River basins

1. Basin concepts and drainage patterns

A river basin, or drainage basin, is the area from which surface runoff reaches a common river system and outlet. Its boundary is a drainage divide or water divide, usually following ridges and other elevated land. A tributary joins a larger stream, whereas a distributary branches away from the main channel, commonly near a delta. A confluence is the meeting point of streams. Groundwater movement does not always follow surface-basin boundaries because aquifers are controlled by subsurface geological conditions.

Catchment and drainage basin are often used interchangeably. In Indian planning, watershed commonly denotes a smaller hydrological unit within a basin, although terminology varies internationally. Basins can be subdivided into sub-basins and micro-watersheds for soil conservation, recharge and local water budgeting. An exorheic basin ultimately drains to the sea; an endorheic basin terminates within the landmass, usually in a lake, wetland or depression where evaporation and infiltration remove water.

Drainage patterns provide clues about relief and rock structure. Dendritic networks resemble tree branches and commonly develop over relatively uniform rocks. Trellis drainage occurs where alternating resistant and softer formations guide streams; rectangular drainage reflects joints and faults. Radial drainage develops outward from an elevated centre, while centripetal drainage converges towards a depression. Pattern describes the spatial arrangement of streams, not whether their water is perennial or seasonal.

• Drainage density is total stream length divided by basin area; it is influenced by permeability, vegetation, slope and rainfall.
• Antecedent rivers maintain their courses while the land rises; superimposed drainage inherits a course established on an earlier cover of rocks.
• River capture transfers drainage from one basin to another through processes such as headward erosion.

2. Himalayan and northern river basins

The Indus, Ganga and Brahmaputra are major transboundary Himalayan river systems. Their flows reflect both precipitation and snow or glacier melt, but the proportions differ across basins and seasons. They are generally perennial, carry substantial sediment and build extensive alluvial plains. Young mountain terrain, steep gradients, landslides and intense rainfall create major erosion and flood hazards. Himalayan rivers are not supplied exclusively by glaciers: monsoon rainfall is particularly important in the Ganga and Brahmaputra systems.

The Indus rises in Tibet near Lake Mapam Yumco, also called Manasarovar, and flows through Ladakh before entering Pakistan. Its major eastern tributaries are the Jhelum, Chenab, Ravi, Beas and Sutlej. The Beas joins the Sutlej at Harike in Punjab. The Chenab forms from the Chandra and Bhaga near Tandi in Himachal Pradesh. The Sutlej enters India near Shipki La. Distinguishing tributaries of the Indus system from those of the Ganga is a recurring map-based examination requirement.

The Ganga takes its name at Devprayag, where the Bhagirathi and Alaknanda meet. Major tributaries include the Yamuna, Ramganga, Gomti, Ghaghara, Gandak, Kosi and Son. The Yamuna joins the Ganga at Prayagraj, while the Chambal, Betwa and Ken join the Yamuna. The Brahmaputra is called Yarlung Tsangpo in Tibet and Siang or Dihang in Arunachal Pradesh; after receiving the Dibang and Lohit, it traverses Assam. The Ganga–Brahmaputra–Meghna system forms an extensive delta shared by India and Bangladesh.

• Left and right banks are identified while facing downstream, not by looking northward on a map.
• The Son is a right-bank tributary of the Ganga; the Ghaghara, Gandak and Kosi enter from the northern side.
• The Teesta joins the Brahmaputra, known as the Jamuna in Bangladesh, rather than directly joining the Ganga.

3. Peninsular and inland drainage

Peninsular drainage developed largely over an ancient, relatively stable geological framework. Many rivers have broad valleys, comparatively fixed courses and strong seasonal variation linked to the monsoon. They are often described as seasonal, but this does not mean every Peninsular river dries up each summer. Groundwater discharge, rainfall distribution and reservoir operations influence dry-season flow. The broad eastward slope of the plateau explains why most large Peninsular systems reach the Bay of Bengal.

The Mahanadi rises near Sihawa in Chhattisgarh and flows through Odisha. The Godavari rises near Trimbakeshwar in Maharashtra; important tributaries include the Pranhita, Indravati, Manjira and Sabari. The Pranhita drains the combined waters of the Wardha–Wainganga system. The Krishna rises near Mahabaleshwar, with the Bhima and Tungabhadra as major tributaries. The Kaveri rises at Talakaveri in Karnataka’s Brahmagiri Hills; its tributaries include the Kabini, Hemavati, Bhavani and Amaravati.

The Narmada rises near Amarkantak and flows west between the Vindhya and Satpura ranges, largely through a structurally controlled rift valley. The Tapi rises near Multai in Madhya Pradesh and also flows west through a structural trough. Both enter the Gulf of Khambhat and have estuarine mouths rather than extensive deltas. Short western coastal rivers include the Mandovi, Zuari, Sharavathi and Periyar. In arid northwestern India, some streams terminate inland; the Luni flows towards the Rann of Kachchh, while Sambhar Lake occupies a closed drainage depression.

• The Mahanadi, Godavari, Krishna and Kaveri build important east-coast deltas.
• West-flowing does not automatically mean rift-valley drainage: the Sabarmati, Mahi and numerous coastal rivers also flow west.
• River-mouth form depends on sediment supply, waves, tides and coastal configuration, not drainage direction alone.

4. Basin water balance and hazards

Basin water availability depends on precipitation, evapotranspiration, runoff and changes in storage. Storage includes soil moisture, groundwater, lakes, wetlands, reservoirs and snow. Annual runoff is not equivalent to water reliably available throughout the year. India’s concentrated monsoon creates a mismatch between wet-season supply and dry-season demand. Water-resource estimates must therefore distinguish average availability, seasonal dependability, usable storage, water quality and ecological requirements.

Land-use changes alter basin responses. Deforestation, road cutting and exposed slopes can increase erosion; urban surfaces accelerate storm runoff and reduce local infiltration. Floodplain occupation increases exposure even where flood magnitude remains unchanged. Excessive groundwater withdrawal may reduce baseflow to rivers. Dams moderate some floods and provide irrigation or electricity, but they also modify sediment transport, aquatic connectivity and downstream flow timing. Delta subsidence, coastal erosion and saline intrusion require attention to both upstream management and coastal processes.

• Flood and drought can occur within the same basin because rainfall, storage and access to water are uneven.
• Environmental flows concern the quantity, timing and quality of water needed to sustain river ecosystems and dependent livelihoods.
• A basin assessment should examine upstream–downstream effects rather than treating each project in isolation.

5. Governance and examination approach

Basin-based planning seeks to coordinate land, surface water, groundwater and ecosystem management across administrative boundaries. Under the Constitution, water appears in State List Entry 17, subject to Union List Entry 56 concerning regulation and development of inter-state rivers and river valleys when Parliament declares Union control expedient in the public interest. Article 262 enables parliamentary provision for adjudicating inter-state water disputes. The Inter-State River Water Disputes Act, 1956 and River Boards Act, 1956 are important statutory reference points.

Effective management requires shared data, flood forecasting, pollution control, demand management and coordination among states. The Central Water Commission and India-WRIS provide useful basin information; the National Water Policy, 2012 supports integrated water-resources management with the river basin or sub-basin as the planning unit. For Prelims, trace each river from source to mouth, identify tributary hierarchies and distinguish states traversed by the main channel from states containing part of its catchment. Also separate a basin’s total transboundary area from its area within India.

• Study river–dam pairs alongside basin maps: Hirakud–Mahanadi, Nagarjuna Sagar–Krishna and Sardar Sarovar–Narmada.
• Avoid assuming that rivers originating near one another belong to the same basin: the Narmada and Son drain in different directions.
• Use physical maps to connect divides, plateaus, mountain passes, confluences and river mouths.

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