In this article we will discuss about the sequent and insequent drainage system of streams.
1. Sequent Drainage Systems:
i. Consequent streams:
Consequent streams are the first streams to be originated in a particular region. These streams have their courses in accordance with the initial slope of land surface. In other words, the consequent streams follow the regional slope. These are also called dip streams.
In a region of folded structure (when the crustal rocks are folded due to lateral compressive forces into parallel anticlines and synclines) consequent streams are formed in the synclinal troughs. Such consequent streams are called synclinal consequent streams, which become the master consequent streams of trellis drainage pattern at much later date.
The first streams to be initiated on a newly emerged coastal plain are consequent streams which are parallel to each other and thus form parallel drainage pattern. The longest stream of the whole system of consequent streams is called master consequent (fig. 17.1).
Most of the streams draining the coastal plains of India are the examples of consequent streams. The most ideal landscapes for the origin and development of consequent drainage system are domes and volcanic cones.
Consequent streams are divided in two types e.g.:
(1) Longtudinal consequent (which follows the axis of the depression or syncline in a folded structure) and consequent streams generally join the master or
(2) Lateral consequent (which follows the sides of the logitudinal or synclinal consequent more or less at depressions or the sides of the anticlines).
Lateral consequent streams generally join the master or longitudinal or synclinal consequent more or less at right angle.
ii. Subsequent streams:
The streams originated after the master consequent stream and following the axis of the anticlines or ridges and the strikes of beds are called subsequent streams. Some of the scientists have opined that the subsequent streams originate on the flanks of the anticlines and join the master consequent at almost right angle (which means that the lateral consequents, as referred to above, are the subsequents) while others maintain that the subsequents are parallel to the master consequent. S. W. Wooldridge and R.S. Morgan (1960) have opined that it should be noted that all the first generation tributaries to consequent streams are subsequent in the sense that they arise subsequently to the establishment and incision of the consequent streams.
According to them ‘such streams, starting as gullies on the sides of the primary consequent valleys, discover and explore belts of structural weakness, due to softer strata, fault, or joint-planes, and shatter zones’.
E Ahmad (1985) has remarked that “the term subsequent may be used generally not to indicate a chronological sequence but a relation between drainage on the one hand and geology, structure and tectonics on the other. Generally, it refers to the streams transverse to the master consequents.” The Asan River, a tributary of the Yamuna River and the Song River, a tributary of the Ganga River in the Dehra Dun valley (in filled alluvial plain) are the examples of subsequent streams while the Yamuna and the Ganga are the master consequents (fig. 17.2).
iii. Obsequent streams:
The streams flowing in opposite direction to the master consequent are called obsequent streams. In fact, obsequent streams are also consequents because they also follow the slopes of the ranges. The streams originating from the northern slopes of the west-east stretching ranges of the Himalayas flow northward to meet the east-west flowing tributaries (subsequent streams) of the southward draining master consequent streams.
For example, the Mahabharat Range of the Lesser Himalaya has issued several streams from its northern slopes. These northward flowing streams join the subsequent stream Sun Kosi which runs west to east, as obsequent streams because these are opposed to the directions of the master consequents like the Ganga and the Yamuna.
Similarly, several streams originating from the northern slopes of the Siwalik Range drain due northward to join east-west subsequent streams of the southward flowing master consequents of the Ganga and the Yamuna, north of the Someshwar Range of Dundwa Range (of the Siwalik Range). These northward flowing tributaries are the examples of obsequent streams.
iv. Resequent streams:
The tributary streams flowing in the direction of the master consequents are called resequents. These are originated at much later date in comparison to the master consequents. Since they are of recent origin, and hence they are called resequent.
The resequent streams are originated during the initiation of second cycle of erosion in a folded structure. The gradual denudation of folded mountains during the first cycle of erosion results into inversion of relief with the passage of time wherein anticlinal ridges and synclinal valleys are converted into anticlinal valleys and synclinal ridges respectively (fig. 17.3).
Thus, logitudinal streams are developed in the anticlinal valleys. These features are peneplaned by the end of the first cycle of erosion. The initiation of second cycle of erosion begins with the excavation of new valleys in the synclines. Thus, the streams developed in the synclinal portions become resequent streams which though analogous to the original longitudinal consequents developed during the first cycle of erosion, but in fact, they are hundreds of metres below the initial surface (fig. 17.3).
2. Insequent Drainage System:
The streams which do not follow the regional slopes and drain across the geological structures are called insequent or inconsequent streams.
Antecedent and superimposed streams are the best representative examples of insequent drainage systems:
i. Antecedent drainage system:
Antecedent streams are those which are originated prior to the upliftment of land surface. In other words, antecedent streams antedate the upliftment of an upland or mountain across which they have maintained their present courses through continuous down-cutting of their valleys.
It is, thus, obvious that if a river has developed in a particular region and if the subsequent uplift or up-warping of the land area across the pre-existing river does not divert or deflect the course of the river and the river maintains its previous course through down-cutting its valley at the rate equivalent to the rate of uplift of the land area, the river is called antecedent or anti-consequent. It may be pointed out that the concept of antecedence of a particular river is based on guesses and conjectures because neither the rate of the uplift of land area nor the rate of down-cutting by the rivers is known.
The nature and the rate of upliftment of land area is very important parameter for the development of antecedent drainage system because rivers cannot maintain their previous courses during all types of upliftment. For example, if the upliftment of the area occurs very rapidly and is completed within a very short period of time, the rivers draining through that area cannot maintain their previous courses because the rate of down-cutting of their valleys cannot keep pace with the rate of the upliftment and thus the rivers are dismembered and adopt different courses depending on local (new) conditions.
On the other hand, if the rate of upliftment is such that the rivers are able to degrade their valleys through active downcutting (valley incision) at the rate equal to the rate of the upliftment, they become able to maintain their previous courses and thus they become antecedent streams. The nature and dimension of the upliftment also determine the antecedence of the rivers. It is the local and not the regional upliftment which gives birth to the development of antecedent streams.
The origin and development of antecedent drainage system can be explained with the help of a diagram (fig. 17.4). ‘A’ river (fig. 17.4 (1) has fully developed its valley and course in a region of almost flat topographic surface. The river is flowing from west to east. At a later date there is upliftment of land area across river ‘A’ at point B. The land is rising in a form of a ridge with slow rate of upliftment. This local upliftment of the land rejuvenates the river ‘A’ due to which it deepens its valley with accelerated rate of downcutting.
If the rate of downcutting (valley deepening) equals the rate of uplift the bed of the river valley remains constant and the river maintains its usual flow direction. River ‘A’ in the second stage (fig. 17.4(2) has deepened its valley so much so to match the rate of upliftment.
It is apparent from figure 17.4(2) that ‘A’ river has cut across the newly uplifted ridge at ‘C’ and has maintained its previous course. The river continues to deepen its valley through active downcutting so long as the upliftment continues. Thus, the river develops very deep and narrow gorges across the uplifted land area (ridge) wherein the valley sides are of convex slope and rise almost vertically from the valley floor (fig. 17.4 (3).
It may be pointed out that tributaries of the master antecedent streams cannot deepen their valleys at par with their master streams and hence their valleys are at higher level than the valleys of their master streams. Thus, the tributary valleys become hanging valleys.
Examples of antecedent streams are found in almost all of the folded mountains of the world. Many of the major Himalayan rivers are the examples of antecedent streams e.g., the Indus, the Sutlej, the Ganga, the Ghaghra, the Kali, the Gandak, the Kosi, the Brahmaputra etc. It may be pointed out that the Himalayan mountain system consists of three parallel ranges running from west to east viz., (from north to south) the Greater Himalaya, the Lesser Himalaya and the Outer Himalaya or Siwalik Range.
Three kinds of antecedent streams are identified in terms of their antecedence to the aforesaid three ranges of the Himalayas:
(1) The Indus, the Sutlej, the Ganga, the Ghaghra, the Kosi, the Brahmaputra etc., are antecedent to all of the three ranges of the Himalayas as they cut across the Greater, the Lesser and the Outer Himalayan ranges. It is believed that these rivers were present before the folding and upliftment of the Himalayan ranges.
With the upliftment of these ranges during Tertiary period these rivers maintained their southward courses through downcutting their valleys in harmony with the rate of upliftment. All of these rivers have formed deep and narrow gorges while cutting across the Himalayan ranges. The Indus River has dug out a 17,000- foot deep gorge across the Greater Himalayas near Gilgit.
It may be pointed out that a group of geologists does not believe in the antecedence of the Indus River. According to them the Indus is an example of consequent stream because consequent streams might have developed on the southern slopes of the Himalayas. These rivers might have extended their courses northward through the process of headward erosion resulting into gradual backward shifting of their water divides.
Ultimately, the streams of the southern slopes of the Himalayas might have captured the streams of the northern slopes and thus they might have extended their courses across the Himalayan ranges. The Arun River has been quoted as the best example of Antecedent River by L.R. Wager (1937).
The Arun River flows from west to east in its uper reaches following the structural grain of the region to the north of the Himalayas. It suddenly turns due southward and drains across the Himalayas through several deeply entrenched narrow gorges. A few geologists opine that the sudden southward bend of the Arun River is due to river capture. The advocates of the antecedent origin of the Arun River maintain that the convex course of the river across the Himalayas could be possible only through active downcutting of the valley in accordance with the rate of upliftment. A series of terraces on either side of the valley of the Arun River also validate its antecedent origin.
(2) The rivers, which are antecedent to only two ranges of the Himalayas viz., the Lesser and the Outer (Siwalik Range) Himalayas, are the Jhelum, the Yamuna, the Ravi, the Ramganga etc.
(3) The streams, which are antecedent only to the Outer Himalaya or Siwalik Range, include the Kamla, the Rapti, the Gola etc.
ii. Superimposed drainage system:
Like antecedent streams superimposed streams are also not adjusted to regional geological structures and slope and thus are insequent or anti-consequent streams. Superimposed stream means a river which, flowing on a definite geological formation and structure, has inherited the characteristics of its previous form developed on upper geological formation of entirely different structural characteristics.
In other words, a superimposed drainage is formed when the nature and characteristics of the valleys and flow direction of a consequent stream developed on the upper geological formation and structure are superimposed on the lower geological formation of entirely different characteristics. It is not necessary that the upper geological formation is conformal to the lower geological formation, rather it happens that sometimes the upper rock cover is entirely different from the underlying geological formation viz., the upper cover may consist of horizontally bedded sedimentary rocks while the lower cover may be composed of folded sedimentary beds, or domed structure or batholithic intrusives.
The consequent streams are developed on almost flat ground surface of the horizontally bedded sedimentary rocks. These streams develop their valleys through vertical erosion (downcutting). With the passage of time the lower structure is exposed to the river which continues its downcutting and extends its valley downward on the lower geological structure (say anticlinal folds, domes or batholiths) and thus the valley developed on the upper structure is superimposed on the lower structure.
The lower structure has to accept the form of the valley already developed on the entirely different upper structure. Thus, the river maintains the form of its valley, the flow direction and its drainage patterns as usual. Such rivers are called superimposed rivers. Had the lower structure been present on the ground, the nature of drainage system would have certainly been entirely different but the buried different geological structure has no alternative other than to adapt the nature of valley and flow direction developed on the upper structure. Such streams are unconformal to the local geological structure and slopes. Such unconformal valleys have been named as ‘superimposed’ by J.W. Powell (1857) and D. Maw (1886) and ‘superposed’ by W.J. McGee (1888).
The mechanism of the development of a superimposed stream and valley can be explained with the help of a diagram (fig. 17.5). Figure 17.5 depicts such a region which is characterized by a flat ground surface consisting of horizontally bedded sedimentary rocks and the lower structure of anticlinal fold. First of all a consequent stream (fig. 17.5(1) develops on the upper horizontally bedded sedimentary formation. The river is flowing in harmony with the geological formation and structure and local slope.
With the passage of time the consequent stream digs out its valley with slow process of valley deepening. In the second stage (fig. 17.5(2) the consequent stream fully develops its valley on horizontally bedded sedimentary rocks. Now the anticlinal part (third stage, fig. 17.5(3) is exposed to the river as the upper horizontal beds of sedimentary rock have been removed due to gradual downcutting.
This anticlinal lower structure is entirely different from the upper structure of the horizontal beds of sedimentary rocks but the river extends its valley on this lower un-conformal structure of anticline in accordance with its already developed valley on the upper structure i.e. the river degrades the anticline through down-cutting and continues to develop its valley further downward.
The development of river valley goes on unhindered because the anticline becomes a passive factor in controlling the development of the river valley as it has no option other than to accept the form of the valley already developed in the upper structure. Now the consequent river and its valley are superimposed on the anticline and the river maintains its usual course. Had this anticline been initially exposed to the ground surface, the drainage system would have been entirely different.
The fundamental difference between superimposed and antecedent drainage systems lies in the fact that the former represents the superimposition of the valley developed in the upper geological formation and structure on the lower structure irrespective of its types and complexities but without any upliftment of the land area while the latter represents the maintenance of former course of the river and its valley inspite of upliftment of the land area across the river course due to concomitant valley deepening and up warping of the land.
Most of the rivers of the Deccan trap (lava) region of Peninsular India are superimposed because new drainage system was evolved on the new surface formed due to cooling and solidification of lavas erupted during late Cretaceous and early Tertiary periods and these rivers were superimposed on the lower formation after the removal of lava covers. For example, the Subarnarekha river is superimposed on Dalma and phyllite hills to the west of Chandil in the south-eastern Chotanagpur plateaue region of Jharkhand.
The Son River flowing across the Khainjua ridges in the southern part of Rewa plateau (Madhya Pradesh) is a fine example of superimposed river (fig. 17.6). The Son River flows parallel and very close to ‘he Khainjua ridge between 81° 15’ Elogitude and Deolond (fig. 17.6). In fact, the Khainjua ridge of quartzitic sandstones makes the southern bank of the Son valley. The river suddenly cuts across the Khainjua ridge at Deolond and then flows in south-easterly direction (fig. 17.6). After its confluence with the Banas River it again turns northward and cuts across the Khainjua ridge and flows in easterly direction.
The Son cannot be an antecedent river, though it cuts across Khainjua ridges at two places, because:
(i) The Khainjua ridges are composed of lower Vindhyans sedimentaries,
(ii) There are no evidences to support any localized rise in the ridges after Cambrian period,
(iii) The evidence of laterites to the north of Kaimur scarps denotes the spread of basaltic lava of the Deccan trappean age of Cretaceous period which might have buried and obi iterated previous drainage lines, and
(iv) The absence of any significant stream to the north of the Son does not support the idea of the Son being older than the Crectaceous lava flow.
Thus, the Son appears to be superimposed river. It may be argued that the Deccan lava flows during Cretaceous period covered the whole of the Rewa Plateau and consequently buried the pre-existing drainage network and reliefs including Kaimurs and Khainjua ridges. With the initiation of sub-aerial conditions during Tertiary period new drainage lines began to develop over the solidified lava cover (basaltic lava) and the Son developed its present course through several stages.
The lava cover might have been removed through continuous but gradual fluvial erosion. Once the Son established its course over the lava covered surface, it continued to erode its valley over the buried structures and thus its valley became superimposed over the Khainjua ridges.