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Drainage Regionalization


Drainage regionalization refers to the systematic classification of river systems based on common physical, hydrological, and geomorphological characteristics. In India, drainage is mainly categorized according to:

  1. Origin and river characteristics

  2. Direction of flow / discharge

  3. Size of catchment area

  4. Physiographic control and geological structure

This framework helps understand:

  • Water resource distribution

  • Flood potential

  • Landform development

  • River behavior and environmental management


Drainage Basin / Catchment Area

The total area drained by a river and its tributaries.
Example: The Ganga basin is India's largest, covering nearly 26% of the country.

Watershed

A smaller hydrological unit within a basin, separated by ridges or highlands.

Perennial Rivers

Flow throughout the year; fed by rainfall + snowmelt.
Example: Ganga, Brahmaputra.

Seasonal (Non-perennial) Rivers

Flow mainly during the monsoon; dry up in summer.
Example: Pennar, Luni.

River Regime

Seasonal variation in river discharge over a year.

Drainage Pattern

The geometric arrangement of streams:

  • Dendritic (tree-like) – Ganga system

  • Trellis – in folded mountains like Chambal

  • Radial – rivers from Amarkantak

  • Rectangular – rivers flowing over jointed rocks

Regionalization Based on Origin and Characteristics

A. Himalayan Drainage System

Examples: Indus, Ganga, Brahmaputra and their tributaries.

Characteristics

  • Perennial rivers fed by glaciers + rainfall.

  • Large catchment areas and alluvial floodplains.

  • Exhibit all major fluvial landforms:

    • V-shaped valleys in upper course

    • Meanders and oxbow lakes in middle/ lower course

    • Floodplains, levees, distributaries in lower course

  • High sediment load due to rapid erosion in Himalayas.

  • Known for river capture and frequent course changes (e.g., Kosi River — "River of Sorrow").

Examples of tributaries

  • Ganga system: Yamuna, Ghaghara, Gandak

  • Indus system: Jhelum, Chenab, Ravi, Sutlej

  • Brahmaputra system: Subansiri, Tista

B. Peninsular Drainage System

Examples: Godavari, Krishna, Kaveri, Narmada, Tapi.

Characteristics

  • Older, mature rivers flowing over stable crystalline rocks.

  • Predominantly seasonal, dependent on monsoon rains.

  • Exhibit straight, structurally controlled river courses.

  • Show less erosional activity and well-developed river terraces.

  • Tend to have broad and shallow valleys due to long geological stability.

  • Peninsular rivers are often superimposed or rejuvenated due to tectonic uplift.

Major examples

  • East-flowing: Godavari, Krishna, Kaveri, Mahanadi

  • West-flowing: Narmada, Tapi, Luni

2. Regionalization Based on Direction of Discharge

India's drainage divides into Arabian Sea and Bay of Bengal systems based on flow direction, controlled by the Western Ghats, Himalayan slopes, and Peninsular plateaus.

A. Arabian Sea Drainage (23%)

Characteristics

  • Shorter rivers on the west coast

  • Steeper gradients and narrower valleys

  • High potential for hydropower

Major Rivers

  • Indus (though originates in Tibet, flows partly along India–Pakistan border)

  • Narmada and Tapi – flow through rift valleys

  • Periyar, Sharavati, Mandovi, Zuari

Example of geomorphic control

  • Narmada and Tapi run parallel to the Vindhya–Satpura ranges, showcasing tectonic origin (rift valleys).

B. Bay of Bengal Drainage (77%)

Characteristics

  • Rivers are longer, form large deltas

  • Carry heavy sediment loads

  • Highly braided and meandering channels

Major Rivers

  • Ganga – Brahmaputra – Meghna system (world's largest delta)

  • Godavari (Peninsular India's largest basin)

  • Krishna, Mahanadi, Kaveri

Important features

  • Famous deltas: Sundarbans, Mahanadi delta, Godavari delta

  • Frequent flooding due to low gradient and high monsoonal discharge

C. Inland Drainage

Found in arid and semi-arid regions where rivers do not reach any sea.

Examples

  • Luni River (flows into the Rann of Kachchh)

  • Seasonal rivers in Thar Desert, parts of Haryana and Punjab

  • Bols (salt lakes) like Sambhar Lake formed by interior drainage

Characteristics

  • High salinity

  • Ephemeral streams

  • Endorheic basins (closed drainage)

3. Regionalization Based on Size of Catchment Area

A. Major River Basins ( >20,000 sq. km )

India has 14 major basins.

Examples

  • Ganga (largest basin)

  • Brahmaputra

  • Godavari (largest in Peninsular India)

  • Krishna, Mahanadi, Indus, Narmada, Tapi, Kaveri

These basins support:

  • Dense population

  • Irrigation networks

  • Large deltas and alluvial plains

B. Medium River Basins ( 2,000–20,000 sq. km )

India has 44 medium basins.

Examples

  • Periyar (Kerala)

  • Meghna (NE India)

  • Subarnarekha, Vamsadhara, Pennar

C. Minor River Basins ( <2,000 sq. km )

Hundreds of small basins, mostly short coastal rivers.

Examples

  • Rivers of Goa (e.g., Chapora)

  • Short rivers in Tamil Nadu coast

  • Streams draining into the Chilika Lake

Additional Points Often Missed

  • Physiography strongly controls drainage (Himalayas vs. Peninsular plateau).

  • Western Ghats act as a major water divide, forcing rivers eastwards.

  • River rejuvenation due to tectonic uplift is common in India (e.g., Narmada).

  • Human activities like dams, barrages alter river regimes (Bhakra, Hirakud).

  • Delta formation is highest in Bay of Bengal due to large sediment loads.

  • Indus is a trans-boundary river, shared with Pakistan.

  • Ganga–Brahmaputra delta is the world's largest delta and home to the Sundarbans.


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