Match the following :
Types of Stream
(a) Consequent
(b) Obsequent
(c) Subsequent
(d) Braided
Explanation
(i) Stream develops valley across outcrop
(ii) Flow controlled by original slope
(iii) Flowing

Question

Match the following :
Types of Stream
(a) Consequent
(b) Obsequent
(c) Subsequent
(d) Braided
Explanation
(i) Stream develops valley across outcrop
(ii) Flow controlled by original slope
(iii) Flowing opposite to the dip of rock
(iv) Maze of interconnected channels

UnlockIAS · Accepted Answer

Correct — option (2). Three of the four terms belong to the family of streams classified by their relation to the underlying structure, and the fourth belongs to a different classification altogether, which is where the question is most easily cracked. Braided at (d) describes a channel pattern, not a structural relationship: where a river carries more load than it can move, has a variable discharge and banks of loose material, it deposits bars in mid-channel and divides and rejoins around them, producing exactly the maze of interconnected channels described at (iv). No other clue in the second column can mean this, and no other item in the first column can be described by it, so (d) pairs with (iv) and that pairing alone identifies option (2) as the answer. The remaining three confirm it. A consequent stream at (a) is the first stream to establish itself on a newly emerged or newly uplifted surface, and it takes the direction the surface itself gives it, which is what (ii) says when it speaks of flow controlled by the original slope. A subsequent stream at (c) develops afterwards, not on the original slope but along a line of weakness that erosion has exposed, cutting its valley where the softer bed outcrops and usually joining the consequent at a sharp angle; that is (i). An obsequent stream at (b) is the one that runs the other way, down the steep face left by that erosion and therefore against the dip of the beds, which is (iii). The completed pairing is (a)-(ii), (b)-(iii), (c)-(i), (d)-(iv), the row printed at option (2).

Why the other options are wrong — (a) This row gives the consequent stream the maze of interconnected channels at (iv), which is a description of a braided channel pattern and has nothing to do with the way a stream relates to the slope or the structure beneath it. A consequent stream is defined by its direction — it takes the slope of the land as first formed — and a stream may be consequent whether it flows in one channel or in fifty. The row then leaves the braided stream at (d) with the outcrop clue at (i), which is the description of a subsequent stream. What makes this row worth studying is the speed with which it can be killed: the braided pairing is the one clue in the question that a candidate can be certain of, and any row that spends (iv) elsewhere is finished before the other three pairs are even read.  (c) This row repeats the error of giving the maze of interconnected channels to the consequent stream, and adds to it by making the braided stream the one that flows against the dip of the rock. Neither statement can stand. Flowing opposite to the dip is the definition of an obsequent stream, a structural description that concerns the attitude of the beds beneath the channel, whereas braiding is a matter of sediment load, discharge variability and bank material and is found on rivers in every structural setting — the Kosi in north Bihar and the Brahmaputra in Assam are the standard Indian examples. The row also gives the subsequent stream the original-slope clue that belongs to the consequent. Every pair in it is wrong, and the braided test disposes of it at once.  (d) This is the row built to defeat a candidate who checks the first half of the list and stops. It correctly gives the consequent stream the original slope at (ii) and the obsequent stream the opposite-to-dip clue at (iii), which are the two pairs most candidates can do, and then reverses the last two: it gives the subsequent stream the braided maze at (iv) and the braided stream the outcrop clue at (i). Both are wrong. A subsequent stream is defined by structure — it follows the outcrop of a weaker bed, which is why it is also called a strike stream — while braiding is a channel pattern produced by load and discharge. The habit that defeats this row is to verify every pair in a matching item, and to be most careful about the last ones, since a setter who leaves the easy pairs intact has done so deliberately.

Concept: Streams are classified in two independent ways and the question mixes them, so the classifications are worth separating. The first is genetic, that is by the relation of the stream to the structure it flows over. The consequent stream is the original one, following the slope of the land as it first emerged from the sea or was uplifted. As it cuts down it exposes beds of different hardness, and tributaries develop along the weaker outcrops at right angles to it; these are subsequent or strike streams. Short streams then flow down the steep scarp faces into the subsequent valleys, and because the scarp faces the opposite way from the dip these are obsequent streams; where a stream flows in the same direction as the consequent but at a lower level, on a bench cut into the weaker bed, it is called resequent. Two further terms belong to the same family: an antecedent stream is one that existed before the uplift and cut down fast enough to maintain its course across it, which is how the Indus, Sutlej and Brahmaputra cross the Himalaya, and a superimposed stream is one that established its pattern on a cover of rock since removed and now cuts incongruously into the structure beneath. The second classification is by channel pattern — straight, meandering, braided and anastomosing — and it depends on gradient, sediment load, discharge and the strength of the banks rather than on structure. Braiding belongs to this second scheme.

State PCS 2022 Q6 — Geography PYQ with Answer

State PCS 2022 Q6 — Geography PYQ with Answer