Previous Year Question
Given below are two statements, one is labelled as Assertion (A) and other as Reason (R):
Assertion (A): Teesta river was earlier a tributary of Ganga, now it is a tributary of Brahmaputra.
Reason (R): River capturing is a major feature of Himalayan rivers.
Select the correct answer using the codes given below:
Both (A) and (R) are true and (R) is the correct explanation of (A)
Both (A) and (R) are true, but (R) is not the correct explanation of (A)
(A) is true, but (R) is false
(A) is false, but (R) is true
Correct Answer: A
Detailed Solution
The Teesta is presently an important tributary of the Brahmaputra river system. It originates in the Sikkim Himalaya, flows through Sikkim and West Bengal, enters Bangladesh and subsequently joins the Brahmaputra-Jamuna system.
Historically, the Teesta flowed southwards and was connected with the Ganga-Padma river system. During the exceptionally severe flood of 1787, its course underwent a major change and the river shifted south-eastwards to establish its present connection with the Brahmaputra system.
River Capture is the process in which one river or drainage system diverts the flow of another river into its own drainage network, generally through headward erosion or other geomorphic processes. In the young and tectonically active Himalayan region, steep gradients, intense erosion and changes in river courses make drainage rearrangement an important phenomenon.
Therefore, in the UPPSC question, the change in the drainage of the Teesta is associated with river capture in Himalayan rivers, and the Reason is treated as an explanation of the Assertion.
An accuracy point should also be remembered—modern geomorphological literature often describes the major 1787 course shift of the Teesta as an “avulsion”, in which a river rapidly abandons one channel and adopts another during a major flood.