewa district), tributary of the Tons river, Kevti falls (98m) on the Mahana river (Rewa district), a tributary of the Tons river, Odda falls (145m) on the Odda river (Rewa district), a tributary of the Belan river (which is itself a tributary of the Tons river), Devdari falls (58m) on the Karamnasha river (Rohtas plateau, Bihar), Telharkund falls (80m) on the Suara West river (Rohtas plateau), Suara falls (120m) on the Suara East river, Durgawati falls (80m) on the Durgawati river (Rohtas plateau), Okharean Kund falls (90m) on the Gopath river (Rohtas plateau), Dhuan Kund falls (30m, Rohtas plateau, near Sasaram) on the Dhoba river, Kuaridah falls (180m) on the Ausane river (a tributary of the Son river, Rohtas Plateau), Rakim Kund falls (168m on the Gayghat river, a tributary of the Ausane river (Rohtas plateau) etc.
Types of Waterfalls:
The waterfalls vary so greatly in terms of their height, shape and size, dimension and volume of water that it becomes difficult to classify them in certain categories.
Generally, waterfalls are divided into two broad categories on the basis of mode of their origin viz.:
(1) Normal waterfalls, and
(2) Minor waterfalls.
Normal waterfalls include those falls which are formed due to variation in the resistance of rocks.
These waterfalls are indicative of youthful stage of stream development and ungraded long profiles of the streams. Minor waterfalls include the falls which are originated due to interruption in the cycle of erosion caused by rejuvenation. Such water falls are called knick-point falls. The following is the detailed scheme of the classification of fluvialiy originated waterfalls.
1. Normal waterfalls:
(i) Step waterfalls,
(ii) Caprock falls,
(iii) Barrier falls, and
(iv) Plateau falls.
2. Minor waterfalls:
(A) Falls originated due to endogenetic forces:
(i) Fault falls,
(ii) Falls due to upliftment.
(B) Falls originated due to changes in the level of valley floors:
(1) Due to lowering of valley floor:
(i) Hanging valley falls,
(ii) Glacial hanging valley falls,
(iii) Falls due to river capture,
(iv) Coastal hanging valley falls. and
(v) Knickpoint falls.
(2) Due to obstructions in the river courses:
(i) Falls due to landslides,
(ii) Falls due to lava dams, and
(iii) Falls due to glacial moraines.
Recession of waterfalls:
It may be pointed out that waterfalls and rapids are not permanent landforms. They disappear when the rivers attain their graded curves and profiles of equilibrium during mature stage of valley development and normal cycle of erosion. In fact, the rivers try to grade themselves through vertical erosion (valley deepening) in relation to base level of erosion (sea level).
The obliteration of waterfalls takes place through two processes viz.:
(1) Horizontal recession through backwasting, and
(2) Lowering of height through downwasting.
Niagra falls are receding at the rate of 1.2 to 1.4 m per year. It has been estimated that Niagra falls have receded upto about 11 km till now. No attempts have been made to record the recession of waterfalls in India.
Erosional Landform # 3. Pot Holes:
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 numersous potholes in Chotanagpur highlands where the rivers have been rejuvenated due to upliftment effected during Tertiary period. The basaltic bed of the Gaur nadi 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.
Erosional Landform # 4. 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 (fig. 18.8).
Erosional Landform # 5. River 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:
(1) Paired terraces, and
(2) 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 (fig. 18.9). 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.
River terraces are also divided into:
(1) Rock terraces, and
(2) Aggradational terraces.
Rock terraces are characterized by bedrock platform covered by fluvial deposits whereas aggradational terraces consist of very thick deposits of fluvial sediments. River terraces are generally formed due to erosion of former flood plains consequent upon rejuvenation caused by either upliftment of the land- mass or fall in sea level. Alternatively, river terraces are divided into alluvial terraces and strath terraces (stony terraces).
The mechanism of the formation of river terraces may be explained in the following manner. The rivers form extensive flood plains during late mature stage and attain their graded curve of profile of equilibrium. The flood plains consist of thick deposits of alluvia and gravels. The rivers are rejuvenated due to sudden negative change (fall) in sea-level. Consequently, the erosive capacity of the rivers is increased substantially.
Thus, rejuvenated rivers deepen their valleys due to accelerated rate of vertical erosion. Now the rivers form their new narrow valleys within former flat valleys and thus terraces (fig. 18.9) are formed on both the sides. The rivers are again rejuvenated due to fall in sea level and new narrow valleys are formed due to vigorous valley deepening consequent upon second rejuvenation. This process leads to the formation of second pair of terraces (fig. 18.9) and so on.
Erosional Landform # 6. River Meanders:
River meanders refer to the bends of longitudinal courses of the rivers. The bends of sinous rivers have been named meanders on the basis of Meander River of Asia Minor (Turkey) because it flows through numerous bends.
Each bend of a meander belt has two types of slopes of valley sides:
(1) One side is characterized by concave slope where the channel strikes the valley sides directly, with the result concave side is subjected to severe erosion resulting into the formation of vertical cliffs. This side of the meander belt is also called as cliff-slope side. The other side of the meander belt is characterized by convex slope which receives deposition mostly of sands and gravels but sometimes alluvium is also deposited. This convex side is characterized by gentle slope and is called the side of slip-off slope (fig. 18.10).
The shape of meander is usually semi-circular but sometimes it is also circular. The length of a meander belt can be found out on the basis of the channel width because meander belt is usually about 15 to 18 times the width of bankfull channel.
It may be pointed out that the meandering is a natural process which is governed by a number of environmental factors viz. lithological characteristics, topographic characteristics, general slope, vegetation, annual precipitation, stream discharge and of course the stage of river development and cycle of erosion (i.e., timal factor).
All streams meander in all types of terrains (e.g., mountainous and hilly terrains, dissected upland, alluvial plains, plateaux, coastal plains etc.) but their magnitude varies according to ground slope and nature of geomaterials. Meandering is most pronounced in the regions characterized by even surface and gentle slope, alluvial deposits and sufficient stream discharge. Theoretically, the streams should adopt straight path (referred to as expected path) but no streams adopt straight paths rather they register significant departures from expected straight paths.
The degree of deviation of observed path (actual course of a stream) from expected path (theoretical straight course), known as sinuosity index tells the magnitude of meandering. If the value ranges between 1 and 1.3, the stream is said to be sinuous and the stream becomes meandering if the value exceeds 1.3.
The gradient of highly meandering streams ranges between 20 cm to 10 m per kilometre. All of the alluvial streams of the Northern Plains of India have developed meandering courses.
The Gomti river (Uttar Pradesh) is a typical example of highly meandering stream of alluvial plains because of the fact that its channel gradient is very low (9 cm per kilometre between Lucknow and Sultanpur). The Ganga River has developed highly meandering course between Allahabad and Varanasi due to very low channel gradient (6 cm per kilometre). Besides, Ramganga, Sai (a tributary of the Gomti River), Rapti, Ghahghra, Punpun, Burhi Gandak, Kosi etc. have also developed highly meandering courses.
Meanders are the result of erosion and deposition both.
Meanders are divided into two major types on the basis of the nature of fluvial erosion e.g.:
(1) Meanders developed through lateral erosion (normal meanders),
(2) Meanders developed by vertical erosion or valley deepening (incised meanders), and
(3) Misfit or unfit meander is also identified as the third type of meanders.
Morphologically, river meanders are divided into 3 types viz.:
(1) Wavy type of meanders,
(2) Horse-shoe type of meanders, and
(3) Ox-bow or bracelet type of meanders.
The wavy meanders are very simple in plan (fig. 18.11(2)) wherein the meander necks are wide apart. Such meanders have been developed by the major streams in the Himalayas. The horse-shoe types of meanders are those in which the beds are highly curved (fig. 18.11 (3)) and the arms of meanders are brought closer to each other with the result the meander necks become very narrow.
The oxbow or bracelet (a type of ornament of women) types of meanders are those which have almost circular bends with high curvature. Most of the alluvial rivers of the Northern Plains of India have developed horseshoe and ox-bow types of meanders.
(1) Simple meanders:
The meanders developed during first cycle of erosion by a stream are called simple meanders. These are formed by lateral erosion.
Simple meanders or say monocyclic meanders are divided into three types on the basis of their morphological characteristics e.g.:
(i) Wavy meander,
(ii) Horseshoe type of meander. and
(iii) Ox-bow or bracelet type of meander.
There are certain necessary conditions for the development of simple meanders, (a) Over-loaded streams cannot form meanders because their total power is spent in the transportation of huge amount of sediments. Such rivers are always engaged in depositional activity.
The streams in youthful stage are also not capable of forming meanders because they are actively engaged in incising their valleys through down-cutting and related valley deepening. Thus, mature streams are more capable of forming meanders because they resort to lateral erosion and consequently valley widening more than valley deepening. The most ideal conditions required for development of meanders are alluvial plains, gentle slope, sufficient amount of precipitation and general absence of vegetation.
A minor obstruction in the free flow of the streams in flood plains diverts their courses from straight course and thus the process of meander formation begins with the initiation of very minor bends in the longitudinal courses of the alluvial streams (formation of wavy meanders, fig. 18.11 (1 and 2)).
The channel currents strike against the concave side of the open meander bends and cut the loose geomaterials (alluvium) and thus there is continuous sharpening of meander bends resulting into high degree of curvature of meander loops (formation of horse-shoe type of meander, fig. 18.11 (3).
The processes of erosion of concave sides and deposition of sediments on convex sides of meander loops, (fig. 18.10) continue and the curvatures of meander loops are made more and more circular (formation of ox-bow or bracelet type of meanders, fig. 18.11 (4) and the river course becomes highly meandering with several ox-bow lakes.
(2) 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 (fig. 18.12).
It may be pointed out that 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.:
(i) Incised meanders,
(ii) Entrenched meanders,
(iii) Intrenched meanders,
(iv) Inclosed meanders and
(v) 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.
Further, incised meanders can be divided on the basis of nature and slope of valley sides into:
(i) Entrenched or intrenched meanders having uniform slopes of both the valley sides of meander loops, and
(ii) Ingrown meanders, which have unequal slopes of valley sides wherein one side of the valley representing concave side is deeply undercut and the other side (convex side, slip-off slope) is characterized by gentle valley side slope and deposition of sediments mainly sands.
Thus, it is apparent that entrenched meanders represent those incised meanders in which the valley floors have been deeply entrenched through vertical erosion consequent upon rejuvenation whereas ingrown meanders are those incised meanders in which one side of the valley has been deeply undercut resulting into the formation of hanging cliff.
Misfit meanders represent those meanders which are formed within the extensive former meanders due to substantial decrease in the volume of water. The rivers develop extensive meander loops and belts in alluvial plains and are braided into several channels which wander in the extensive broad and flat valleys. When, by any reason, the volume of water in the concerned rivers decreases substantially, the channels become narrow.
These narrow channels becomes unable to fit themselves in the broader former valleys and hence they develop their own meandering course of narrow valleys within the older wider meanders. Such narrow meanders within the wider meanders are called misfit meanders because they cannot fit with the latter.
Erosional Landform # 7. Ox-Bow Lakes:
The lakes formed due to impounding of water in the abandoned meander loops are called ox-bow or horse-shoe lakes. When the curvature of the meander loops is so accentuated due to lateral erosion, the meander loops become almost circular and the two ends of meander loops come closer, consequently, the streams straighten their courses and meander loops are abandoned to form ox-bow lakes (fig. 18.13).
It may be pointed out that the formation of oxbow lakes owes to erosion (straightening of river course through the intersection of two ends of meander loops at the meander neck due to lateral erosion) and deposition both (filling and plugging of cut off ends of meander loops through deposition (fig. 18.13). There is frequent sedimentation (alluviation) of oxbow lakes during floods and thus they are converted into swamps in due course of time.
The meandering course of the Ganga River in Uttar Pradesh and its gradual southward shifting has left out a series of palaeochannels and oxbow lakes to the north of the present course of the Ganga. Such palaeochannels and oxbow lakes are now seen in the forms of tanks, ponds and lakes. Several examples of palaeochannels and palaeo-oxbow lakes are still observable in Pratapgarh district of Uttar Pradesh.
Erosional Landform # 8. 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.
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.