The average height of the Kosi plain from mean sea level is

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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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2023 Q97 — Geography PYQ with Answer

State PCS 2023 Q97 — Geography PYQ with Answer