Which of the following rivers is famous for its fault valley drainage ?

Question

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Correct — B, Damodar. A fault valley, or rift valley, is a trough that a river inherits from tectonics rather than carves for itself: a block of crust drops between two roughly parallel faults, and the river simply occupies the ready-made depression. The Damodar is the standard Indian example among the east-flowing rivers. It rises on the Chhotanagpur plateau in Jharkhand and runs east through a long, narrow, fault-bounded trough — a graben produced by the down-warping and faulting that accompanied the break-up of Gondwanaland — before turning south to join the Hooghly below Kolkata. Two consequences give the answer away. First, the trough subsided long enough to preserve thick Gondwana sedimentary beds that would otherwise have been stripped away, and those beds carry India's richest coal seams: Raniganj, Jharia, Bokaro, Karanpura, Ramgarh and Giridih all lie in this one valley. Second, the structural control shows in the river's plan — an unusually straight, east-west course cutting across the general grain of the plateau, quite unlike the meandering, purely erosional valleys of its neighbours. UPSC asked this same fact in almost the same words in 1998, calling it a rift valley formed by down-warping.

Why the other options are wrong — (a) The Chambal rises at Janapav in the Vindhyan hills of Madhya Pradesh and flows north to join the Yamuna. Its valley is famous for something quite different — the badlands or beehad, a maze of deep ravines gouged out of soft alluvium by gully erosion. That is erosion by running water on weak material, not a structural trough between faults.  (c) The Gandak is a Himalayan river, the Kali Gandaki and Narayani of Nepal, which enters Bihar and joins the Ganga near Sonepur and Hajipur. Its signature landform is the great alluvial megafan it has built across north Bihar — a depositional feature of the Himalayan foredeep. It flows in an alluvial and antecedent gorge setting, not in a fault-bounded rift.  (d) The Ramganga rises in the Dudhatoli range of Uttarakhand, emerges onto the plains at Kalagarh on the edge of Corbett, and joins the Ganga near Kannauj in Uttar Pradesh. It is an ordinary Himalayan tributary flowing in a valley cut by erosion; no rift or graben is associated with it.

Concept: Indian river valleys fall into two broad classes by origin. Most are erosional — the river cuts its own valley into the rock, so the valley's shape reflects the rock's resistance and the river's energy. A minority are structural: the depression already existed as a tectonic feature and the river merely took it over. Peninsular India carries several such rifts, formed when the Gondwana supercontinent began to break apart and long blocks of crust sank between parallel normal faults. The Narmada and the Tapi are the famous west-flowing examples, which is why they alone among the large peninsular rivers refuse to drain into the Bay of Bengal. The Damodar is the east-flowing counterpart — a Gondwana graben on the Chhotanagpur plateau. Because these sunken troughs protected the coal-bearing Gondwana strata from erosion, India's coal geography is essentially a map of its rift valleys: Damodar, Son, Mahanadi and Godavari.

State PCS 2019 Q124 — Geography PYQ with Answer

State PCS 2019 Q124 — Geography PYQ with Answer