Drainage

1. Drainage — Basin and Watershed

The term drainage describes the river system of an area. A river system consists of a main river and all its tributaries (rivers joining it from the side) and distributaries (smaller branches near the mouth).

Key terms

• Drainage basin / Catchment area: The entire area drained by a single river and all its tributaries. Every drop of rain falling within the basin eventually finds its way into that river.
• Watershed (water divide): The elevated land — a ridge or highland — that separates two adjacent drainage basins. Rain falling on one side of the watershed flows into one basin; rain on the other side flows into a different basin. Example: the Western Ghats act as a watershed between rivers draining to the Arabian Sea and those draining to the Bay of Bengal.
• Right bank / Left bank tributaries: Determined when facing downstream. Example — the Yamuna joins the Ganga from the right bank at Prayagraj.

Classification of drainage patterns

• Dendritic: Tree-like branching pattern — forms on uniform slopes (e.g., rivers of the northern plains).
• Trellis: Tributaries join the main river at right angles — forms on alternating hard and soft rocks.
• Rectangular: Forms on strongly jointed rocky terrain.
• Radial: Rivers flow outward in all directions from a central hill — e.g., rivers originating from the Amarkantak plateau (Narmada west, Son north, Mahanadi east).

Two major drainage systems of India

• Himalayan drainage: Indus, Ganga, Brahmaputra systems — perennial, fed by snowmelt and monsoon rain, carry heavy silt, form large plains and deltas.
• Peninsular drainage: Rivers rise from the Western Ghats or Deccan plateau — seasonal, rain-fed, shallower valleys, smaller discharge.

2. Himalayan Rivers — Perennial and Antecedent

Why perennial?

Himalayan rivers receive water from two sources throughout the year: (a) monsoon rainfall in summer and (b) melting snow and glaciers in spring and summer. This dual supply keeps them flowing even in the dry season, unlike peninsular rivers.

Why antecedent?

Himalayan rivers are older than the Himalayan mountains themselves. When the Himalayas began to rise due to tectonic forces, these rivers — already flowing southward from the Tibetan plateau — kept cutting downward through the rising rock, forming deep gorges. This is called antecedent drainage. The Indus, Satluj, and Brahmaputra all display this feature.

The Indus System

• Originates in Tibet, near Lake Mansarovar. Enters India through Ladakh, flows through Jammu and Kashmir, then into Pakistan and drains into the Arabian Sea.
• Total length: ~2900 km (one of the longest rivers in Asia); only a small part flows through India.
• Major tributaries in India: Jhelum, Chenab, Ravi, Beas, Satluj — collectively called the "Five Rivers" of Punjab (Punjab means "land of five rivers").
• The Indus Waters Treaty (1960) between India and Pakistan allocated the three eastern rivers (Ravi, Beas, Satluj) to India and the three western rivers (Indus, Jhelum, Chenab) to Pakistan.

3. The Ganga River System

Origin and course

• Rises from the Gangotri glacier in Uttarakhand as the Bhagirathi. At Devprayag it merges with the Alaknanda to become the Ganga.
• Enters the plains at Haridwar and flows east through Uttarakhand, Uttar Pradesh, Bihar and West Bengal before emptying into the Bay of Bengal.
• Total length: ~2500 km. The Ganga basin covers about one-fourth of India's total area.
• The Ganga is India's National River.

Major tributaries

The Sunderbans Delta

• Near the sea, the Ganga splits into many distributaries. In India, the main distributary is the Hugli; in Bangladesh, it is the Padma, which joins the Brahmaputra (called Jamuna in Bangladesh).
• Together they form the Ganga-Brahmaputra delta — the Sunderbans, the world's largest delta and a UNESCO World Heritage site.
• The Sunderbans is famous for the Royal Bengal Tiger and mangrove forests. It straddles India and Bangladesh.

4. The Brahmaputra River System

Names along its course

• Tibet: Tsangpo (flows east for ~1600 km, parallel to the Himalayas but north of them)
• Arunachal Pradesh: Dihang (after the sharp U-turn around Namcha Barwa)
• Assam: Brahmaputra
• Bangladesh: Jamuna

Key features

• Antecedent river: Like the Indus, the Brahmaputra pre-dates the Himalayas. It makes a dramatic hairpin bend (U-turn) around Namcha Barwa peak (7782 m), cutting one of the world's deepest gorges through the Himalayas.
• Braided channels and flood plains: In Assam, the river is very wide (up to 15 km in places) and flows in multiple braided channels. Floods deposit silt on the flood plains, making the land very fertile. Majuli in Assam is the world's largest river island, formed mid-stream by the Brahmaputra.
• Annual floods: Heavy monsoon rainfall + glacial meltwater cause severe annual flooding in Assam. Yet flood silt keeps the plains extremely productive.
• The Brahmaputra carries the highest sediment load of any river in the world — due to fragile young Himalayan geology and heavy monsoon rainfall.
• Total length: ~2900 km; length in India ~916 km.

5. Peninsular Rivers — Seasonal and Rain-fed

General characteristics

• Seasonal: Fed mainly by monsoon rain. Flow high in July-September; low or dry in summer.
• Older, mature rivers with gentler gradients; flow in wider, shallower valleys.
• Hard rock beds limit erosion; rivers carry less silt compared to Himalayan rivers.
• The Western Ghats act as the main watershed. East-flowing rivers are longer (drain into Bay of Bengal); west-flowing rivers are shorter and steeper.

East-flowing rivers (into Bay of Bengal)

West-flowing rivers (into Arabian Sea)

Why do Narmada and Tapi flow westward? They occupy down-faulted rift valleys between uplifted horst blocks — the structural trend directs them westward toward the Arabian Sea, unlike most other peninsular rivers.

Why is Cauvery nearly perennial?

Cauvery's upper catchment (Karnataka) receives rain from the south-west monsoon (June-September) while its lower delta region (Tamil Nadu) gets rain from the north-east monsoon (October-December). This overlapping monsoon coverage keeps the river flowing for more months than other peninsular rivers.

6. Lakes — Freshwater and Saltwater

Lakes are important for local climate moderation, tourism, fisheries, and as water reserves. India has both freshwater and saltwater (brackish) lakes.

How are lakes formed?

• Tectonic: depression caused by faulting — Wular Lake (J and K).
• Glacial: scouring by glaciers — Nainital, Bhimtal.
• Lagoon / coastal bar: sand bar cuts off a coastal bay — Chilika, Pulicat.
• Ox-bow: meander cut off by river migration — common in Ganga plains.
• Crater: volcanic crater filled with water — Lonar Lake, Maharashtra (but not in NCERT Ch 3).

7. Role of Rivers in India's Economy

Rivers have been the cradle of human civilisation in India. Their contributions span agriculture, energy, transport, and daily life.

• Irrigation: The most critical use. River water is diverted through canals to irrigate farmland. The Indus basin canal system is one of the largest irrigation networks in the world. The Cauvery and Krishna basins have extensive canal networks in peninsular India. India's food security depends heavily on river-fed irrigation.
• Drinking water supply: Most Indian cities are located on river banks — Delhi (Yamuna), Patna (Ganga), Varanasi (Ganga), Kolkata (Hugli), Agra (Yamuna), Nashik (Godavari). Rivers supply water to municipal treatment plants.
• Hydroelectric power (HEP): Fast-flowing Himalayan rivers and steep west-flowing peninsular rivers are ideal for HEP dams. Key examples: Bhakra-Nangal Dam (Satluj) — one of India's largest; Hirakud (Mahanadi); Nagarjuna Sagar (Krishna); Tehri Dam (Bhagirathi-Ganga).
• Navigation / Inland waterways: Rivers provide cheap, low-energy transport. National Waterway No. 1 (Ganga-Hugli: Allahabad to Haldia, 1620 km) and NW No. 2 (Brahmaputra: Sadiya to Dhubri) are the most important. Historically, rivers were the main arteries of trade.
• Fisheries: Rivers and flood plains support millions of fisherfolk. Freshwater fish species are commercially important; lakes like Chilika are major fishery hubs.
• Fertile soil / Agriculture: Annual river floods deposit alluvial silt, naturally replenishing soil fertility. The Ganga-Brahmaputra delta has some of the world's most fertile land because of this continuous silt deposition.
• Tourism and pilgrimage: Rivers — especially the Ganga, Yamuna, and Cauvery — have deep religious significance. Ghats at Varanasi, Haridwar Kumbh, and Prayagraj Sangam attract millions of pilgrims annually.
• Industrial use: Water-intensive industries (textile, steel, chemicals, thermal power plants for cooling) locate near rivers for a ready water supply.

8. Rivers and Pollution

Causes of river pollution

• Domestic sewage: Most cities discharge untreated or partially treated sewage directly into rivers. The Ganga receives sewage from over 100 towns and cities; only a small fraction is treated before discharge.
• Industrial effluents: Factories discharge chemical waste, heavy metals (mercury, chromium, lead), dyes, and toxic solvents. The tanneries of Kanpur and paper mills are major polluters of the Ganga.
• Agricultural runoff: Chemical fertilisers (nitrates, phosphates) and pesticides wash into rivers during rain, causing eutrophication — excessive algal growth that depletes dissolved oxygen and kills aquatic life.
• Religious and social practices: Immersion of idols, flowers, ashes, and unburnt bodies in rivers adds to pollution load.
• Solid waste dumping: Plastic and solid waste dumped on river banks eventually enters the water.
• Thermal pollution: Warm water released by thermal power plants raises river temperatures, reducing dissolved oxygen.

Consequences

• Loss of aquatic biodiversity — the Ganga river dolphin (India's National Aquatic Animal), gharials, turtles, and fish are endangered.
• Drinking water quality deterioration — waterborne diseases (cholera, typhoid, hepatitis) spread among communities dependent on river water.
• Soil contamination — polluted irrigation water affects crop quality and enters the human food chain.

Government measures

• Ganga Action Plan (GAP), 1985: India's first major river conservation programme — aimed to intercept and treat sewage, set up electric crematoria, and reduce industrial discharge into the Ganga. Results were limited.
• Namami Gange Programme (National Mission for Clean Ganga), 2014: Comprehensive approach — sewage treatment plants in all towns on the Ganga's banks, river-front development, biodiversity conservation, and public awareness campaigns. Targets both a clean Ganga and "Aviral Dhara" (continuous natural flow).
• Pollution Control Boards: The Central Pollution Control Board (CPCB) and State PCBs regulate industrial discharge under the Water (Prevention and Control of Pollution) Act, 1974.
• Similar action plans for Yamuna, Gomti, and other polluted rivers.

9. Solved NCERT Exercise Questions

Q1. Multiple choice — choose the right answer.

• (i) Which one describes the drainage pattern resembling branches of a tree?
  (a) Radial (b) Dendritic (c) Centrifugal (d) Trellis
  Answer: (b) Dendritic — the word comes from the Greek word for tree. Tributaries join the main river at acute angles giving a tree-like branching appearance, typically on uniform slopes.
• (ii) In which state is Wular lake located?
  (a) Rajasthan (b) Punjab (c) Uttar Pradesh (d) Jammu and Kashmir
  Answer: (d) Jammu and Kashmir
• (iii) The river Narmada has its source at:
  (a) Satpura (b) Brahmagiri (c) Amarkantak (d) Slopes of the Western Ghats
  Answer: (c) Amarkantak — Amarkantak plateau in Madhya Pradesh. (Brahmagiri is the source of Cauvery.)
• (iv) Which is the longest river of Peninsular India?
  (a) Narmada (b) Krishna (c) Godavari (d) Mahanadi
  Answer: (c) Godavari — length ~1465 km; also called "Dakshin Ganga."
• (v) Which one of the following lakes is a saltwater lake?
  (a) Sambhar (b) Dal (c) Wular (d) Gobind Sagar
  Answer: (a) Sambhar — an inland saltwater lake in Rajasthan used for commercial salt production.

Q2. Brief answers.

(i) What is meant by a water divide? Give an example.
Answer: A water divide (or watershed) is any elevated land — ridge, hill range or plateau — that separates two drainage basins, directing rainfall into different river systems. The Western Ghats form the major water divide in Peninsular India: rivers on the western slopes flow into the Arabian Sea (Narmada, Tapi), while rivers on the eastern slopes (Godavari, Krishna, Cauvery, Mahanadi) flow into the Bay of Bengal.

(ii) Which is the largest river basin in India?
Answer: The Ganga basin is the largest river basin in India. It covers about one-fourth of the country's total area, spanning from Uttarakhand and Himachal Pradesh in the north to West Bengal in the east, including Uttar Pradesh, Bihar, Jharkhand and parts of Delhi.

(iii) Where do the rivers Indus and Ganga have their origin?
Answer: The Indus originates in Tibet (China), near Lake Mansarovar in the Kailash range. The Ganga originates from the Gangotri glacier in Uttarakhand, flowing initially as the Bhagirathi; it merges with the Alaknanda at Devprayag to become the Ganga.

(iv) Name the two headstreams of the Ganga. Where do they meet to form the Ganga?
Answer: The two headstreams are Bhagirathi and Alaknanda. They meet at Devprayag in Uttarakhand to form the river Ganga.

(v) Why does the Brahmaputra in its Tibetan part have less silt, despite a longer course?
Answer: In Tibet, the Brahmaputra (Tsangpo) flows through a cold, dry, semi-arid plateau where rainfall is very low, vegetation is sparse and the terrain is relatively stable. There is little material available for erosion. But once it enters Arunachal Pradesh and Assam, it passes through the young, fragile Himalayas under very heavy monsoon rainfall, causing intense erosion and carrying enormous silt loads.

(vi) Which two Peninsular rivers flow through trough (rift valley)?
Answer: Narmada and Tapi (Tapti) flow through rift valleys — structural depressions formed by down-faulting between uplifted blocks. This is why they flow westward towards the Arabian Sea instead of eastward like most peninsular rivers, and why they form estuaries rather than deltas.

(vii) State some economic benefits of rivers and lakes.
Answer: Rivers and lakes provide: (a) water for irrigation; (b) drinking water for cities; (c) hydroelectric power (Bhakra-Nangal on Satluj, Hirakud on Mahanadi, Tehri on Ganga); (d) inland navigation (NW-1 on Ganga, NW-2 on Brahmaputra); (e) fisheries — livelihoods for fisherfolk; (f) tourism and religious pilgrimage; (g) fertile alluvial soil renewal by floods; (h) water for water-intensive industries.

Q3. Classify the given lakes as Natural or Man-made (reservoir).

Natural lakes: Wular, Dal, Nainital, Bhimtal, Loktak, Barapani, Chilika, Sambhar, Pulicat
Man-made (reservoir) lakes: Gobind Sagar (Bhakra Dam on Satluj), Rana Pratap Sagar (Chambal), Nizam Sagar (Manjira), Nagarjuna Sagar (Krishna), Hirakud (Mahanadi)

Q4. Significant differences between Himalayan and Peninsular rivers.

Q5. Compare east-flowing and west-flowing peninsular rivers.