Drainage System

2.1 Core concepts

• Drainage and drainage system: Flow of water through well-defined channels is 'drainage'; the network of such channels is a 'drainage system'. The drainage pattern of an area is determined by geological time period, nature and structure of rocks, topography, slope, amount of water flowing, and periodicity of flow. (NCERT p. 17)
• Four drainage patterns: (i) Dendritic — tree-branch pattern, e.g. rivers of the northern plain; (ii) Radial — rivers flow outward from a central hill, e.g. rivers from the Amarkantak range; (iii) Trellis — primary tributaries flow parallel, secondary tributaries join at right angles; (iv) Centripetal — rivers discharge into a central lake or depression. (NCERT p. 17 box)
• Catchment area, river basin, watershed: A river drains water from its catchment area. The area drained by a river and its tributaries is a drainage basin. The boundary separating one basin from another is the watershed. Watersheds are small; basins cover larger areas. River basins and watersheds are used as micro, meso, or macro planning regions. (NCERT p. 19)
• Classification by orientation to sea: ~77% of India's drainage (Ganga, Brahmaputra, Mahanadi, Krishna, etc.) flows into the Bay of Bengal; ~23% (Indus, Narmada, Tapi, Mahi, Periyar) flows into the Arabian Sea. The water divide runs along the Delhi ridge, Aravalis, and Sahyadris. (NCERT p. 19)
• Classification by watershed size: (i) Major river basins — catchment >20,000 sq. km, 14 basins including Ganga, Brahmaputra, Krishna, Tapi, Narmada, Mahi, Pennar, Sabarmati, Barak; (ii) Medium river basins — 2,000–20,000 sq. km, 44 basins including Kalindi, Periyar, Meghna; (iii) Minor river basins — <2,000 sq. km. (NCERT p. 19)
• Evolution of Himalayan drainage — Indo-Brahma theory: Geologists believe a mighty river called Shiwalik or Indo-Brahma traversed the entire Himalayan length from Assam to Punjab during the Miocene period (5–24 million years ago), discharging into the Gulf of Sind. It was later dismembered into Indus (west), Ganga (central), and Brahmaputra (east) systems due to Pleistocene upheaval. The Potwar Plateau (Delhi Ridge) uplift divided Indus from Ganga drainage; downthrusting of the Malda gap diverted Ganga and Brahmaputra toward Bay of Bengal. (NCERT p. 20)
• Himalayan rivers — general characteristics: Fed by both snowmelt and precipitation, hence perennial. Pass through giant gorges, V-shaped valleys, rapids. In plains they form ox-bow lakes, flood plains, braided channels, meanders, and deltas. They shift course frequently (e.g. Kosi, the 'sorrow of Bihar'). (NCERT pp. 19–20)
• The Indus System: One of the largest river basins in the world — 11,65,000 sq. km total (321,289 sq. km in India); total length 2,880 km (1,114 km in India). Originates near Bokhar Chu (31°15'N, 81°40'E) in the Kailash range, Tibet, at 4,164 m. Known as 'Singi Khamban' (Lion's mouth) in Tibet. Flows northwest between Ladakh and Zaskar ranges, enters Pakistan near Chilas. Key Himalayan tributaries: Shyok, Gilgit, Zaskar, Hunza, Nubra, Shigar, Gasting, Dras. Receives Kabul river at Attock. Panjnad (five Punjab rivers: Satluj, Beas, Ravi, Chenab, Jhelum) joins above Mithankot. Discharges into Arabian Sea east of Karachi. (NCERT pp. 20–21)
• Jhelum: Rises from spring at Verinag, foot of Pir Panjal, Kashmir; flows through Srinagar and Wular lake; joins Chenab near Jhang, Pakistan. (NCERT p. 21)
• Chenab: Largest Indus tributary; formed by Chandra and Bhaga at Tandi near Keylong, HP; also called Chandrabhaga; 1,180 km long. (NCERT p. 21)
• Ravi: Rises west of Rohtang Pass, Kullu hills, HP; flows through Chamba valley; joins Chenab near Sarai Sidhu. (NCERT p. 21)
• Beas: Originates from Beas Kund near Rohtang Pass at 4,000 m; flows through Kullu valley; meets Satluj near Harike. (NCERT p. 21)
• Satluj: Originates in 'Raksas tal' near Mansarovar at 4,555 m, Tibet (called Langchen Khambab); antecedent river; passes through Shipki La; feeds Bhakra Nangal canal system. (NCERT p. 21)
• The Ganga System: Most important river of India. Rises in Gangotri glacier near Gaumukh, Uttarkashi, Uttarakhand, at 3,900 m (known as Bhagirathi here). Bhagirathi meets Alaknanda at Devprayag — thereafter called Ganga. Alaknanda source: Satopanth glacier above Badrinath. Length: 2,525 km; basin: ~8.6 lakh sq. km. Shared by Uttarakhand (110 km), UP (1,450 km), Bihar (445 km), West Bengal (520 km). Discharges into Bay of Bengal near Sagar Island. (NCERT pp. 21–22)
• Left bank tributaries: Ramganga, Gomati, Ghaghara, Gandak, Kosi, Mahananda.
• Right bank tributary: Son (major).
• Yamuna: Westernmost and longest tributary of Ganga; source in Yamunotri glacier, Banderpunch range (6,316 m); joins Ganga at Prayag (Allahabad). Right bank: Chambal, Sind, Betwa, Ken (from Peninsular plateau). (NCERT p. 22)
• Chambal: Rises near Mhow, Malwa plateau, MP; famous for Chambal ravines (badland topography). (NCERT p. 22)
• Kosi: Antecedent river; source north of Mt. Everest in Tibet; forms Sapt Kosi; known as 'sorrow of Bihar' for frequently changing course due to heavy sediment load. (NCERT pp. 20, 22)
• Damodar: Flows through rift valley on eastern margins of Chotanagpur Plateau; once 'sorrow of Bengal'; now managed by Damodar Valley Corporation. (NCERT p. 22)
• The Brahmaputra System: Originates in Chemayungdung glacier, Kailash range, near Mansarovar lake. Traverses ~1,200 km eastward in Tibet as Tsangpo ('the purifier'). Enters India west of Sadiya, Arunachal Pradesh, as Siang/Dihang. Receives Dibang/Sikang and Lohit; then called Brahmaputra. Travels 750 km through Assam valley. Enters Bangladesh near Dhubri, called Jamuna; merges with Padma into Bay of Bengal. Known for floods, channel shifting, bank erosion due to heavy sediment load. (NCERT pp. 22–23)
• Peninsular drainage — general characteristics: Older than Himalayan drainage. Broad, shallow, graded valleys indicate maturity. Rivers have fixed course, absence of meanders, non-perennial flow. Most flow west to east. Exceptions: Narmada and Tapi flow in rift valleys from east to west. (NCERT p. 23)
• Evolution of Peninsular drainage — three geological events: (i) Subsidence of western flank of Peninsula (early Tertiary) disturbed original watershed symmetry; (ii) Himalayan upheaval caused subsidence of northern Peninsular flank, creating trough faults in which Narmada and Tapi flow (hence no alluvial/deltaic deposits in these rivers); (iii) Slight tilting of Peninsular block northwest to southeast oriented drainage toward Bay of Bengal. (NCERT p. 23)
• Major Peninsular rivers:
• Mahanadi: Rises near Sihawa, Raipur district, Chhattisgarh; 851 km long; catchment 1.42 lakh sq. km; drains into Bay of Bengal through Odisha. (NCERT p. 23)
• Godavari: Largest Peninsular river; called 'Dakshin Ganga'; rises in Nasik district, Maharashtra; 1,465 km long; catchment 3.13 lakh sq. km; discharges into Bay of Bengal. Tributaries: Penganga, Indravati, Pranhita, Manjra. Forms large delta after Rajamundri. (NCERT pp. 23–24)
• Krishna: Second largest east-flowing Peninsular river; rises near Mahabaleshwar, Sahyadri; 1,401 km long. Tributaries: Koyna, Tungabhadra, Bhima. (NCERT p. 24)
• Kaveri: Rises in Brahmagiri hills (1,341 m), Kodagu, Karnataka; 800 km long; catchment 81,155 sq. km. Carries water throughout the year because upper catchment receives SW monsoon (summer) and lower catchment receives NE monsoon (winter). Tributaries: Kabini, Bhavani, Amravati. (NCERT p. 24)
• Narmada: Originates on western flank of Amarkantak plateau at 1,057 m; flows in rift valley between Satpura (south) and Vindhyan range (north); 1,312 km long; catchment ~98,796 sq. km; forms marble rocks gorge and Dhuandhar waterfall near Jabalpur; meets Arabian Sea south of Bharuch forming a 27 km estuary. Sardar Sarovar Project on this river. (NCERT p. 24)
• Tapi: Originates at Multai, Betul district, MP; 724 km long; catchment 65,145 sq. km; flows westward into Arabian Sea. (NCERT p. 24)
• Luni: Largest river of Rajasthan west of Aravali; originates near Pushkar; ephemeral; flows into Rann of Kutch. (NCERT p. 24)
• Usability of river water: River water in India is unevenly distributed in time and space. Perennial rivers carry water year-round; non-perennial carry very little in dry season. Inter-basin water transfer schemes discussed include Periyar Diversion Scheme, Indira Gandhi Canal Project, Kurnool-Cuddapah Canal, Beas-Satluj Link Canal, Ganga-Kaveri Link Canal. (NCERT pp. 24–25)
• River regimes — Himalayan vs Peninsular: A river regime is the pattern of seasonal variation in discharge. Himalayan rivers have a dual regime — they are fed by both snowmelt (summer maximum from April–June) and the southwest monsoon (June–September), giving them perennial flow with high discharge throughout summer and early autumn. Peninsular rivers depend almost entirely on the southwest monsoon and are therefore seasonal in regime, with sharp peaks in July–September and very low flow in the dry months. The Kaveri is the major exception because its lower basin receives the retreating north-east monsoon. (NCERT pp. 19–20, 24)
• Antecedent vs subsequent vs consequent drainage: Drainage that pre-dates the uplift through which it now cuts is called antecedent — the Indus, Satluj, Brahmaputra and Kosi are textbook Himalayan examples that maintained their southward courses by deep down-cutting as the Himalayas rose. Consequent streams flow down the original slope of the new land surface, while subsequent streams follow weak rock structures and join the consequent at right angles, producing the trellis pattern seen in the Vindhyan country. (NCERT pp. 20–22)
• Deltas vs estuaries — Peninsular contrast: Mahanadi, Godavari, Krishna and Kaveri build extensive deltas in the Bay of Bengal because the eastern coast is emergent, gradient is gentle, and sediment load is high. Narmada and Tapi, in contrast, flow through structural rift valleys with steep gradients into the submerged western coast; they cannot drop their load and instead form long estuaries — Narmada's 27 km estuary south of Bharuch is the standard example. (NCERT pp. 23–24)
• River pollution and inter-basin transfer: NCERT notes that the increasing demand of water for irrigation, industry and domestic use creates the twin challenges of pollution from untreated effluents and over-extraction. Inter-basin transfer schemes (Periyar Diversion to Tamil Nadu; Indira Gandhi Canal carrying Beas–Satluj water into the Thar desert) attempt to redistribute the surplus of perennial Himalayan and west-flowing rivers to deficit Peninsular basins. The Ganga Action Plan and Namami Gange are policy responses to pollution in the Ganga basin. (NCERT p. 25)
• Hierarchical drainage classification (Strahler order conceptual): A first-order stream has no tributaries; two first-order streams join to form a second-order stream; the Ganga at its mouth is approximately a tenth-order trunk stream. NCERT does not present the Strahler scheme directly but uses an equivalent hierarchical idea while describing major/medium/minor basins (>20,000 / 2,000–20,000 / <2,000 sq km).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 3.1 (p. 17): A river in the mountainous region — visualise gorges, V-shaped valleys, rapids characteristic of Himalayan upper course.
• Figure 3.2 (p. 18): Map of Major Rivers of India — must-memorise for location-based MCQs: note water divide (line separating Arabian Sea from Bay of Bengal drainage), positions of Indus, Ganga, Brahmaputra (north) and Mahanadi, Godavari, Krishna, Kaveri, Narmada, Tapi (Peninsula).
• Figure 3.3 (p. 19): Rapids — characteristic feature of Himalayan rivers in their upper course where gradient is steep.
• Confluence points of Ganga tributaries (p. 21): Vishnu Prayag (Dhauli + Vishnu Ganga = Alaknanda), Karna Prayag (Pindar + Alaknanda), Rudra Prayag (Mandakini + Alaknanda), Devprayag (Bhagirathi + Alaknanda = Ganga) — these are repeatedly tested.
• Rift valley rivers (p. 23–24): Narmada and Tapi both flow in rift valleys between parallel ranges — Satpura + Vindhyas for Narmada; no alluvial/deltaic deposits hence estuaries not deltas.

2.5 Key data table (NCERT figures to memorise)

2.4 Common confusions / NTA trap points

• 'Sorrow of Bihar' vs. 'Sorrow of Bengal': Kosi = sorrow of Bihar (changes course due to sediment); Damodar = sorrow of Bengal (floods; now managed by DVC). NTA frequently swaps these.
• Indus tributaries vs. Ganga tributaries: Panjnad rivers (Satluj, Beas, Ravi, Chenab, Jhelum) are all Indus tributaries, NOT Ganga tributaries. Yamuna, Ghaghara, Gandak, Kosi are Ganga tributaries. Students often confuse Chambal — it joins Yamuna (a Ganga tributary) not the Indus.
• Narmada and Tapi flow west, not east: Almost all Peninsular rivers flow west to east into Bay of Bengal. Narmada and Tapi are the major exceptions flowing into Arabian Sea. Luni is ephemeral and flows into Rann of Kutch — neither Bay of Bengal nor Arabian Sea proper.
• Devprayag vs. Vishnu Prayag: Devprayag is where Bhagirathi meets Alaknanda (river becomes 'Ganga'). Vishnu Prayag is where Dhauli and Vishnu Ganga meet to form Alaknanda. NTA uses these as distractors for each other.
• Kaveri's year-round flow: Unlike other Peninsular rivers, Kaveri does not dry up because its upper catchment receives SW monsoon and lower catchment receives NE monsoon — a standard 'reason'-type question. Students often attribute this only to snowmelt (which is incorrect for Peninsular rivers).
• Indo-Brahma vs Indo-Gangetic — different concepts: Indo-Brahma is the hypothetical Miocene mega-river that allegedly traversed the entire Himalayan length; Indo-Gangetic refers to the modern plain or geological trough between the Himalayas and the Peninsula. NTA may swap the labels to test reading carefulness.
• Source of Ganga at Devprayag, not Gangotri: The river officially named "Ganga" begins only at Devprayag where the Bhagirathi (from Gangotri) meets the Alaknanda (from Satopanth above Badrinath). Upstream of Devprayag, the headstream is the Bhagirathi, not the Ganga. NTA exploits this in source-identification questions.
• Yamuna ≠ a Peninsular tributary: Yamuna is a Himalayan tributary of the Ganga (source Yamunotri glacier). However, its own right-bank tributaries — Chambal, Sind, Betwa, Ken — are Peninsular. NTA mixes these in match-the-following.
• Tsangpo is Brahmaputra in Tibet, not Indus: Indus in Tibet is "Singi Khamban"; Brahmaputra in Tibet is "Tsangpo"; Satluj in Tibet is "Langchen Khambab". Swapping these Tibetan names is a high-frequency trap.
• Drainage pattern in northern plains: The Ganga and its tributaries in the northern plain exhibit a dendritic (tree-branch) pattern. The trellis pattern appears in the Vindhyan country, the radial pattern around Amarkantak, and the centripetal pattern around inland-drainage basins like Sambhar Lake.