Indo-Gangetic-Brahmaputra Plain

At a Glance

• Length About 3,200 km; about 2,400 km in India
• Alluvium depth Up to about 6,000 m in places
• Gradient About 20 cm per km
• Belts from north Bhabar, terai, bhangar, khadar
• Share About a quarter of the area, nearly half the people
• Exam link UPSC Prelims, Geography: India: Plains, Plateau, Coasts and Islands

Where Does It Fit?

This is the first topic of the macro on India’s plains, plateau, coasts and islands. The previous macro described the Himalayas that supply the sediments, and this one starts with the plain they built. The next topic covers the Peninsular Plateau and its minor plateaus. Previous topic: Passes of the Himalayas and Their Significance. Next topic: Peninsular Plateau and Minor Plateaus.

How Was the Great Plain Formed?

In short: the Himalayas rose, the rivers cut them down, and the sediment filled the trough in front of them to make an alluvial plain.

When the Indian plate collided with Eurasia, the Tethys sea between them was squeezed, and a long depression, the Indo-Gangetic trough or foredeep, formed in front of the rising Himalayas, as in Continent-Continent Convergence – Himalayas and Tibetan Plateau. Rivers coming from the young mountains carried huge amounts of gravel, sand and silt into the trough, and the glaciers and new rivers added more. Over millions of years, the Tethys receded and the trough was filled to form a monotonous, featureless plain. Peninsular rivers, such as the Chambal, Betwa, Ken and Son, added a small share. Today the plain is built mainly by the Indus, the Ganga and the Brahmaputra with their tributaries, and so it is named after them. The alluvium is thick: the depth to the basement rocks reaches about 6,000 m in places, though it varies greatly.

What Are the Features of the Plain?

• Size and form: an arcuate plain about 3,200 km from the Indus mouth to the Ganga mouth, about 2,400 km within India, up to about 500 km wide in the west, narrowing in the east. The Shiwaliks mark the north, an irregular line along the Peninsula marks the south, and the Purvanchal the east.
• Flatness: the average elevation is about 200 m, with the highest point of about 291 m near Ambala, which forms the divide between the Indus and Ganga systems. The gradient is about 20 cm per km from Saharanpur to Kolkata and falls to about 15 cm per km from Varanasi to the delta, so rivers flow slowly, meander and shift their courses.
• Depositional landforms: levees, bluffs, oxbow lakes, marshes, ravines and braided channels, as in Fluvial Landforms and Drainage Patterns.

What Are the Regional Divisions of the Plain?

• Rajasthan Plain: the Thar desert or Marusthali, an undulating plain at an average height of about 325 m. It is a part of the Peninsula geologically, with outcrops of old rocks. Shifting dunes called dhrian cover the west, and the eastern Bagar is semi-arid. The Luni flows into the Rann of Kutch, and saline lakes such as Sambhar, the largest, Didwana and Degana yield salt.
• Punjab-Haryana Plain: formed by the five rivers of the Indus system, with the land between rivers called doabs, at an average height of about 250 m. The northern edge is gullied by seasonal streams called chos. The Ghaggar, thought to be a successor of the Saraswati, lies between the Sutlej and the Yamuna.
• Ganga Plain: the largest unit, from Delhi to Kolkata, about 375,000 sq km, with the sub-regions of Rohilkhand, Awadh, Mithila and Magadh. Rivers are sluggish, shift their courses and cause floods; the Kosi is the Sorrow of Bihar.
• Brahmaputra Plain: the Assam valley, built by the Brahmaputra and its tributaries from the north, with alluvial fans, braided channels, meanders, oxbow lakes known as beels, and marshes.
• Ganga-Brahmaputra Delta: also called the Ganga delta, the largest delta in the world, with a slope of about 2 cm per km; two-thirds is below 30 m above sea level, so it is extremely vulnerable to sea-level rise and cyclones. Its seaward part has mudflats, islands and the tidal mangrove forest of the Sundarbans, named after the sundari tree, a UNESCO World Heritage Site shared by India and Bangladesh.

Why Is the Plain So Important, and What Are Its Problems?

The plain is about a quarter of India’s area and supports nearly half of its population. Its fertile soils, flat land, perennial rivers and canal and groundwater irrigation make Punjab, Haryana and western Uttar Pradesh the granary of India, as in Ganga River System. It has a dense network of roads and railways, large cities, and the religious centres of Hinduism, Buddhism and Sikhism.

It also faces serious pressures. Floods occur regularly in Bihar, eastern Uttar Pradesh and Assam; groundwater in the northwest has been depleted by irrigation for rice and wheat; arsenic is found in the groundwater of the Ganga and Brahmaputra plains from West Bengal to Bihar; the salinity of the soil is increasing in irrigated areas; and winter air pollution, affected by crop-residue burning and the temperature inversions of the plain, as in Vertical Distribution of Temperature and Temperature Inversion, affects health. The sediment-filled basin is also seismically vulnerable, since it lies on the front of the Himalayan thrust, and soft alluvium amplifies the shaking. As an analytical point, the plain is the base of India’s food security and its main environmental stress zone, so changes in crop patterns, groundwater management and flood plain zoning matter more here than anywhere else.

Why Is This Topic Important?

• Facts Bhabar, terai, khadar asked
• Concept Formation by deposition
• Issues Groundwater, floods, pollution

What Came Next?

The next topic describes the Peninsular Plateau and its minor plateaus. Next topic: Peninsular Plateau and Minor Plateaus.

Key People and Terms

• Bhabar Porous pebble belt at the foot of the Shiwaliks
• Terai Marshy belt south of the bhabar
• Bhangar Old alluvium
• Khadar New alluvium
• Kankar Lime nodules in old alluvium
• Doab Land between two rivers

Exam Corner

Mains Angle

Questions ask for formation and features. These points add depth.

• Formation Foredeep filling
• Belts Four alluvial zones
• Stress Groundwater and flood