In this article we will discuss about:- 1. Meaning of River Capture 2. Evidence of River Capture 3. Conditions 4. Types.
Meaning of River Capture:
River capture is a natural process which is more active in the youthful stage of the valley development because the streams are actively engaged in head-ward erosion and valley lengthening but river capture also occurs during mature and senile stages of the valley development through the process of lateral erosion and meander intersection.
The stronger and more powerful streams (in terms of channel gradient, stream velocity and discharge and kinetic energy) capture the upper courses of weak and sluggish streams. Figure 17.17 depicts the stages of the capture of the Saraswati River by the Yamuna River.
The diversion of the part of the course of a river by another river is called stream diversion or stream capture or stream piracy. The river which captures the course of another river is called the capturing or captor stream while the part of the stream which has been divested of its course and water is called the captured stream.
Evidence of River Capture:
There are four major evidences of river capture viz.:
(i) Elbow of capture,
(ii) Cols, or wind gaps,
(iii) Watrgaps, and
(iv) Misfit or underfit streams and valleys.
The elbow of capture denotes the point (fig. 17.18 (E)) where the course of the captured stream has been diverted to the course of the captor stream. Generally, the elbow of capture denotes sharp turn in the course of a river almost at right angle. The water gap denotes the deep and narrow valley in the form of a gorge formed by the captor stream through headward erosion across the ridge (fig. 17:18 w.g.).
Wind gap is the dry portion of the beheaded stream just below the elbow of capture (fig. 17.18 w.g). The wind gap is also called as col. The misfit or under-fit stream is the lower course of the captured stream. It is called misfit because of the fact that the former valley of the captured stream becomes too large and wide for the beheaded stream because of substantial decrease in the volume of water due to diversion of its water to the captor stream.
Conditions for River Capture:
Though the river capture is a natural process, but it does not take place in all circumstances rather it requires certain necessary conditions. In fact, the process of river capture depends on channel gradient, depth of river valley, volume of water, velocity and discharge, lithological characteristics and geological structures, stage of cycle of erosion or the stage of river development.
A particular river of a locality having deeper valley, more volume of water, steeper channel gradient and hence higher velocity and kinetic energy and flowing over less resistant and softer rocks than the other river of that region resorts to more powerful headward erosion than the latter, and thus may usurp the water and upper reaches of the weaker river.
It may be, thus, inferred that river capture occurs under the following conditions:
(1) Steep channel gradient,
(2) Relatively narrow valley so that water may not spread in the otherwise wide and flat valleys,
(3) Higher volume of water so that velocity and discharge may be sufficiently high,
(4) Soft rocks so that the river may resort to rapid rate of headward erosion,
(5) Deeper valley than the valleys of other neighbouring rivers, and
(6) Low sediment load so that the river may resort to active erosion etc.
Types of River Capture:
It is apparent from the above discussion that the process of river capture is effected by erosion of different sorts viz., headward erosion (valley lengthening), vertical erosion (valley deepening or downward cutting), lateral erosion (valley widening), and intersection of meanders (lateral erosion). Headward and lateral erosion is the most powerful geomorphological process of river capture. It may also be pointed out that headward erosion is more effective in terms of river capture during juvenile stage (youthful stage) of river development while lateral erosion becomes more operative during mature stage.
The capture of the course of a particular river by the other river through the intersection of meanders occurs mostly during late maturity and senile stage (old stage).
Thus, the forms of river capture may be grouped in 3 broad categories viz.:
(1) Capture through headward erosion,
(2) Capture through latral erosion, and
(3) Capture through the intersection of meanders.
(1) River capture through headward erosion:
Most of the river captures occur due to headward erosion. In the initial stage of their development most of the streams and their tributaries are engaged in active headward erosion resulting into continuous creeping or shifting of water divides and lengthening of their valley thal wegs. The nature and intensity of headward erosion of any stream largely depends on the potential energy (height of the divide) and the steepness of the side slope of the water divide.
Generally, the side slopes of the divide are unequal. The streams originating from the steeper slopes of the divide having relatively softer rocks and more precipitation and relatively short channel lengths degrade their valleys through the process of valley deepening more powerfully and resort to headward erosion at more accelerated rate than those streams which originate from the other side of the divide having less steep slope relatively resistant rocks and low precipitation.
Consequently, the erosive power of the former becomes much more than the latter. The powerful stream pushes the water divide backward towards the side of gentle slope through active headward erosion. Prolonged headward erosion by more powerful stream flowing on the steeper hillside of the divide results in the coalescence of the sources of both the streams on opposite sides of the divides.
Since the valley floor of the stream of the steeper side of the divide is lower than the valley floor of the stream of the gentler side of the divide and hence the former captures the headwater of the latter.
The process of river capture may be explained with the help of an example. Consequent streams originate on the slopes of any uplifted landmass. The most active and the longest consequent is called the master consequent. ‘A’ is the master consequent (fig. 17.18-1) while ‘B’ is the other consequent stream shown in figure 17.18.
‘A’ stream is flowing through steeper slope and channel gradient than B stream and thus the former has deepened its valley much more than the latter, with the result the valley floor of A stream is lower than the valley floor of B stream. It is, thus, apparent that A stream is more active than B stream. A few subsequent or lateral consequent streams emerge from the ridge (fig. 17.18-1) and join the longitudinal consequent A and B streams at almost right angles.
For example, C and D are the tributaries of streams A and B respectively. These two tributaries take their sources on both the slopes of the same ridge. The valley of C would be also deeper than the valley of D stream because the valley of the master stream of C(A) is deeper than the valley of the receiving stream of D(B).
Thus, the headward erosion by C stream would be more active and vigorous than the headward erosion by D stream. The water divide is gradually pushed back (towards the source of the stream D) because of more active headward erosion by C stream. A time comes when the C stream cuts across the ridge and extends its course through deep and narrow valley (gorge) and captures the course of D stream (fig. 17.18,II).
Now the water of the upper course of the longitudinal consequent B stream also flows into the master consequent A stream via the integrated D and C tributary streams. Now the water of BEDC in the form of one channel drains into A stream. This example illustrates the capture of two streams at two stages.
First, D stream, a tributary of B stream, was captured by C stream, a tributary of A stream through active headward erosion. Secondly, the headwaters of B stream (from the source to E point, the elbow of capture) were diverted towards A stream via D and C streams due to fallout of the first stage. C-D streams now flow through deeply entrenched narrow valley known as gorge (fig. 17.18,II).
This narrow passage through the ridge is called water gap (WG in Fig. 17.18). B-E portion of the former B consequent stream has become captured stream which turns at right angle forming an elbow of capture (E in fig. 17.18 II). H-B portion of the former B consequent has now become a beheaded stream, the upper part of which is called wind gap (wg. in fig. 17.18 II) because of dry bed of the river due to capture of the upper portion of the river.
The H-B portion of the former B consequent stream has now become misfit or under fit river because now the existing H-B stream is unable to adjust itself in its former valley because of marked reduction in the volume of water due to diversion of its headwaters to A stream via D-C streams as a result of river capture.
There are two evidences which enable the investigators to identify the captured streams in the field viz.:
(i) elbow of capture and
(ii) wind gap just to the downstream side of the elbow.
The erosional work of the beheaded stream becomes almost nil because of marked reduction in the volume of water.
Sometimes, the valley of the beheaded stream becomes almost dry. On the other hand, the captor streams (fig. 17.18, C and A) resort to more vertical erosion resulting into accelerated rate of valley deepening because of marked increase in the volume of water due to additional supply of water of D stream and headwaters of B stream (B-E portion) because of river capture.
It may be pointed out that C and D tributary streams were formerly flowing in opposite directions (fig. 17.18, I) but now the waters of D stream flow in the direction of C stream. Thus, such streams of reversed flow direction are called inverted streams.
A group of geologists and geomorphologists believe that the present drainage system of the Himalayas is the outcome of progressive river piracy during various stages of drainage development. The Arun Kosi, a head tributary of the Kosi river, has captured the Phung Cho, a southern tributary of the Tsangpo (the upper part of the Brahmaputra river is called Tsangpo) river.
Two head-tributaries of the Ganga e.g. the Bhagirathi and the Vishnuganga have captured the source tributaries of the Sutlej River. The water divide between the tributaries of Song River (a tributary of the Ganga River) and Asan River (a tributary of the Yamuna River) is only a few metres wide near Dehra Dun.
It is expected that the Song River may capture the Asan River and thus the upper course of the Yamuna may be diverted to the Ganga via the Asan and the Song rivers.
(2) River capture through lateral erosion:
Lateral erosion and consequent valley widening becomes more active and significant during mature stage of river development than vertical erosion and valley deepening. The water divides between parallel streams developed on sedimentary rocks of the coastal plains are gradually narrowed down due to lateral erosion and valley widening.
The streams having more volume and discharge of water and relatively steeper channel gradient than the other streams resort to more lateral erosion due to which smaller-parallel tributaries are consumed by the larger parallel streams. This process of river capture is called stream abstraction or natural selection.
(3) River capture through the intersection of meanders:
The streams adopt highly sinuous and meandering courses during their late mature and old stages of development because of the development of level to gentle slopes (0° -5°) over major part of the area concerned. The meanders of two closely spaced streams are gradually sharpened due to continuous lateral erosion and ultimately they intersect each other and thus relatively more powerful stream captures the waters of the other stream.
The Belan river, a tributary of the Tons river (which is itself a tributary of the Ganga river), has captured the lower course of its tributary, the Seoti river near Deoghat (about 80 km south of Allahabad city) through meander intersection and now has pushed its course through the course of the Seoti while its older course, now an example of a palaeochannel, has become quite narrow due to sedimentation and anthropogenic processes (cultivation).
Due to this unique process of river capture the confluence of the Belan-Seoti Rivers has been pushed about 6 km upstream. The older (palaeo) valley of the Belan now has become misfit valley.
Numerous cases of river capture have occurred in the Himalayan region. In fact, the present drainage system of the Himalayas is, to greater extent, the result of progressive stream piracy. The headward erosion has been the most active process of river capture in the Himalayas and the Western Ghats.
The water divide between the headwaters of the Savitri river (draining into the Arbian Sea) and the Krishna river) draining into to the Bay of Bengal) at the margin of the Mahabalesh war plateau (Maharashtra) is very narrow. The Krishna River may capture the headwaters of the Savitri in near future. The process of river capture through lateral erosion and interesection of meanders is more active in the coastal plains and the Great Northern Plains (India).