BPSC 69th CCE Prelims General Studies · Question 97 of 144

Correct — C, 30 m. The Kosi plain is the Indian part of the Kosi megafan, the tract of Supaul, Saharsa, Madhepura, Araria, Purnia and Katihar through which the river spreads before joining the Ganga near Kursela. It is among the lowest-lying land in India, and every independent check puts it in the tens of metres, not the hundreds. A Government of India document puts the number on it directly. The Ganga Basin Report prepared by the Central Water Commission with the National Remote Sensing Centre under the India-WRIS programme describes the Middle Ganga Plain — which it defines as the Bihar plains together with eastern Uttar Pradesh — as "in general … below 100m above the sea level", rising to 130 m in the north-west near Domariaganj and "up to 150m in the south" where the peninsular uplands push into the plain. Of the eastern end it says, in as many words: "in the east the Kosi plain ranges between 30m in the south to 75m in the extreme north." Thirty metres is the figure a Central Water Commission basin report attaches to the Kosi plain, and it is almost certainly where the paper's option came from. The same sentence also explains why 150 m is sitting on the list — it is the report's own southern margin figure, for the opposite end of Bihar. Start with the state envelope. Bihar's average elevation is about 53 m above mean sea level; its lowest point is 11 m, on the Ganges; its highest is 880 m at Someshwar Fort on the Shivalik ridge in the extreme north-west of West Champaran — a sliver of hill country at the opposite end of the state from the Kosi. A tract that is still receiving fresh silt every monsoon cannot sit above the state mean, let alone at three or six times it. Then the settlements themselves. Across the Kosi country the ground stands at roughly 33 m at Supaul, 35 m at Katihar, 36 m at Purnia, 36 m at Khagaria, 41 m at Saharsa, 46 m at Forbesganj and 47 m at Araria, with Nirmali on the northern rim at 59 m. The whole tract falls from the high fifties where the Kosi enters Bihar to the mid-thirties at the Ganga confluence — barely 25 metres of drop across some 130 km of country, a gradient of roughly a fifth of a metre per kilometre. That near-absence of slope is not an incidental fact; it is the physical reason the Kosi drops its load, splits into more than twelve channels across a fan of about 15,000 sq km, and earns its name as the sorrow of Bihar. One caveat, because this answer is ours and carries medium rather than high confidence. The 30 m in the basin report is the southern, downstream end of a stated range of 30–75 m; it is not an average of that range, whose mid-point would be about 52 m, and the settlement data average nearer 35–40 m. Read (c) as the Government's own low-end figure for the Kosi plain rather than as a computed mean. That is a rounding argument, though, and the other two options are wrong by factors of five and ten.

• (a)300 m — Nearly six times Bihar's statewide average elevation of 53 m, and higher than almost anything in the state outside the Shivalik ridge of West Champaran. 300 m is plateau country — the sort of elevation you meet on the Chhotanagpur fringe south of the Ganga — and it is simply not available to an actively aggrading river plain in the north-east of the state. This is the throwaway option.
• (b)150 m — The real trap, and it is on the list for a precise reason. In the standard physiographic description, the Bihar Plain is the tract enclosed by the 150-metre contour on both its northern and its southern side — 150 m is where the plain stops and the hills begin. A candidate who has learned that sentence and reads a boundary value as an average lands exactly here. It is roughly three times the state's mean elevation, and in Bihar it is exceeded only in the Shivalik tract and parts of the southern hills. The India-WRIS basin report is the cleanest illustration of the error: the one sentence that gives 30 m for the Kosi plain in the east also gives 150 m for the south of the Middle Ganga Plain, where the peninsular projections intrude. Both numbers are in the source; only one of them is about the Kosi.
• (d)None of the above — This is the option a careful candidate can argue for, and it deserves a straight answer. Settlement elevations across the Kosi tract cluster a little above 30 m — Supaul 33 m, Katihar 35 m, Purnia 36 m, Khagaria 36 m, Saharsa 41 m, Araria 47 m — so a strict arithmetic average lands nearer 35–40 m than the round figure the paper wants, and the Central Water Commission's own range for the tract, 30–75 m, has a mid-point around 52 m. What settles it against (d) is that 30 m is not a guess: it is the number a Government of India basin report prints for the Kosi plain, so the option is quoting a real source rather than approximating. The other two numbers are out by factors of five and ten. Reaching for (d) costs a mark over whether the report's low-end figure counts as an average.

The Kosi plain is the Indian portion of the Kosi megafan — one of the largest alluvial fans on Earth, about 15,000 sq km — built where the river leaves the Himalayan foothills through the Chatra gorge and spills onto the Gangetic plain. A fan of this kind is made entirely of sediment the river has dropped on itself, so its surface slopes away from the apex at a very gentle and steadily decreasing gradient, and the river wanders across it rather than cutting down into it. In Bihar the fan covers the Kosi–Seemanchal districts — Supaul, Saharsa, Madhepura, Araria, Purnia, Katihar and part of Khagaria — before the Kosi joins the Ganga near Kursela in Katihar district. Physiographically the tract belongs to the North Bihar plain, and Bihar's plain as a whole is defined as the land enclosed by the 150-metre contour on both its northern and southern edges. Inside that envelope the country is remarkably low: the state averages about 53 m, bottoms out at 11 m on the Ganges, and reaches 880 m only at Someshwar Fort in the Shivaliks of West Champaran, far to the north-west of the Kosi. The Kosi tract sits in the lowest part of that range.

Two moves settle this without knowing any published figure. First, bracket the plain. Bihar's mean elevation is about 53 m and its floor is 11 m on the Ganges; the only ground in the state above 150 m is the Shivalik sliver in West Champaran and parts of the southern hills, neither anywhere near the Kosi. So 300 m and 150 m are not merely high — they are impossible for a river tract still taking on silt every monsoon. Second, see why 150 m is on the list at all: it is the contour that bounds the Bihar plain in the standard physiographic definition, the line at which the plain ends. Mistaking a boundary for an average is the single discriminating error the question is built around, and it is the reason a well-read candidate is more likely to fall for (b) than an ill-read one. The positive check is the settlement data, and it is easy to carry: Supaul 33 m, Katihar 35 m, Purnia 36 m, Khagaria 36 m, Saharsa 41 m, Forbesganj 46 m, Araria 47 m, Nirmali 59 m. Only one of the four options belongs to that world, and the same near-flatness explains the Kosi's behaviour — a river with a fifth of a metre of fall per kilometre cannot carry its Himalayan silt load, so it deposits, raises its own bed and jumps channel.

• Bihar's average elevation is about 53 m above mean sea level; the lowest point is 11 m on the Ganges and the highest 880 m at Someshwar Fort on the Shivalik ridge in West Champaran
• The Bihar Plain is defined as the tract enclosed by the 150-metre contour on both its northern and southern sides — 150 m marks the plain's boundary, not its average height, which is exactly why option (b) is offered
• Elevations across the Kosi tract: Supaul about 33 m, Katihar about 35 m, Purnia 36 m, Khagaria 36 m, Saharsa 41 m, Forbesganj 46 m, Araria 47 m, Nirmali 59 m
• The Kosi megafan covers roughly 15,000 sq km, among the largest alluvial fans in the world, and the river breaks across it into more than twelve distinct channels
• The Ganga Basin Report of the Central Water Commission and NRSC (India-WRIS) states that "the Kosi plain ranges between 30m in the south to 75m in the extreme north", and that the Middle Ganga Plain is in general below 100 m, rising to 130 m in the north-west and 150 m in the south
• The same report places the Kosi sub-basin between 85°E and 87°21′E and between 25°25′N and 26°48′N, entirely inside Bihar, with a total catchment of 95,156 sq km of which 18,413 sq km lie in India; nearly 80% of the catchment is in Nepal and Tibet and about 77% of the area is cultivated
• The Kosi is formed by the confluence of the Sun Kosi, the Arun Kosi and the Tamur Kosi; the Arun (Phungchu in Tibet) is the largest of the three and carries both Mount Everest and Kanchenjunga in its catchment. The basin report gives the river a length of 730 km and a drainage area of 86,900 sq km, 21,500 sq km of it in India, meeting the Ganga 32 km west of Manihari
• The Kosi joins the Ganga near Kursela in Katihar district. Its published catchment figures differ by section even inside the same official report — 74,500 sq km for the river, 95,156 sq km for the basin — so quote the figure with its source rather than treating any one of them as the number
• The Koshi Barrage at Bhimnagar, just inside Nepal, has 56 gates, was built between 1958 and 1962 following the Kosi Agreement of 25 April 1954, and is maintained by India
• The Kosi catchment runs through six belts from above 8,000 m on the Tibetan Plateau down to about 95 m in the Terai — the river is already near plain level before it crosses into Bihar

• Reading the 150-metre contour that bounds the Bihar plain as the plain's average height. It marks where the plain ends, not what it averages.
• Carrying Bihar's extreme figures across the state. The 880-metre high point is in the Shivaliks of West Champaran, at the far north-western corner, hundreds of kilometres from the Kosi.
• Treating a flat plain as a level one. The Kosi plain does slope — by roughly a fifth of a metre per kilometre — and that tiny gradient is the whole explanation of the river's behaviour.

BPSC asks Bihar physiography as a bare numerical with a "None of the above" escape, and expects the state's contour framework to be at your fingertips: the 150-metre bounding line, the three physiographic divisions, the north–south split by the Ganga. UPSC does not ask Bihar elevations at all; it comes at the same terrain through process instead — the order in which the Himalayan tributaries meet the Ganga, why the northern plains are alluvial, why a river on a low-gradient fan shifts course. Learn the numbers for BPSC and the mechanism for both.

• practice — not a real PYQ

  The Bihar plain is bounded on both its northern and its southern side by which contour line?

  • (a)100 m
  • (b)150 m
  • (c)300 m
  • (d)500 m

  Answer(b) 150 m — the standard physiographic definition encloses the Bihar plain within the 150-metre contour, beyond which the Shivaliks rise to the north and the plateau fringe to the south.

• practice — not a real PYQ

  The Kosi joins the Ganga near which one of the following places?

  • (a)Kursela
  • (b)Sonepur
  • (c)Chapra
  • (d)Mokama

  Answer(a) Kursela, in Katihar district. Sonepur marks the Gandak confluence opposite Patna and Chapra sits near the Ghaghara confluence; Mokama is a Ganga town with no major tributary junction.