Previous Year Question
The Brahmaputra, Irrawaddy and Mekong rivers originate in Tibet and flow through narrow and parallel mountain ranges in their upper reaches. Of these rivers, Brahmaputra makes a ‘U’ turn in its course to flow into India. This ‘U’ turn is due to
uplift of folded Himalayan series
syntaxial bending of geologically young Himalayas
geo-tectonic disturbance in the tertiary folded mountain chains
Both (a) and (b)
Correct Answer: B
Detailed Solution
The ‘U’-shaped bend of the Brahmaputra is associated with the Eastern Himalayan Syntaxis. A syntaxis is a region where mountain ranges or geological structures bend sharply and converge towards one another.
Option (b) is correct: At its eastern end, the Himalayan arc bends sharply southward around Namcha Barwa. This syntaxial bending of the geologically young Himalayas provides the structural route along which the Brahmaputra takes its sharp ‘U’ turn.
Course of the Brahmaputra:
In Tibet, the river flows eastward as the Yarlung Tsangpo or Tsangpo.
In its upper course, it flows approximately parallel to the Himalayas.
Near Namcha Barwa, within the Eastern Himalayan Syntaxis, its course turns sharply southward.
Here, it cuts the extremely deep Tsangpo Gorge.
After entering Arunachal Pradesh, it is known as the Siang or Dihang.
After meeting the Dibang and Lohit rivers, it is known as the Brahmaputra in Assam.
The Brahmaputra is also regarded as an antecedent river. Its river system existed before the complete development of the present Himalayan relief. As the Himalayas rose, the river maintained its course by intense vertical erosion and progressively deepened its valley.
Why are the other options incorrect?
Option (a): Himalayan uplift contributed to the regional relief and the formation of deep gorges, but the specific ‘U’ turn is directly explained by the Eastern Himalayan syntaxial bend.
Option (c): Tertiary mountain-building and tectonic activity are related to the evolution of the Himalayas, but this option does not identify the specific structural cause of the bend.
Option (d): Since option (b) gives the direct explanation, both (a) and (b) cannot be treated as equally correct.
Related fact: The Western Himalayan Syntaxis occurs near Nanga Parbat, where the Indus takes a sharp bend and cuts a deep gorge. Namcha Barwa is associated with the Eastern Syntaxis, while Nanga Parbat is associated with the Western Syntaxis.
Therefore, the correct answer is option (b).