Structure, Physiography and Drainage

The One Thing

India’s physiography and drainage have developed through the interaction of geological structure, plate movement, folding, faulting, uplift, erosion, deposition, relief and climate. These processes explain both the contrast between the young, tectonically active Himalayas and the older, more stable Peninsular Plateau, and the differing characteristics of their river systems.

Who and What

• Geological Structure: The arrangement, age and nature of rocks and tectonic formations. It influences relief, soils, minerals and drainage.
• Physiography: The study and classification of the major physical divisions and landforms of a region.
• Plate Tectonics: The theory that Earth’s lithosphere consists of moving plates. The collision of the Indian Plate with the Eurasian Plate formed and continues to uplift the Himalayas.
• Six major physiographic divisions: The Northern and North-Eastern Mountains, Northern Plains, Peninsular Plateau, Indian Desert, Coastal Plains and Islands.
• Himalayan Mountains: A young fold mountain system extending in an arc from the Indus valley to the Brahmaputra valley. It consists mainly of the Greater Himalaya, Lesser Himalaya and Shiwaliks.
• Trans-Himalayas: Ranges north of the Greater Himalaya, including the Karakoram, Ladakh, Zaskar and Kailash ranges.
• Greater Himalaya: Also called Himadri; the highest and most continuous Himalayan range, with an average height of about 6,000 metres, permanently snow-covered peaks and glaciers.
• Lesser Himalaya: Also called Himachal; the middle Himalayan range, including the Pir Panjal, Dhauladhar and Mahabharat ranges.
• Shiwaliks: The southernmost and lowest Himalayan range. They consist largely of unconsolidated sediments and contain longitudinal valleys called duns.
• Northern Plains: A vast level depositional plain formed mainly by the alluvial deposits of the Indus, Ganga and Brahmaputra systems. It comprises the Punjab Plain, Ganga Plain and Brahmaputra Plain.
• Bhabar: A narrow belt along the Himalayan foothills where streams disappear underground after depositing coarse pebbles and sediments.
• Terai: A marshy belt south of the Bhabar where underground streams re-emerge, producing wet and forested conditions.
• Bhangar: Older alluvium forming slightly elevated terraces above active floodplains. It may contain calcareous nodules called kankar.
• Khadar: Newer alluvium deposited annually in active floodplains; it is generally more fertile than Bhangar.
• Peninsular Plateau: An ancient, stable block of crystalline, igneous and metamorphic rocks, divided mainly into the Central Highlands and Deccan Plateau.
• Central Highlands: The plateau north of the Narmada, including the Malwa Plateau and associated uplands. It has a general eastward and north-easterly slope.
• Deccan Plateau: The triangular plateau south of the Narmada, bounded by the Western and Eastern Ghats and containing ancient rocks and basaltic lava formations.
• Western Ghats: A relatively continuous, high escarpment along the western edge of the Deccan Plateau. It is an important water divide.
• Eastern Ghats: A discontinuous and eroded range cut by the Mahanadi, Godavari, Krishna and Kaveri.
• Aravalli Range: One of India’s oldest fold mountain systems, extending broadly from Gujarat through Rajasthan toward Delhi.
• Indian Desert: The arid region west of the Aravalli Range, characterised by sand dunes, sparse vegetation, low rainfall and seasonal streams.
• Coastal Plains: Plains between the Peninsular Plateau and the sea. They are divided into the Western Coastal Plain and Eastern Coastal Plain.
• Western Coastal Plain: A relatively narrow strip divided into the Konkan, Karnataka Coast and Malabar Coast. It contains estuaries and some backwaters.
• Eastern Coastal Plain: A broader and more level plain divided into the Northern Circars and Coromandel Coast. Large rivers form extensive deltas here.
• Islands: India’s main island groups are the Andaman and Nicobar Islands in the Bay of Bengal and the Lakshadweep Islands in the Arabian Sea. Lakshadweep Islands are mainly coral islands, while the Andaman and Nicobar Islands are largely tectonic and volcanic. Barren Island is India’s active volcano.
• Drainage: The river network of an area, including its main river, tributaries, distributaries, drainage basins and water divides.
• Drainage Basin: The area drained by a river and its tributaries.
• Water Divide: An upland or ridge separating two drainage basins.
• Himalayan Drainage: A system fed by rainfall, snowmelt and glaciers. It contains generally perennial rivers with deep gorges, meanders, floodplains and deltas.
• Peninsular Drainage: An older and more stable system with largely fixed courses, shallow valleys and seasonal variation in discharge.
• Antecedent River: A river that existed before mountain uplift and maintained its course by cutting downward as the land rose. The Indus, Sutlej and Brahmaputra show this characteristic.
• Consequent River: A river that follows the natural slope of newly formed land, as seen in many Peninsular rivers flowing generally eastward.
• Dendritic Drainage: A tree-like pattern developing on uniform rock or gently sloping terrain.
• Trellis Drainage: A pattern in which parallel main rivers are joined by tributaries at nearly right angles, commonly associated with folded or alternating hard and soft rocks.
• Radial Drainage: A pattern in which streams flow outward from a central highland or dome.
• Inland Drainage: Drainage in which streams do not reach the sea but terminate in lakes, marshes or inland depressions, as in parts of western Rajasthan.
• River Regime: The seasonal pattern of variation in a river’s discharge.
• Delta: A depositional landform formed where a river divides into distributaries and deposits sediment near its mouth.
• Estuary: A funnel-shaped tidal river mouth where freshwater mixes with seawater. Many west-flowing Peninsular rivers form estuaries.
• Discharge: The volume of water passing through a river channel, expressed as Q = A × V, where Q is discharge, A is the cross-sectional area of the channel and V is mean flow velocity.

Causes and Consequences

• The collision of the Indian Plate with the Eurasian Plate formed the Himalayas.

The Himalayas are therefore young fold mountains, remain tectonically active and continue to undergo uplift and erosion.

• The Himalayas extend from the Indus valley to the Brahmaputra valley.

Their great elevation and steep relief produce deep gorges, meanders, large sediment loads and extensive floodplains.

• The Himalayas receive both rainfall and melting snow and glaciers.

Himalayan rivers are consequently generally perennial and maintain flow throughout the year.

• The Shiwaliks consist largely of unconsolidated sediments.

Their erosion contributes sediment to foothill streams and helps produce duns and the Bhabar–Terai sequence.

• Coarse sediment is deposited along the Himalayan foothills.

In the Bhabar, streams disappear underground; farther south, they re-emerge in the Terai, creating marshy, wet and forested conditions.

• Repeated deposition by the Indus, Ganga and Brahmaputra systems formed the Northern Plains.

The resulting deep alluvial soils make the plains highly fertile, supporting intensive agriculture and dense settlement.

• Differences in the age of alluvium produce Bhangar and Khadar.

Bhangar forms older, slightly elevated terraces and may contain kankar, whereas Khadar is renewed annually and is generally more fertile.

• The Peninsular Plateau is an ancient and stable land block.

Its rivers have largely fixed courses, shallow valleys and drainage patterns strongly influenced by long-term denudation and resistant crystalline rocks.

• The Western Ghats form the principal water divide of the Deccan Plateau.

Rivers on the western side generally flow toward the Arabian Sea, while most rivers on the eastern side flow toward the Bay of Bengal.

• Rift valleys guide the Narmada and Tapi westward.

The Narmada flows between the Vindhya Range to the north and the Satpura Range to the south, while the Tapi flows south of the Satpura Range.

• Most major Peninsular rivers follow the general eastward slope of the plateau.

The Mahanadi, Godavari, Krishna and Kaveri flow into the Bay of Bengal and form deltas. The Godavari is the largest Peninsular river basin and is often called the “Dakshin Ganga.”

• The Eastern Ghats are discontinuous and eroded.

The Mahanadi, Godavari, Krishna and Kaveri cut through them before reaching the Bay of Bengal.

• The Western Coastal Plain is narrow and includes the Konkan, Karnataka Coast and Malabar Coast.

Its topography favours estuaries and backwaters rather than the large deltas characteristic of the eastern coast.

• The Eastern Coastal Plain is broader and more level.

The Northern Circars and Coromandel Coast contain extensive deltas formed by large east-flowing rivers.

• Low rainfall and high aridity produce the Indian Desert.

Streams are generally seasonal, and in parts of western Rajasthan they form inland drainage, ending in lakes, marshes or depressions rather than reaching the sea.

• The Brahmaputra changes name across its course.

It is known as the Tsangpo in Tibet, enters India through Arunachal Pradesh as the Siang or Dihang, and becomes the Brahmaputra in Assam.

• The Ganga system is supplied by major tributaries.

These include the Yamuna, Ghaghara, Gandak, Kosi, Son and Damodar, which enlarge the drainage basin and contribute to floodplain formation.

• The Indus system includes the Jhelum, Chenab, Ravi, Beas and Sutlej.

These tributaries form an extensive Himalayan drainage network.

• The Ganga and Brahmaputra deposit large quantities of sediment near their mouths.

Their distributaries form the Ganga-Brahmaputra delta, one of the world’s largest deltas. Its southern part includes the Sundarban mangrove region.

• Peninsular rivers depend mainly on the southwest monsoon.

They are generally seasonal, although reservoirs and groundwater can maintain substantial perennial sections in some large rivers.

• River discharge varies with rainfall, snowmelt, basin size, slope, vegetation, soil, rock type and human use of water.

These factors determine the annual range of a river’s regime and its flood or low-flow conditions.

• Relief, geological structure, slope, rock resistance, climate and the history of river development shape drainage patterns.

Uniform rocks and gentle slopes may produce dendritic drainage; folded or alternating hard and soft rocks may produce trellis drainage; central highlands may produce radial drainage.

• Rivers provide water for agriculture, transport, hydropower, settlements, ecosystems and cultural activities.

Their use must be balanced with flood control, pollution prevention and sustainable water management because low-relief plains and large sediment supplies increase flood vulnerability.

What Gets Asked

• Compare Himalayan and Peninsular rivers in terms of age, sources of water, permanence, valley form, sediment load, drainage basins and seasonal discharge.
• Explain how the collision of the Indian Plate with the Eurasian Plate produced the Himalayan Mountains and why the Himalayas remain tectonically active.
• Distinguish between Bhabar and Terai, and between Bhangar and Khadar, with reference to deposition, relief, soil and fertility.
• Compare the Western Ghats and Eastern Ghats, including their continuity, elevation, erosion, role as a water divide and influence on river flow.
• Explain the different origins and characteristics of deltas and estuaries, using the Ganga-Brahmaputra delta, the Sundarban mangrove region and west-flowing Peninsular rivers as examples.
• Explain how geological structure, relief, slope, rock resistance, climate and river history produce dendritic, trellis, radial and inland drainage patterns.