Consider the following statements: 1. The Great Northern Plains are formed by basins of three distinct river systems — the Indus, the Ganga and the Brahmaputra. 2. They are one of the most densely populated areas on Earth. 3. Between the Yamuna at Delhi and the Bay of Bengal, nearly 1600 km away, there is a drop of only 200 metres in elevation. Which of the statements given above are correct?

Correct — A, 1, 2 and 3. All three statements hold. The Great Northern Plains are an alluvial fill laid down by three separate river systems — the Indus in the west, the Ganga in the middle and the Brahmaputra in the east — which is why they are often called the Indus-Ganga-Brahmaputra plains. They carry some of the densest rural populations anywhere on Earth: at the 2011 Census, Bihar recorded 1,106 persons per square kilometre and West Bengal 1,028, against an all-India figure of 382. And the gradient really is that gentle. Delhi stands only about 200 metres above sea level, and the Bay of Bengal is at zero, so over the roughly 1,600 km from the Yamuna at Delhi to the sea the land falls barely 200 metres — a slope of about 12 centimetres per kilometre, which is why the rivers of the lower plain meander, braid and change course so freely.

• (b)2 and 3 only — Rejects statement 1, but the three-river origin of the plains is the standard description and the reason they are called the Indus-Ganga-Brahmaputra plains.
• (c)1 and 2 only — Rejects statement 3, presumably on the feeling that 200 metres over 1,600 km sounds too flat to be true. It is arithmetically right: Delhi's elevation is around 200 metres and the sea is at zero.
• (d)1 and 3 only — Rejects statement 2, but the plains include Bihar, Uttar Pradesh and West Bengal, three of the most densely settled large territories on the planet.

The Great Northern Plains are an aggradational landform — built up by deposition rather than carved out by erosion. The Himalayan rivers dropped their sediment load into a foredeep between the rising mountains and the old peninsular block, filling it with alluvium to depths that run into thousands of metres. The result is one of the largest and most fertile stretches of flat land in the world, about 2,400 km long and 240 to 320 km wide.

The three claims in this item reinforce each other. The three river systems supply the alluvium; the alluvium plus a reliable monsoon and easy groundwater supports the density; and the near-absence of gradient is what let the rivers spread that alluvium so evenly in the first place. The gradient figure is the one candidates doubt, so it is worth fixing with a number you can check independently — Delhi at roughly 200 metres above sea level and the Bay of Bengal at zero. That fall of about 12 cm per kilometre is also the reason the Ganga and its distributaries meander so extravagantly across Bihar and Bengal and why floods spread so far when embankments fail. The plains are conventionally divided from north to south into the Bhabar of coarse pebbles at the foot of the Shiwaliks, the marshy Terai below it, the older alluvium of the Bhangar, and the newer flood-plain alluvium of the Khadar.

• The Great Northern Plains are formed by the Indus, Ganga and Brahmaputra river systems, and stretch roughly 2,400 km from west to east.
• They are built of alluvium several thousand metres deep, deposited in a foredeep between the Himalaya and the peninsular block.
• Census 2011 density: Bihar 1,106 and West Bengal 1,028 persons per square kilometre against an all-India 382.
• Delhi lies about 200 metres above sea level, so the fall to the Bay of Bengal over roughly 1,600 km is about 200 metres — some 12 cm per kilometre.
• North to south the plains are divided into Bhabar, Terai, Bhangar (older alluvium) and Khadar (newer alluvium).

• Rejecting a statement because a figure sounds implausible, without checking it against a known elevation.
• Restricting the Northern Plains to the Ganga basin and forgetting the Indus and Brahmaputra.
• Confusing the Terai with the Bhabar — the Bhabar is the porous pebble belt above, the Terai the marshy re-emergence below.

Asked as a three-statement item in which the numerical claim is the one designed to make you doubt an otherwise straightforward set.

• practice — not a real PYQ

  The narrow belt of porous pebbly deposits at the foot of the Shiwaliks, where streams disappear underground, is called the

  • (a)Terai
  • (b)Bhabar
  • (c)Bhangar
  • (d)Khadar

  Answer(b) Bhabar — the streams sink into its coarse deposits and re-emerge in the marshy Terai just to the south.

• practice — not a real PYQ

  The newer alluvium of the flood plains of the Northern Plains is known as

  • (a)Bhangar
  • (b)Khadar
  • (c)Bhabar
  • (d)Regur

  Answer(b) Khadar — the newer, more fertile alluvium renewed by floods, as against the older, terrace-forming Bhangar.