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Drainage and Drainage System

NCERT Class 11 · Geography Based on NCERT Class 11 Geography textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains the concept of drainage, drainage systems, and drainage patterns in India. It covers the classification of Indian rivers into Himalayan and Peninsular systems, their evolution, major river systems, and the extent of usability of river water. The one big idea is that India's drainage system is a product of its geological history, physiography, and climate, and understanding it is essential for water resource management and flood control.
Key Concepts (23)
Drainage
The flow of water through well-defined channels.
e.g. Water flowing through rivers, nalas, and channels during the rainy season.
Drainage system
The network of drainage channels.
e.g. The network of the Ganga and its tributaries.
Drainage pattern
The outcome of the geological time period, nature and structure of rocks, topography, slope, amount of water flowing and the periodicity of the flow.
e.g. Dendritic pattern resembling branches of a tree.
Catchment area
The specific area from which a river drains the water collected.
e.g. The area from which the Ganga collects its water.
Drainage basin
An area drained by a river and its tributaries.
e.g. The Ganga basin covers about 8.6 lakh sq. km in India alone.
Watershed
The boundary line separating one drainage basin from the other. Also refers to small catchments of rivulets and rills.
e.g. The Delhi ridge acts as a water divide between the Arabian Sea and Bay of Bengal drainage.
Dendritic drainage pattern
A drainage pattern resembling the branches of a tree.
e.g. The rivers of the northern plain.
Radial drainage pattern
When rivers originate from a hill and flow in all directions.
e.g. Rivers originating from the Amarkantak range.
Trellis drainage pattern
When primary tributaries flow parallel to each other and secondary tributaries join them at right angles.
e.g. Found in areas with folded topography.
Centripetal drainage pattern
When rivers discharge their waters from all directions into a lake or depression.
e.g. Streams flowing into a crater lake.
Perennial river
A river that flows throughout the year, fed by both melting of snow and precipitation.
e.g. The Ganga, the Indus, the Brahmaputra.
Ephemeral river
A river that has water only during the rainy season and is dry otherwise.
e.g. Luni river in Rajasthan.
Antecedent river
A river that existed before the uplift of the Himalayas and continued to flow, cutting deep gorges.
e.g. The Satluj, the Kosi, the Subansiri.
River basin
The catchment area of a large river.
e.g. The Ganga basin.
Water divide
An elevated area that separates two drainage basins.
e.g. The Delhi ridge, the Aravalis, and the Sahyadris.
Estuary
A broad, funnel-shaped mouth of a river where it meets the sea, formed where the river is not building a delta.
e.g. The Narmada and Tapi form estuaries.
Delta
A depositional feature formed at the mouth of a river where it splits into several branches before entering the sea.
e.g. The Ganga-Brahmaputra delta.
Rift valley
A valley formed by the subsidence of a block of the earth's crust between two faults.
e.g. The Narmada and Tapi flow through rift valleys.
Panjnad
The name given to the five rivers of Punjab, namely the Satluj, the Beas, the Ravi, the Chenab and the Jhelum.
e.g. The Panjnad joins the Indus a little above Mithankot.
Sorrow of Bihar
The river Kosi, notorious for frequently changing its course and causing floods.
e.g. The Kosi brings huge quantities of sediments and deposits them in the plains.
Sorrow of Bengal
The river Damodar, which used to cause frequent floods before the Damodar Valley Corporation tamed it.
e.g. The Damodar now has multipurpose projects.
Dakshin Ganga
Another name for the Godavari, the largest Peninsular river.
e.g. The Godavari rises in Nasik district of Maharashtra.
Namami Gange Programme
An Integrated Conservation Mission approved by the Union Government in June 2014 with the twin objectives of effective abatement of pollution, conservation and rejuvenation of the National River Ganga.
e.g. Main pillars include Sewerage Treatment Infrastructure, River-Front Development, River-Surface Cleaning, Bio-Diversity, Afforestation, Public Awareness, Industrial Effluent Monitoring, and Ganga Gram.
How It Works
Evolution of the Himalayan Drainage System
  1. A mighty river called Shiwalik or Indo-Brahma traversed the entire longitudinal extent of the Himalaya from Assam to Punjab and onwards to Sind, discharging into the Gulf of Sind during the Miocene period (5-24 million years ago).
  2. The Indo-Brahma river was dismembered into three main drainage systems: the Indus and its five tributaries in the west, the Ganga and its Himalayan tributaries in the central part, and the Brahmaputra in Assam and its Himalayan tributaries in the east.
  3. The dismemberment was due to the Pleistocene upheaval in the western Himalayas, including the uplift of the Potwar Plateau (Delhi Ridge), which acted as the water divide between the Indus and Ganga systems.
  4. The down-thrusting of the Malda gap area between the Rajmahal hills and the Meghalaya plateau during the mid-Pleistocene period diverted the Ganga and Brahmaputra systems to flow towards the Bay of Bengal.
💡 The dismemberment of the Indo-Brahma river is the key event; the Delhi Ridge and Malda gap are the specific causes.
Evolution of the Peninsular Drainage System
  1. Subsidence of the western flank of the Peninsula led to its submergence below the sea during the early tertiary period, disturbing the symmetrical plan of rivers.
  2. Upheaval of the Himalayas caused the northern flank of the Peninsular block to subside, leading to trough faulting. The Narmada and Tapi flow in these trough faults.
  3. Slight tilting of the Peninsular block from northwest to the southeast gave orientation to the entire drainage system towards the Bay of Bengal.
💡 Three events: western subsidence, Himalayan upheaval (trough faulting), and southeast tilting.
Formation of Depositional Features by Himalayan Rivers
  1. Himalayan rivers, upon entering the plains, lose velocity due to reduced gradient.
  2. They deposit sediments, forming flat valleys, ox-bow lakes, flood plains, braided channels, and deltas near the river mouth.
  3. The Kosi river brings huge quantities of sediments from its upper reaches, blocking its course and causing it to change course frequently.
💡 Depositional features are formed in the plains, not in the mountains.
Classification of Indian Drainage System
  1. On the basis of discharge of water (orientation to the sea): Arabian Sea drainage (23%) and Bay of Bengal drainage (77%), separated by the Delhi ridge, Aravalis, and Sahyadris.
  2. On the basis of size of watershed: Major river basins (>20,000 sq. km, 14 basins), Medium river basins (2,000-20,000 sq. km, 44 basins), Minor river basins (<2,000 sq. km).
  3. On the basis of mode of origin, nature, and characteristics: Himalayan drainage and Peninsular drainage.
💡 77% of drainage area flows into the Bay of Bengal; the Narmada and Tapi are exceptions flowing west.
Exam Traps — Don't Make These Mistakes
⚠ Confusing catchment area with drainage basin
✗ Catchment area and drainage basin are the same thing.
→ Catchment area refers to the area from which a river collects water, while a drainage basin is the area drained by a river and its tributaries. The text states: 'A river drains the water collected from a specific area, which is called its catchment area. An area drained by a river and its tributaries is called a drainage basin.'
✓ Catchment area is the area of collection for a single river; drainage basin is the entire area drained by a river and all its tributaries.
⚠ Mixing up watershed and river basin
✗ Watershed and river basin are identical terms.
→ The text explicitly says: 'The catchments of large rivers are called river basins while those of small rivulets and rills are often referred to as watersheds. Watersheds are small in area while the basins cover larger areas.'
✓ River basins are large catchments of major rivers; watersheds are small catchments of rivulets and rills.
⚠ Calling the Kosi the 'Sorrow of Bengal'
✗ The Kosi is known as the 'Sorrow of Bengal'.
→ The Kosi is called the 'Sorrow of Bihar' due to its frequent course changes and flooding in Bihar. The Damodar is the 'Sorrow of Bengal'.
✓ The Kosi is the 'Sorrow of Bihar'; the Damodar is the 'Sorrow of Bengal'.
⚠ Thinking all Peninsular rivers flow east
✗ All Peninsular rivers flow from west to east into the Bay of Bengal.
→ The Narmada and Tapi are exceptions; they flow westward into the Arabian Sea through rift valleys.
✓ Most Peninsular rivers flow east into the Bay of Bengal, but the Narmada and Tapi flow west into the Arabian Sea.
⚠ Confusing the origin of the Indus and the Ganga
✗ The Indus originates from the Gangotri glacier.
→ The Indus originates from a glacier near Bokhar Chu in the Kailash Mountain range in Tibet. The Ganga originates from the Gangotri glacier.
✓ The Indus originates from a glacier near Bokhar Chu in Tibet; the Ganga originates from the Gangotri glacier in Uttarakhand.
⚠ Misidentifying the Panjnad rivers
✗ The Panjnad includes the Indus.
→ The Panjnad is the name given to the five rivers of Punjab: the Satluj, the Beas, the Ravi, the Chenab, and the Jhelum. The Indus is not included.
✓ The Panjnad consists of the Satluj, Beas, Ravi, Chenab, and Jhelum.
Compare & Contrast
Age and origin
Himalayan Drainage
Peninsular Drainage
Himalayan drainage is younger, evolved through long geological history; Peninsular drainage is older, with broad, largely-graded shallow valleys and mature rivers.
Nature of flow
Himalayan Drainage
Peninsular Drainage
Himalayan rivers are perennial, fed by snowmelt and precipitation; Peninsular rivers are non-perennial (except Narmada and Tapi), with fixed course and absence of meanders.
Formation at river mouth
Delta
Estuary
A delta is formed when a river deposits sediments at its mouth, splitting into distributaries (e.g., Ganga); an estuary is a broad, funnel-shaped mouth where the river meets the sea without building a delta (e.g., Narmada).
Shape and tributary arrangement
Dendritic Drainage Pattern
Trellis Drainage Pattern
Dendritic resembles branches of a tree with irregular tributary joining; trellis has primary tributaries flowing parallel to each other with secondary tributaries joining at right angles.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA (3 marks), 1 LA (5 marks)
Most asked: Classification of Indian drainage (Himalayan vs Peninsular) · Major river systems: Indus, Ganga, Brahmaputra, and Peninsular rivers · Drainage patterns (dendritic, radial, trellis, centripetal) · Differences between river basin and watershed, delta and estuary · Evolution of Himalayan and Peninsular drainage · Exceptions: Narmada and Tapi (west-flowing, rift valley) · Sorrow of Bihar (Kosi) and Sorrow of Bengal (Damodar) · Antecedent rivers (Satluj, Kosi, Subansiri)
Don't skip: Compare and contrast Himalayan and Peninsular drainage systems (5 marks); Describe the evolution of the Himalayan drainage system (3 marks); Explain the major river systems of India with their tributaries (5 marks)
One-Line Revision (read aloud before the exam)
  1. Drainage is the flow of water through well-defined channels; a drainage system is the network of such channels.
  2. The drainage pattern of an area depends on geological time, rock structure, topography, slope, water volume, and periodicity of flow.
  3. Dendritic pattern resembles tree branches; radial pattern flows from a central hill; trellis has parallel primary tributaries with right-angle secondary joins; centripetal flows into a lake or depression.
  4. A catchment area is where a river collects water; a drainage basin is the area drained by a river and its tributaries.
  5. A watershed is the boundary between drainage basins; small catchments are called watersheds, large ones are river basins.
  6. 77% of India's drainage flows into the Bay of Bengal; 23% flows into the Arabian Sea.
  7. The Himalayan drainage is perennial, fed by snow and rain; the Peninsular drainage is older, non-perennial, with fixed courses.
  8. The Indus originates from a glacier near Bokhar Chu in Tibet; the Ganga from the Gangotri glacier; the Brahmaputra from the Chemayungdung glacier.
  9. The Kosi is the 'Sorrow of Bihar' due to frequent course changes; the Damodar is the 'Sorrow of Bengal' due to floods.
  10. The Narmada and Tapi are exceptions: they flow westward through rift valleys and form estuaries.
  11. The Godavari is the largest Peninsular river, also called Dakshin Ganga.
  12. The Satluj, Kosi, and Subansiri are antecedent rivers that existed before the Himalayan uplift.
  13. The Panjnad is the collective name for the Satluj, Beas, Ravi, Chenab, and Jhelum.
  14. The Indo-Brahma river was dismembered into the Indus, Ganga, and Brahmaputra systems due to Pleistocene upheaval and the Malda gap down-thrusting.
  15. The Peninsular drainage evolved through western subsidence, Himalayan upheaval causing trough faults, and southeast tilting.
  16. The Namami Gange Programme aims at pollution abatement, conservation, and rejuvenation of the Ganga.

MCQs — 10 Questions with Answers

Q1. The boundary line separating one drainage basin from another is called:

  • A. Catchment area
  • B. Watershed ✓
  • C. Tributary
  • D. Delta

Answer: B — A watershed is the precise geographic boundary that divides drainage basins, whereas catchment area is the region itself.

Q2. Which drainage pattern is observed in the rivers of the northern Indian plains?

  • A. Radial
  • B. Trellis
  • C. Dendritic ✓
  • D. Centripetal

Answer: C — Dendritic drainage resembles tree branches and is characteristic of the northern plains' gentle slope and homogeneous rock structure.

Q3. What percentage of India's drainage area flows towards the Bay of Bengal?

  • A. 23 per cent
  • B. 50 per cent
  • C. 77 per cent ✓
  • D. 85 per cent

Answer: C — The Ganga, Brahmaputra, Mahanadi, and Krishna systems account for approximately 77 per cent of Indian drainage towards Bay of Bengal.

Q4. Which of the following is NOT a major river basin of India (>20,000 sq. km)?

  • A. Ganga
  • B. Krishna
  • C. Kalindi ✓
  • D. Brahmaputra

Answer: C — Kalindi is a medium river basin (2,000-20,000 sq. km), whereas Ganga, Krishna, and Brahmaputra are major basins exceeding 20,000 sq. km.

Q5. Rivers originating from the Amarkantak range and flowing in all directions exhibit which drainage pattern?

  • A. Dendritic
  • B. Radial ✓
  • C. Trellis
  • D. Centripetal

Answer: B — Radial drainage occurs when rivers originate from a central elevated region (like Amarkantak) and disperse outward in multiple directions.

Q6. Why is the Kosi River referred to as the 'sorrow of Bihar'? Consider the drainage behaviour and sediment transport.

  • A. It has very low water discharge
  • B. It frequently changes its course due to heavy sediment deposition blocking the channel ✓
  • C. It flows in a dendritic pattern
  • D. It receives water only during winter

Answer: B — The Kosi brings enormous sediment quantities from Himalayan reaches; when deposits block the channel, the river shifts course, causing frequent flooding and distress in Bihar.

Q7. Both statements: (I) Himalayan rivers are perennial because they are fed by snowmelt and precipitation. (II) Peninsular rivers are always perennial like Himalayan rivers. Which is true?

  • A. Both I and II are correct
  • B. Only I is correct ✓
  • C. Only II is correct
  • D. Both I and II are incorrect

Answer: B — Statement I is correct — Himalayan rivers depend on dual water sources ensuring year-round flow, while Statement II is false because Peninsular rivers are mostly seasonal.

Q8. If a drainage basin is classified as a major river basin, what is its minimum catchment area in sq. km?

  • A. 2,000 sq. km
  • B. 10,000 sq. km
  • C. 20,000 sq. km ✓
  • D. 50,000 sq. km

Answer: C — Major river basins in India are defined as those exceeding 20,000 sq. km of catchment area and include 14 drainage systems.

Q9. Study the map in Figure 3.2. Which of the following Arabian Sea drainage rivers originates from the Western Ghats and flows westward?

  • A. Ganga and Brahmaputra
  • B. Krishna and Godavari
  • C. Narmada and Tapi ✓
  • D. Mahanadi and Barak

Answer: C — The Narmada and Tapi are exceptional peninsular rivers that originate from the Deccan Plateau/Central highlands and discharge westward into the Arabian Sea, unlike most southern rivers flowing eastward.

Q10. HOTS: Ancient Indo-Brahma river (Miocene period) fragmented into three drainage systems. Which geological event caused this dismemberment, and how does this explain the current orientation of Himalayan rivers?

  • A. Erosion by tributaries; explains meandering in plains
  • B. Pleistocene uplift of Himalayan blocks; created physical barriers forcing rivers into separate basins ✓
  • C. Climate change; reduced snowmelt
  • D. Volcanic activity in Deccan; blocked westward flow

Answer: B — Pleistocene tectonic uplift fragmented the unified Indo-Brahma system into the Indus (west), Ganga (central), and Brahmaputra (east) basins, explaining how regional tectonics control modern river orientation and basin structure.

Flashcards

What is drainage?

Drainage is the flow of water through well-defined channels that drain excess water from a region.

Define catchment area.

A catchment area is the specific region from which a river collects water along with its tributaries.

What is a watershed?

A watershed is the boundary line that separates one drainage basin from another.

Name one dendritic drainage pattern example.

Rivers of the northern Indian plains display dendritic drainage patterns resembling tree branches.

What percentage of Indian drainage flows to Bay of Bengal?

Approximately 77 per cent of India's drainage area is oriented towards the Bay of Bengal.

Which Indian river is called 'sorrow of Bihar'?

The Kosi River is called the 'sorrow of Bihar' because it frequently changes its course.

What is the difference between Himalayan and Peninsular drainage?

Himalayan drainage is perennial (fed by snowmelt and rain), while Peninsular drainage is mostly seasonal.

Name the three major Himalayan river basins.

The three major Himalayan river basins are the Indus, Ganga, and Brahmaputra.

What is radial drainage pattern?

Radial drainage pattern occurs when rivers originate from a hill and flow in all directions, exemplified by rivers from Amarkantak range.

How many major river basins does India have?

India has 14 major river basins with catchment area exceeding 20,000 sq. km.

Important Board Questions

Define 'drainage' and explain how it differs from 'catchment area'. [2 marks]

Drainage = flow through channels; catchment area = region supplying water. Provide one geographical example like Ganga.

Explain why Himalayan rivers are perennial while most Peninsular rivers are seasonal. Support your answer with two specific features of each drainage system. [5 marks]

Himalayan: snowmelt + rainfall = year-round water; V-shaped valleys; Peninsular: rainfall only, seasonal flow. Mention one river example for each.

Analyze the evolution of the Himalayan drainage system from the Miocene Indo-Brahma river to the present-day three-basin system. Explain how Pleistocene tectonic activity caused this fragmentation and how the resulting basins function as drainage systems with distinct orientations towards the Arabian Sea and Bay of Bengal. [6 marks]

Trace timeline: Miocene unified river → Pleistocene uplift → three separate basins (Indus west, Ganga centre, Brahmaputra east); explain role of Delhi Ridge, Aravalis, Sahyadris as divides; discuss 77% vs 23% discharge split and river examples for each.

Assertion-Reason Questions for Drainage and Drainage System

Q1. Assertion (A):

Himalayan rivers are perennial in nature despite seasonal variations in discharge.

Reason (R):

Himalayan rivers are fed by both melting snow and precipitation throughout the year, maintaining continuous water flow even during dry seasons.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains why Himalayan rivers remain perennial. Snow and precipitation provide consistent water supply year-round, unlike peninsular rivers that depend solely on monsoon rainfall.

Q2. Assertion (A):

The River Kosi is known as the 'sorrow of Bihar' because it has the highest discharge among all Indian rivers.

Reason (R):

Kosi frequently changes its course due to deposition of huge quantities of sediments from upper reaches blocking the river channel.

Answer: D — A is false but R is true — The assertion is false—Kosi is called 'sorrow of Bihar' due to frequent course changes and flooding, not because of highest discharge. The reason is true; sediment blockage causes course shifts. However, since the assertion is false, the answer is C.

Q3. Assertion (A):

Approximately 77% of India's drainage area drains into the Arabian Sea.

Reason (R):

Rivers like Ganga, Brahmaputra, Mahanadi, and Krishna flow towards the Arabian Sea due to the western slope of the Peninsular Plateau.

Answer: D — A is false but R is true — The assertion is false—77% drains into the Bay of Bengal, not Arabian Sea. The reason is also false because the mentioned rivers (Ganga, Brahmaputra, Mahanadi, Krishna) flow to the Bay of Bengal, not Arabian Sea. Both are incorrect.

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Case-Based Questions (CBQ) for Drainage and Drainage System

Case Study:

The government of Bihar conducted a study on the Kosi River, which is responsible for devastating floods in the region nearly every five years. Researchers found that during monsoon season (June-September), the Kosi discharges 12,000-15,000 cubic meters of water per second, transporting massive quantities of sediment from its upper Himalayan reaches. The river deposits this sediment in its lower course, gradually raising the riverbed. When the riverbed rises, the existing embankments become ineffective, causing breaches and sudden course changes. The 2008 flood alone displaced over 2 million people and destroyed agricultural lands across multiple districts.

Q1. Why does the Kosi River carry such huge quantities of sediment during monsoon season?

  • A. The monsoon increases atmospheric pressure, causing erosion of riverbanks
  • B. High slope in upper Himalayan reaches and intense rainfall enable greater erosion and sediment transport ✓
  • C. Sediment is artificially added upstream to increase river flow
  • D. The river's velocity decreases, allowing more sediment to settle

Answer: B

Q2. According to the passage, how does sediment deposition lead to flooding and course changes?

  • A. Sediment blocks the river completely, forming a lake
  • B. Sediment raises the riverbed, making embankments ineffective and causing breaches and course shifts ✓
  • C. Sediment reduces water volume in the river
  • D. Sediment creates hard rock formations that divert water sideways

Answer: B

Q3. Which classification of the Kosi basin (based on catchment area) does the study suggest, given its massive discharge of 12,000-15,000 cu.m/s?

  • A. Minor river basin (<2,000 sq. km)
  • B. Medium river basin (2,000-20,000 sq. km)
  • C. Major river basin (>20,000 sq. km) ✓
  • D. Cannot be determined from discharge data alone

Answer: C

Q4. The 2008 Kosi flood demonstrates which principle mentioned in the study material about drainage basins?

  • A. Watersheds are smaller than river basins
  • B. Arabian Sea drainage is more prone to flooding
  • C. Radial drainage patterns cause flooding
  • D. What happens in one part of the basin affects other parts and the whole unit ✓

Answer: D

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