Indo Gangetic Plains: Indo-Gangetic-Brahmaputra – UPSC

• The Indo-Gangetic-Brahmaputra Plain, or Great Plain of North India, is a curved (arcuate) surface of deposition, an aggradational plain, laid down by three river systems: the Indus, Ganga and Brahmaputra.
• It is the largest alluvial tract in the world, running about 3,200 km between the Indus mouth and the Ganga mouth. The Indian sector is about 2,400 km long.
  • It covers about 7.8 lakh sq km, and its average width is 150–300 km.
  • The width changes sharply along its length.

• Boundaries:
  • The 300 m contour divides the Himalaya from the plain, and the Shiwalik foothills give it a clean northern edge.
  • The southern limit is a wavy line along the edge of the Peninsula. It follows the 75 m contour over most of its length and the 35 m contour in the north-eastern section towards the delta.
• Extreme flatness is the chief trait of this rolling, monotonous surface.
  • The mean height is about 200 m. The highest point, 291 m, lies between Ambala and Saharanpur, on the low watershed separating Indus and Ganga drainage.
  • Surveyed gradients are about 20 cm per km from Saharanpur to Kolkata, about 1:5,000, and 15 cm per km on the Varanasi-delta stretch, about 1:6,700.
  • The figure of 1:1,000 to 1:2,000 often quoted for the plain is too steep for it as a whole. It suits, at most, the fans at the mountain foot: the Tista fan slopes about 0.43 m per km, about 1:2,300.

• On this map, “Mithala” is the Mithila plain and “Kutch Marsh” is the Rann of Kachchh.

Origin of the Plain

Formation of the Indo-Gangetic-Brahmaputra Plain

• The making of this plain is closely tied to the making of the Himalaya.
• Rivers that once drained into the Tethys Sea, before the Indian Plate met the Eurasian Plate, dumped vast sediment in the Tethys Geosyncline.
• The northward push of the Indian Plate uplifted, folded and compressed this sediment into the Himalaya. The same movement left a trough south of the rising mountains.

Depositional Activity

• As uplift began, older rivers changed course repeatedly and were rejuvenated each time, staying in a kind of perpetual youth.
• Rejuvenation brought intense headward and vertical cutting into the soft strata lying over harder rock.
  • Headward erosion works at the origin of a stream channel. It moves that origin back, against the direction of flow, so the channel lengthens.
• Headward and vertical erosion in the early stages, and lateral erosion later, released huge volumes of detritus: rock debris, gravel, silt and clay. These were carried downslope.
  • Older accounts call this material “conglomerates”. Strictly, a conglomerate is the cemented rock, and the plain’s fill is mostly loose.
• The load settled in the Indo-Gangetic Trough (or syncline), the depression between Peninsular India and the convergent boundary of the present Himalaya. Its floor is hard crystalline rock.

New Rivers and More Alluvium

• The rising Himalaya and its new glaciers gave birth to many new rivers. Together with glacial erosion, they supplied more alluvium and speeded the filling.
• Older accounts then have the Tethys receding and finally vanishing as the trough filled.
  • The current view separates the two: the sea closed through plate collision, and the trough that rivers filled was a later dip in the Indian Plate in front of the mountains.
• The result is a monotonous aggradational plain of alluvium, gravel and rock debris.
  • Monotonous means featureless topography.
  • Aggradational plain means a plain built by deposition, here fluvial deposition.
• Rivers from the northern Peninsula contributed too, but only a small share.
• In the last few million years, the Indus, Ganga and Brahmaputra have dominated the work of deposition, hence the name Indo-Gangetic-Brahmaputra Plain.
• Most of the surface is of Pleistocene and Recent age, and it hides older Peninsular and other formations.

Views on the Depression

• All accounts agree that a deep depression between the two uplands was filled by rivers. They differ on how the depression itself arose.

Depth of the Alluvium

• Richard Dixon Oldham put the greatest depth at about 5,000 m near the southern edge, and later seismic soundings gave about 6,100 m to the basement.
• Current surveys place the greatest thickness, about 6 km, at the northern side, under the piedmont, thinning southward. That is the wedge shape expected of a bent plate.
  • Near Delhi the fill is only 1–2 km thick, and across much of the plain it is 2–5 km.
• The deepest stretch lies between Delhi and the Rajmahal Hills. The fill is thinner in Rajasthan and Assam.
• The floor is uneven. Basement ridges, the Delhi-Haridwar, Faizabad and Munger-Saharsa ridges, separate depressions such as the Sarda and Gandak deeps.
• Thickness also follows the fans: it is greater under the Kosi cone in the north and the Son cone in the south, and shallower between cones.

Longitudinal Divisions: Bhabar, Terai, Bhangar, Khadar

• Four belts run lengthwise along the plain. These are its longitudinal divisions, and one meets them in turn on a north-south traverse.
  • Bhabar (also written Bhabhar)
  • Terai (Tarai)
  • Bhangar (Bangar)
  • Khadar

Bhabar Region

• The narrow, porous, northernmost strip, right against the Shiwalik foothills. It holds cobbles, pebbles, gravel and boulders with coarse sand.
• It is about 8–16 km wide and runs east-west with striking continuity, Indus to Tista.
  • The same type of zone continues in stretches along the foot of the Assam Himalaya, so the belt is commonly described as running from Punjab to Assam.
• Origin: Himalayan rivers lose speed at the mountain foot and drop their load as alluvial fans. These fans have merged sideways to make the belt.
  • Luminescence dating places the piedmont fan surface of the Ganga plain in the latest Pleistocene, about 25,000–10,000 years ago.
• Porosity is its defining feature, a result of the pebbles and rock debris laid down across the fans.
  • Streams disappear underground on reaching it, so the belt shows dry channels outside the rains.
• It is narrower in the east and broader in the west and north-west.
• The profile is complex. The slope of 1:6,000 sometimes given for it matches the plain’s general gradient rather than this belt, which is the steepest part of the plain.
• Not suited to farming. Only big trees with large roots do well.

Terai Plains

• An ill-drained, damp, marshy, extremely flat strip south of the bhabar and parallel to it, from Punjab to Assam. The word comes from the Hindi tar, meaning wet.
• It is about 15–30 km wide, broadest along Bihar and eastern Uttar Pradesh and narrowest further east.
• The underground streams of the bhabar re-emerge here and saturate the ground.
• The belt is better marked in the east, because rainfall there is higher.
• It was once thickly forested and rich in wildlife. Most of it has now been cleared and reclaimed.
• Terai soils are rich in nitrogen and humus.
• In Punjab, Uttar Pradesh and Uttarakhand in particular, reclaimed land grows sugarcane, rice and wheat well.

Bangar Region

• Bangar, also written Bhangar, extends over the whole Gangetic plain.
• It is older alluvium, of Middle to Upper Pleistocene age, standing as terraces above the floodplain along the river beds.
  • Shifting and meandering channels keep eating into these terraces and lowering them.
• The terraces often hold calcareous concretions called kankar.
  • Kankar is made of limestone fragments, so it acts as a natural supply of bases to the soil.
• Fossils of rhinoceros, hippopotamus and elephant occur in it.
• Regional variants of bhangar:
  • The Barind plains of the Bengal delta region.
  • The bhur formations of the middle Ganga and the Yamuna doab.
    • Bhur is raised land along the Ganga banks, chiefly in the upper Ganga-Yamuna Doab. Wind heaped its sand during the hot, dry months.
• It is seriously degraded in parts of Uttar Pradesh, where such land is called usar: infertile from salinisation, with very little productivity.

Barind Tract (Barind Plains)

• Also called the Varendra Tract, and Borendro Bhumi in Bengali. It is the largest Pleistocene physiographic unit of the Bengal Basin, about 7,770 sq km by the standard Bangladeshi count.
• It lies in north-western Bangladesh and north-central West Bengal.
• It sits north-west of the confluence of the upper Padma (Ganga) and the Jamuna (the Brahmaputra’s name in Bangladesh).
  • The floodplains of the Mahananda bound it on the west and the Karatoya on the east, tributaries of the upper Padma and the Jamuna respectively.
• It is a fairly high, undulating tract of reddish and yellowish clay, long recognised as old alluvium.

Khadar Region

• Newer alluvium forming the floodplains along river banks, renewed by a fresh layer in almost every flood.
• This renewal makes it the most fertile soil of the Ganga plain.
• It stays close to present channels, holds less kankar than bhangar, and its organic remains belong to species still living. It passes without a clear break into deltaic deposits.

Reh or Kallar

• Reh, also called kallar or kollar, is a barren saline efflorescence of the drier parts of Uttar Pradesh and Haryana.
• It has spread with irrigation: capillary action draws salts up to the surface.

Transverse Divisions: The Regional Plains

• Across its length the plain falls into four regional units, separated by river patterns, flow direction and landforms. These are its transverse divisions.
  • Rajasthan Plain
  • Punjab Plain
  • Gangetic Plain
  • Assam Plain (Brahmaputra Plain)

Rajasthan Plain

• Limits: the Ghaggar basin in the north and the Aravalli Range in the east.
• It is part of the Thar Desert, the Great Indian Desert, but carries alluvial deposits of the Indus and its tributaries.
  • As the Himalaya rose, these channels moved westward.
• Extent of the Thar: about 2 lakh sq km, with 1.75 lakh sq km on the Indian side.
  • About two-thirds of the Indian part lies in Rajasthan west of the Aravalli, and the rest in Haryana, Punjab and Gujarat.
  • Wider geomorphic definitions take the desert to about 2.9 lakh sq km. India’s whole hot arid zone covers 31.7 million hectares, about 62% of it in Rajasthan.
• Relief: an undulating, wave-like plain averaging about 325 m, dropping to about 150 m along the Pakistan border and on the slopes down to the Rann of Kachchh and the Indus valley.
• Origin: bedrock outcrops of gneiss, schist and granite show that this is geologically a part of the Peninsular block, and only its surface looks aggradational.
  • Aridity is natural and old: luminescence dating shows sand-building for over 1,50,000 years, in pulses that follow swings of the monsoon. The desert is not a recent human creation, although disturbance now speeds dune movement.
• The 25 cm isohyet divides the plain into Marusthali in the west and the Rajasthan Bagar in the east.
  • Marusthali: the desert proper, averaging 200–250 m, with a rocky east and a west of shifting dunes called dhrian.
    • Rainfall is about 25 cm or less, and the characteristic tree is the khejri (Prosopis cineraria). It is not the only tree, but it dominates.
    • The Bishnoi community is tied to the khejri: 363 Bishnois, led by Amrita Devi, died at Khejarli in 1730 resisting its felling.
  • Rajasthan Bagar: a semi-arid belt from the Aravalli edge to the 25 cm line, with short seasonal streams and fertile green patches called rohi.
  • Thali: the sandy plain north of the Luni, itself a seasonal river flowing to the Rann of Kachchh.
• Dunes: satellite imagery shows six types: barchan, longitudinal, transverse, parabolic, obstacle and shrub-coppice.
  • Mean dune height climbs westward, about 8 m in Jaipur and Sikar and about 30 m in Barmer. Some dunes run 5–10 km.

Saline Lakes

• North of the Luni lies inland drainage with several saline lakes, a source of common salt and other salts.
• Important ones are Sambhar, Didwana, Degana, Kuchaman, Sargol and Khatu.
• Sambhar, south-west of Jaipur, is the largest, and India’s largest inland salt lake.
  • It lies at about 360 m, its basin is about 22.5 km long and 3–11 km wide, and it spreads over 190–230 sq km with the seasons.
  • It has been a Ramsar site since 1990.

Punjab Plain

• The land built by five rivers of the Indus system. The name Punjab means “(the land of) five waters”: the Jhelum, Chenab, Ravi, Sutlej and Beas.
• It consists mainly of doabs, the land between two rivers, and there are five:

• Long deposition has fused these doabs into one homogeneous surface.
  • Rivers have cut broad khadar floodplains, locally called bet, flanked by bluffs called dhaya, 3 m or more high and much gullied.
  • The bet lands flood easily but are valuable farmland.
• The plain is extremely fertile but poorly drained and flood-prone.
• Size and height: about 1.75 lakh sq km, 640 km north-west to south-east and about 300 km wide. It averages about 250 m, close to 300 m at the northern end and 200 m at the south-eastern end.
• Eastern limits: the 291 m contour, parallel to the Delhi-Ambala ridge, separates it from the Gangetic basin. The subsurface Delhi-Aravali ridge and, in Haryana, the Yamuna mark the same edge.
• The north, along the Shiwalik foot, is cut by torrents called chos, which cause severe gullying.
  • The damage is worst in Hoshiarpur district, where nearly a hundred chos reach the plain in about 130 km.
• South of the Satluj lies the Malwa plain of Punjab.
• The land between the Ghaggar and the Yamuna in Haryana is the Haryana Tract, the water divide separating the Satluj and Yamuna drainage.
• The Ghaggar, the lone river in the gap between those two, is traditionally regarded as the present successor of the legendary Saraswati.
  • Dating has tested the claim. The Ghaggar-Hakra palaeochannel was a former course of the Satluj, abandoned shortly after about 8,000 years ago. The rivers active there in Harappan times were monsoon-fed.

Gangetic Plain

• The biggest of the four units, stretching Delhi to Kolkata over about 3.75 lakh sq km, in three States: Uttar Pradesh, Bihar, West Bengal.
• The Ganga and its many Himalayan tributaries, such as the Yamuna, Gomati, Ghaghara, Gandak and Kosi, brought the alluvium that built it.
• Peninsular rivers, the Chambal, Betwa, Ken and Son, have added to it.
• The general slope is east and south-east.
• In the lower sections rivers flow sluggishly, leaving fluvial landforms such as levees, bluffs, oxbow lakes and marshes as low local relief, along with ravines and other arid landforms further west.
• Almost every river keeps shifting its course, so floods are frequent. The Kosi, long called the “Sorrow of Bihar”, is the classic case.
  • It has moved westward by more than 113 km across its megafan in about 200 years, and its level has been known to rise 10 m within 24 hours. It flowed near Purnea in 1736 and now runs about 110 km west of it.
  • On 18 August 2008 it broke the eastern embankment at Kusaha in Nepal, 12 km above the barrage, and jumped about 120 km eastward into an old channel.
    • The flow was far below the barrage’s design discharge. The bed had silted up above the floodplain inside the embankments, so the breach came at an ordinary discharge.
    • More than 3 million people were affected in Nepal and north Bihar.

Divisions of the Ganga Plain

Upper Gangetic Plain

• Boundary: the 291 m contour in the west, close to the Yamuna; the 300 m contour in the north; the 75 m contour in the south along the Peninsular edge; and the 100 m contour in the east.
  • The eastern edge is debated. Laurence Dudley Stamp, followed by Oskar Spate, used roughly the 100 cm isohyet, and the 100 m contour is the more practical line.
• It is drained by the Ganga, Yamuna, Ramganga, Sarda, Gomati and Ghaghara, nearly all flowing north-west to south-east with the land.
• The monotony is broken by the terai-bhabar belt, and locally by bluffs, meanders, oxbow lakes, levees, abandoned channels and bhur.
• It includes:
  • Rohilkhand Plains: named after the Rohilla Afghans, with Bareilly and Moradabad as its centres. Very fertile, with the Sharda and Ramganga doabs, and lying low.
    • Older accounts place Muzaffarnagar here. It is west of the Ganga, in the upper Ganga-Yamuna Doab.
  • Ganga-Yamuna Doab: the largest doab of India and the higher, western part of this plain. It holds fine wind-blown dust, the bhur, and is famous for sugarcane.
  • Yamuna-Chambal Basin: a badland of gullies and ravines, among the worst-degraded land in India.
    • Older accounts call it India’s largest degraded area. No survey supports that, but it is the best-known ravine tract: India’s ravines cover about 3.1–3.2 million hectares, with about 2.4 lakh hectares in Madhya Pradesh alone, in Gwalior, Bhind and Morena.

Middle Gangetic Plain

• A transitional plain par excellence, covering eastern Uttar Pradesh and Bihar, and opening without any physical break into the plains on either side.
• It ranks among the most fertile and densely settled tracts in the world.
  • Older accounts call it “the most fertile tract of the world”, able alone to sustain the major part of India’s population. That is rhetoric, not a measurement.
• No part stands above 150 m, and the Ghaghara, Gandak and Kosi fill a trough about 2,000 m deep below the Nepal Himalaya.
• Kankar is scarce because khadar predominates. Local features include tals, shallow lakes.
• Three doabs make up its units: Ganga-Ghaghara, Ghaghara-Gandak and Gandak-Kosi. By region:
  • Awadh Plain: between the Ghaghara and Gomati, in eastern Uttar Pradesh, flood-prone. The Ghaghara’s khadar reaches 55 km wide here. Some schemes count Awadh with the Upper plain, because the 100 m contour passes through it.
  • Mithila Plain: between the Gandak and Kosi, flood-prone, with a gradient of only about 6 cm per km.
  • Magadh Plain: east of the Son, on the steeper southern side, and not flood-prone.
• These plains have ideal loam deposits and a very high groundwater level.

Lower Gangetic Plain

• It takes in Kishanganj in Bihar, West Bengal without Purulia and the Darjeeling hills, and much of Bangladesh.
• From the Rajmahal Hills across to the Bangladesh border it is just 16 km wide.
• Three parts:
  • Paradelta in the north, a zone of erosion shaped like an inverted triangle, in northern West Bengal. The Tista, Jaldhaka and Torsa built it, and it holds the Duars (the Darjeeling tarai) as well as the Barind.
  • Rarh Plains in the west, next to the Chotanagpur Plateau, with laterite deposits. Older accounts draw the line against the plateau at the 35 m contour, while the plain’s western limit overall follows roughly the 50 m contour.
  • Delta Plains: the most extensive part, with braided channels, lakes and marshes, known for inland fishing and jute. Raised land here is called char and marshy basins bil. The Sundarbans, the mangrove or tidal forests, lie towards the coast.

Ganga-Brahmaputra Delta

• The largest delta in the world, about two-thirds of the Lower Ganga Plain.
• The Ganga splits into several channels here. The land falls by barely 2 cm per km, and two-thirds of the area stands below 30 m, which makes the delta highly vulnerable to sea-level change.
• Facing the sea it breaks into estuaries, mudflats, mangrove swamps, sandbanks, islands and forelands.
• A large part of the coastal delta carries tidal forest, the Sundarbans (Sunderbans), named after the dominant sundri tree.
  • The mangrove tract covers about 10,000 sq km. About one-third (34%) of the mangrove forest is in India on one UNESCO count, and about 40% of the whole tract on another.
  • It is a UNESCO World Heritage Site in both India and Bangladesh, and it is the world’s largest mangrove forest.

Brahmaputra Plain

• Also known as the Brahmaputra Valley, Assam Valley or Assam Plain, since most of the valley lies in Assam.
• Boundaries: the India-Bangladesh border and the edge of the Lower Ganga Plain in the west, and the Purvanchal hills (Patkai and Naga) in the east. To the north rises the Eastern Himalaya of Arunachal Pradesh, and to the south the Mikir, Garo, Khasi and Jaintia Hills.
• It is an aggradational plain built by the Brahmaputra and its tributaries, about 720 km long and 56,000 sq km in area.
• The river enters near Sadiya and turns south near Dhubri into Bangladesh. The land drops westward from 130 m to 30 m, at about 12 cm per km, and the 150 m contour marks the edge of the surrounding hills.
• The north side is steep and the south falls gently.
• Countless northern tributaries meet the valley abruptly and form alluvial fans. They then split into many channels and meander, leaving bils and oxbow lakes.
  • Some southern tributaries meander too.
• Large marshy tracts occur here, and the fans of coarse debris create terai or semi-terai conditions.
• Majuli, on the Brahmaputra, is the world’s largest river island by Guinness World Records and India’s first island district (2016). Erosion cut it from 733.79 sq km in 1914 to 502.21 sq km in 2004, with official imagery showing 524.29 sq km in 2016.

• This map shows the drainage basins of the Ganga, Brahmaputra and Meghna rather than a profile of the plain, so its shading runs well beyond the plain, onto the plateau and into Tibet.

Significance of the Plain

• Population: about one-fourth of the country’s land hosts about half its people.
  • The official figure for the Ganga basin alone is 26% of the land and about 43% of the population.
• Farming: fertile alluvial soils, a flat surface, slow perennial rivers and a favourable climate support intense agriculture.
• Granary: heavy use of irrigation has made Punjab, Haryana and western Uttar Pradesh the granary of India, much as the Prairies are called the granaries of the world.
• Transport and growth: roads and railways form a dense web everywhere but in the Thar, and they have drawn large-scale industrialisation and urbanisation, and trade has followed.
• Cultural tourism: many religious centres line the sacred Ganga and Yamuna, rivers held dear by Hindus.
  • The faiths of the Buddha and Mahavira flourished here, as did the Bhakti and Sufi movements.