UPPSC 2022 Prelims GS Paper-I · Question 79 of 150

Correct — D, Mahanadi. Of the four rivers offered, the Mahanadi drains by far the largest basin, and the order of the four is Mahanadi, then Narmada, then Cauvery, then Tapti. One honest caution about the number itself: standard sources do not agree on the Mahanadi basin's exact area. It is given as about 1,32,100 square kilometres in one widely used figure and as about 1,41,589 square kilometres in the Central Water Commission and India-WRIS series, which count the whole basin including its minor coastal drainage. Nothing in this question turns on the discrepancy, because the Mahanadi comes first under either figure — the Narmada, its nearest rival here, is about 98,796 square kilometres, well below both. The Mahanadi rises near Sihawa in Dhamtari district, Chhattisgarh, and flows east across Chhattisgarh and Odisha to the Bay of Bengal, spreading over the broad saucer-shaped Chhattisgarh plain that gives it its width.

• (a)Narmada — Second of the four, with a basin of about 98,796 square kilometres shared between Madhya Pradesh (roughly 82 per cent), Gujarat, Maharashtra and Chhattisgarh. It is the tempting answer because the Narmada is the LONGER river — about 1,312 km from Amarkantak to the Gulf of Khambhat, against the Mahanadi's roughly 850 to 900 km — but length and basin area are different things. The Narmada runs west in a narrow rift valley penned between the Vindhya and Satpura ranges, so it gathers few large tributaries and its basin stays long and thin.
• (b)Cauvery — Third of the four, with a basin usually given as roughly 81,155 square kilometres — an approximate standard figure — spread across Karnataka, Tamil Nadu, Kerala and Puducherry. Large and economically vital, but well short of the Mahanadi's.
• (c)Tapti — The smallest of the four, with a basin usually given as roughly 65,145 square kilometres. Like the Narmada it is a west-flowing river confined to a rift valley, which is exactly why the two westward rivers on this list have narrower basins than the eastward Mahanadi despite their length.

A river basin is the whole area of land whose runoff drains into a single river system, so its size is set by how widely the river's tributaries spread, not by how far the main channel runs. That distinction explains this question. The Mahanadi and the Cauvery flow east across the open peninsular slope and fan out into wide tributary networks; the Narmada and the Tapti flow west through structural troughs — rift valleys between the Vindhya, Satpura and Ajanta ranges — which physically confine them and leave them long, straight rivers with narrow catchments. It also explains the coastal difference examiners often pair with this: the east-flowing peninsular rivers build large deltas, while the Narmada and Tapti empty into estuaries.

Read the word 'area' in the stem carefully, because the same four rivers are asked about by length just as often, and the two rankings are not the same. By length the Narmada beats the Mahanadi comfortably; by basin area the Mahanadi wins. If you are unsure of the numbers, reason structurally instead: two of these four are rift-valley rivers flowing west into the Arabian Sea, which caps their catchments, and of the remaining two the Mahanadi drains the whole Chhattisgarh plain and a large part of Odisha while the Cauvery is confined to the southern peninsula. Note also that this is a question about four specific rivers, not about India as a whole: neither the Ganga nor the Godavari appears among the options, and both drain basins far larger than any of these four.

• Among the four options the ranking by basin area is Mahanadi > Narmada > Cauvery > Tapti
• Sources differ on the Mahanadi basin — about 1,32,100 sq km in one widely used figure, about 1,41,589 sq km in the Central Water Commission and India-WRIS series; the ranking is identical under either
• The Mahanadi rises near Sihawa in Dhamtari district, Chhattisgarh, and flows east through Chhattisgarh and Odisha to the Bay of Bengal
• The Narmada basin is about 98,796 sq km — Madhya Pradesh about 82 per cent, then Gujarat, Maharashtra and Chhattisgarh — and the river runs about 1,312 km west from Amarkantak to the Gulf of Khambhat
• The Narmada is the longer river but has the smaller basin, because its rift valley between the Vindhya and Satpura ranges leaves it few large tributaries
• Cauvery (about 81,155 sq km) and Tapti (about 65,145 sq km) are the standard approximate figures for the other two

• Ranking by length when the question asks for basin area — the Narmada is longer than the Mahanadi but drains less
• Assuming the answer is the largest basin in India or in the peninsula; the Ganga and the Godavari are both far larger and neither is an option here
• Quoting a single figure for the Mahanadi basin as though it were settled; the commonly cited numbers, 1,32,100 and 1,41,589 sq km, come from different conventions

UPPSC asks it as a bare superlative — largest basin, longest river, biggest lagoon — and expects the ranking to be held ready. UPSC has asked the same rivers as an ordering problem, sequencing Godavari, Mahanadi, Narmada and Tapti by length in 2003 and the east-flowing peninsular rivers from north to south in 2002, so prepare these four as a small table with both length and basin area rather than as isolated facts.

• practice — not a real PYQ

  Which one of the following rivers rises near Sihawa in the Dhamtari district of Chhattisgarh?

  • (a)Godavari
  • (b)Mahanadi
  • (c)Narmada
  • (d)Tapti

  Answer(b) Mahanadi — it rises in the Sihawa area of Dhamtari district and flows east through Chhattisgarh and Odisha to the Bay of Bengal; the Narmada rises at Amarkantak and the Godavari near Nasik.

• practice — not a real PYQ

  The Narmada is a longer river than the Mahanadi, and yet its drainage basin is the smaller of the two. The best explanation is that

  • (a)the Narmada flows through a narrow rift valley between the Vindhya and Satpura ranges and so receives few large tributaries
  • (b)the Narmada is a seasonal river that dries up for much of the year
  • (c)the Narmada flows entirely within a single State
  • (d)the Mahanadi is fed by Himalayan snowmelt in addition to rainfall

  Answer(a) the Narmada flows through a narrow rift valley between the Vindhya and Satpura ranges and so receives few large tributaries — length and catchment area are governed by different things.