In this article we will discuss about:- 1. Meaning of Rejuvenation 2. Types of Rejuvenation 3. Causes 4. Topographic Expressions.
Meaning of Rejuvenation:
Rejuvenation simply means acceleration of erosive power of the fluvial process (rivers) caused by a variety of factors.
Rejuvenation lengthens the period of cycle of erosion. For example, if the cycle of erosion is passing through senile stage (old stage) characterized by gentle channel gradient, sluggish river flow and broad and shallow alluvial valleys, after rejuvenation (caused either due to substantial fall in sea level or due to upliftment of landmass) the cycle is interrupted and is driven back to juvenile (youth) stage characterized by steep channel gradient and accelerated valley incision.
Types of Rejuvenation:
Rejuvenation is of three types:
1. Dynamic rejuvenation:
(i) Upliftment in the landmass.
(ii) Tilting of land area.
(iii) Lowering of outlet.
2. Eustatic rejuvenation:
Changes in sea level due to:
(i) Diastrophic events (subsidence of sea floor or rise of coastal land).
(ii) Glaciation causing fall in sea level.
3. Static rejuvenation:
(i) Decrease in the river load.
(ii) Increase in the volume of water and consequent stream discharge due to increased rainfall or melt-water.
(iii) Increase in water volume of the main river due to river capture.
Causes of Rejuvenation:
As pointed out earlier, the basic cause for rejuvenation of fluvial cycle of erosion (acceleration of erosive power of the river) is negative change in the base level of erosion (which is determined by the sea level) which is caused by a host of factors. Negative change of base level of erosion is always related to negative change in sea level (fall in sea-level) which is also called as eustatic movement as it is widespread and global phenomenon.
It may be pointed out that fall in sea level (and hence causing negative movement/change in base level of erosion) steepens the channel gradient resulting into increased kinetic energy of the fluvial process which resorts to valley incision with renewed vigour.
The eustatic negative change in sea level is caused during glacial ages when most of seawater is locked on the continents as thick cover of ice sheets. The consequent lowering of sea level causes steepening of channel gradient of streams which are infact rejuvenated and are engaged in active down-cutting of their valleys.
The Pleistocene glaciation of the northern parts of N. America and Eurasia caused widespread rejuvenation in the temperate and tropical zones. Four river terraces of the Red river of the U.S.A. have been related to four periods of advancement of ice sheets i.e. Nebraskan, Kansan, Illinoin and Wisconcin glacial periods of the Pleistocene Ice Age.
Negative change in sea level causing rejuvenation locally and regionally is also caused because of subsidence of sea floor in relation to coastal land due to tectonic factors.
Local or regional upliftment of landmass causes interruption in the flivial cycle of erosion and rejuvenates the fluvial processes (streams). Such type of regional rejuvenation caused by secular rise in the landmass has been reported from several parts of the Chotanagpur Highlands of Jharkhand which experienced 3 phases of upliftment in response to three episodes of upliftment of the Himalayas during Tertiary period.
The Patlands of the Ranchi plateau and Palawan uplands (Jharkhand) were subjected to an upliftment of 305 m resulting in the interruption of fluvial cycle of erosion and rejuvenation of N. Koel river and its tributaries. The nick points and resultant waterfalls on Burha river (Burhaghaugh falls 142 m, Gutamghaugh falls, 36.57 m and Ghoraghughra falls 7.62 m), a tributary of the N. Koel river, on Pandra river (Ghagri falls of 43 m), on Sankh ri ver(Sadnighaugh falls of 61 m). on Jori river (Jalimghaugh falls of 37 m), on Ghaghra river (Nindighaugh falls of 45.72 m) etc. indicate rejuvenation.
Lowering of outlets of streams also causes rejuvenation due to release of extra volume of water in the concerned river. Such rejuvenation (due to increase in the volume of water) also occurs when the water supply suddenly increases due to river capture (supply of extra water of the captured stream to the captor stream).
Topographic Expressions of Rejuvenation:
The typical landforms resulting from interruptions in the fluvial cycle of erosion and from rejuvenation resulting in the formation of mossaic of poly or multi-cyclic landforms include topographic discordance, valley in valley or multi-storeyed valleys, uplifted peneplains, incised meanders, paired terraces, nick points etc.
Topographic discordance refers to the creation of older topographic forms above and younger forms below. In other words, when the topographic accordance or uniformity from the top of the river valley to its bottom is not maintained rather is disturbed due to interruption in fluvial cycle of erosion caused by rejuvenation, the resultant topographic expression is called topographic unconformity or discordance wherein the upper part of the valley reveals the sign of senile or mature stage whereas the lowest part of the valley belongs to youth stage.
The river develops flat and shallow valley at the end of mature and beginning of senile stage but if there is sudden negative change in the base level of erosion caused either by fall in sea level or upliftment of landmass, the river is rejuvenated and begins active downcutting of its valley due to increased erosive power. Thus, a deep and narrow valley is formed within flat and broad valley.
This new deep and narrow valley is flanked by terraces on its either side which represent earlier older valley. Such topography is called ‘valley in valley topography’ or ‘two storeyed valley’ or ‘two cycle valley’. By the march of time the rejuvenated river deepens its valley to the new base level and thus forms second broad and flat valley within the first broad and flat valley formed during first cycle of erosion.
Suppose, if there is again upliftment of landmass or fall in sea level, the cycle is again interrupted and the river is rejuvenated. Consequently, again a narrow and deep valley is formed within previously formed two storeyed valley. Now there are two paired terraces on either side of the new deep and narrow valley. Thus, the resultant form of the valley is called three storeyed valley. Such topographic forms, where there are more than one valley in a single river’s cross profile, are called ‘multi storeyed or multi- cyclic valleys’.
The Damodar valley at Rajroppa in Hazaribagh (Jharkhand) is a typical example of polycyclic valley or topographic discordance which is characterized by two-storeyed valley. The Damodar River developed its broad and flat valley of senile stage before the onset of Tertiary upliftment.
The river was rejuvenated due to upliftment of landmass during Tertiary period caused by the side effects of the Himalayan orogeny and thus the Damodar excavated its new deep and narrow valley of youthful stage within its broad and flat valley of senile stage.
The Bhera river coming from over the Ranchi plateau makes a water fall while joining the Damodar river and thus presents an example of a hanging valley. Such topographic discordance is also observable in the valley of the Narmada River at Bheraghat (downstream from Dhunwadhar falls, 15 km away from Jabalpur city in M.P.).
Three storeyed valleys have been located in the Himalayas where the rivers were rejuvenated thrice due to three major episodes of upliftment of the Himalayas during Tertiary orogeny. Nearly all the major rivers of the Uttaranchal Himalayas are flanked by three sets of terraces on their either side.
Uplifted peneplains are formed due to interruption caused by rejuvenation consequent upon regional upliftment. The uplifted peneplains are represented by their remnants of accordant summit levels which rise above the general ground surface of the present-day planation surface. Uplifted peneplains are in fact the results of successive cycles of erosion wherein several fluvial cycles of erosion are completed in succession.
Three uplifted peneplains have been identified in the Applachians (which are indicative of successive phases of upliftment, consequent rejuvenation and cycles of erosion) e.g. (from older to younger) Schooley peneplain (after Schooley mountain), Harrisburg peneplain (after Harrisburg mountain), and Sammerville peneplain.
The Patlands of the Ranchi plateau is a typical example of uplifted peneplain which is higher than the central Ranchi plateau (610m a.m.s. 1). The granitic-gneissic surface (910 m. a.m.s. 1) of the western highlands has a capping of 154m thick basaltic lava (now weathered to laterites) cover of Cretaceous period. Prior to Cretaceous lava flow the entire Ranchi plateau, including the present western highlands, was peneplained by Jurassic period, the western part of which received lava cover of 154 m thickness during Cretaceous period.
This western part (610 m + 154 m lava) was uplifted by 305 m in Tertiary epoch and thus the granitic-gneissic surface of 915 m height lying below 154 m thick lateritic basalt is an example of uplifted peneplain (fig. 16.7). The North Koel and its numerous tributaries have dissected patlands and segmented them into several small tableaux locally known as ‘pats’ (which are fine examples of mesas and buttes) such as Netarhat pat, Khamar pat, Rudni pat, Jamira pat, Raldami pat, Bangru pat etc.
Five alternative terms are in use for river meanders viz. incised meanders, entrenched meanders, intrenched meanders, inclosed meanders and ingrown meanders. Incised meanders are the representative features of rejuvenation and polycyclic reliefs and are developed through vertical erosion leading to valley incision consequent upon renewed erosive power due to rejuvenation.
The narrow and deep meanders formed due to accelerated rate of valley incision caused by rejuvenation within simple broad meanders (having wide and shallow valleys) developed by lateral erosion during 1st cycle of erosion are called incised meanders.
Which are further divided into:
(i) entrenched 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 outer side(convex side) is characterized by gentle valley side slopes.
The meandering valley of the Karo River downstream from Pheruaghaugh falls at the southern margin of the Ranchi plateau has been considerably incised due to rejuvenation and hence presents an ideal example of incised meander. The Damodar gorge near Rajroppa is typical example of incised meander. Similarly, Bheraghat gorge of the Narmada near Jabalpur (M.P.) is fine example of incised meander.
Knick point, simply called as nick point or only ‘nick’ represents breaks in slope in the longitudinal profile of a river caused by rejuvenation (fig. 16.6). This is why nick point is also called as head of rejuvenation which registeres gradual recession upstream. These breaks in channel gradient or nick points denote sudden drops of elevation in the longitudinal profile of the rivers and allow water to fall down vertically giving birth to waterfalls of varying dimensions.
These are called as nick point falls or simply nickfalls. Hundru falls (76.67 m) on the Subarnarekha river (near Ranchi city), Johna or Gautamdhara falls at the confluence of the Raru and the Gunga rivers (to the east of Ranchi city), Dassam falls (39.62 m and 15.24 m) on the Kanchi river (east of Ranchi city), Burhaghaugh falls (148 m) on the Burha river, a tributary of the North Koel river, Dhunwadhar falls on the Narmada river (near Jabalpur, M.P.), major falls of Rewa plateau, M.P. (e.g. Chachai falls, 127 m on the Bihar river, Kevti falls, 98 m on the Mahana nadi, Tons or Purwa falls, 75 m on the Tons river, Odda falls, 145 m on the Odda nadi etc.) are the examples of nick points caused by rejuvenation.
Paired terraces (fig. 16.5) are also significant features of poly-cyclic reliefs. Three pairs of terraces ire found in the Himalayas indicating three phases of uplift and consequent rejuvenation.