Drainage System | CBSE Class 11 Geography Notes

This note covers drainage concepts and patterns, river-basin classification, the evolution of Himalayan and Peninsular drainage, major Indian river systems, river-course changes, seasonal water availability, river-water use, pollution and conservation.

What do drainage, catchment area, river basin and watershed mean?

Definition: Drainage is the flow of water through well-defined channels, or paths carrying flowing water. A drainage system is the network of these channels. Where channels are ill-defined or choked, flooding is a common phenomenon.

A river's catchment area is the specific area from which it collects water. A tributary is a stream or river that joins another river. The area drained by a river together with its tributaries is its drainage basin.

The boundary separating neighbouring drainage basins is a watershed. The word also has an area-based use: catchments of small rivulets and rills, meaning small streams, are often called watersheds. Catchments of large rivers are called river basins.

There is a slight difference between the two area-based terms. Watersheds cover small areas, while river basins cover larger areas. A water divide separates drainage flowing towards different basins or outlets.

River basins and watersheds are unified areas. Events in one part directly affect other parts and the whole unit. They are therefore appropriate planning regions at small, intermediate and large scales, corresponding to micro, meso and macro planning.

How are the four drainage patterns recognised?

A drainage pattern is the arrangement of channels. It reflects geological time, the nature and structure of rocks, topography or surface relief, slope, the amount of flowing water and the periodicity, or timing and recurrence, of flow.

Radial and centripetal patterns differ in direction: radial flow spreads outwards from a hill, while centripetal flow converges inwards. Dendritic and trellis patterns differ in the arrangement of their branches, especially the parallel tributaries and right-angle junctions of trellis drainage.

How is India's drainage classified by outlet, size and origin?

Which seas receive the drainage?

By the sea towards which water is discharged, Indian drainage falls into Bay of Bengal drainage and Arabian Sea drainage. The Delhi ridge, Aravalis and Sahyadris separate them. The Sahyadris are the Western Ghats.

Nearly 77 per cent of the drainage area, including the Ganga, Brahmaputra, Mahanadi and Krishna systems, is oriented towards the Bay of Bengal. The remaining 23 per cent includes the Indus, Narmada, Tapi, Mahi and Periyar systems, which discharge into the Arabian Sea.

The map labels the Arabian Sea west of India and the Bay of Bengal to the east. A water-divide line is identified in the legend; the Narmada and Tapi are shown flowing towards the western coast.

See Fig. 3.2 in your NCERT textbook

How does catchment size classify river basins?

Area is expressed in square kilometres, written as km². Length is expressed in kilometres, written as km; elevation is expressed in metres, written as m. One lakh means one hundred thousand.

What distinguishes classification by origin?

By mode of origin, nature and characteristics, drainage is classified as Himalayan or Peninsular. A peninsula is land surrounded by water on three sides. This is the most accepted basis, although it has a difficulty: the Chambal, Betwa and Son are much older than rivers originating in the Himalayas, yet belong to the Ganga system.

Thus, a river's place of origin and the larger system it joins must be distinguished. Indian drainage reflects the evolution of the major physical regions and the nature and characteristics of precipitation, meaning water received from the atmosphere, including rain and snow.

What are the characteristics and proposed evolution of Himalayan drainage?

How do the mountain and plain courses differ?

The Himalayan system mainly includes the Indus, Ganga and Brahmaputra basins. Snowmelt and precipitation feed these rivers, making them perennial, or water-bearing throughout the year. An ephemeral river has water during the rainy season and is dry otherwise.

In the mountains, rivers cut giant gorges, meaning deep, narrow valleys, as erosion proceeds alongside Himalayan uplift. Erosion is the removal of rock or soil by agents such as flowing water. The rivers also form valleys with a V-shaped cross-section.

Photograph: A River in the Mountainous Region (NCERT Class 11 Figure 3.1). The photograph shows a river beside rocky banks, with wooded slopes and mountains behind it.

The Himalayan courses are highly tortuous, meaning winding. In the plains, the rivers strongly tend to meander, or form bends, and frequently shift course. Deposition, the laying down of transported material, produces flat valleys and other features.

Sediment is material such as sand and silt carried or deposited by water; silt consists of fine particles. These features include ox-bow lakes, curved lakes left by cut-off meanders; flood plains, low areas beside rivers built by flood deposits; and braided channels, interlacing channels separated by sediment deposits. Deltas are depositional landforms near river mouths, where rivers discharge their water.

Photograph: Rapids (NCERT Class 11 Figure 3.3). The photograph shows foaming water flowing around exposed rocks. Rapids are stretches of fast, turbulent river flow.

What is the Indo-Brahma explanation?

There are differences of opinion about Himalayan river evolution. Geologists believe that the Shiwalik or Indo-Brahma river once ran along the Himalayas from Assam through Punjab to Sind, entering the Gulf of Sind near lower Punjab during the Miocene period, some 5 to 24 million years ago.

This viewpoint is supported by Shiwalik continuity, its lacustrine, or lake-related, origin, and alluvial deposits, meaning river-deposited material. These include sand, silt, clay, boulders and conglomerates, rocks made of cemented rounded fragments.

It is opined that Indo-Brahma was later divided into the Indus and its five tributaries in the west, the Ganga and its Himalayan tributaries centrally, and the Brahmaputra stretch in Assam with its Himalayan tributaries in the east.

This division was probably caused by upheaval during the Pleistocene, a geological period, including uplift of the Potwar Plateau, which acted as an Indus-Ganga divide. A plateau is an elevated area of relatively level land. Down-thrusting of the Malda gap between the Rajmahal hills and Meghalaya plateau during the mid-Pleistocene diverted the Ganga and Brahmaputra towards the Bay of Bengal.

How do the Indus and its major tributaries form a river system?

Where does the Indus flow?

The Indus, also called Sindhu, is India's westernmost Himalayan river. It starts from a glacier near Bokhar Chu in the Tibetan Kailash range at 4,164 m. A glacier is a slowly moving mass of ice. In Tibet the river is called Singi Khamban, or Lion's mouth.

It flows northwest between the Ladakh and Zaskar ranges, through Ladakh and Baltistan, and forms a gorge near Gilgit. It enters Pakistan near Chilas, emerges from the hills near Attock, and ultimately reaches the Arabian Sea east of Karachi.

The Indus flows through Ladakh and Jammu and Kashmir in India. Himalayan tributaries include the Shyok, Gilgit, Zaskar, Hunza, Nubra, Shigar, Gasting and Dras. At Attock it receives the Kabul on its right bank, the right-hand side when facing downstream.

The left bank is correspondingly the left-hand side when facing downstream. Other right-bank tributaries are the Khurram, Tochi, Gomal, Viboa and Sangar, originating in the Sulaiman ranges. Above Mithankot, the southward-flowing Indus receives the Panjnad.

Which five rivers make up the Panjnad?

Panjnad names the five Punjab rivers: Satluj, Beas, Ravi, Chenab and Jhelum. Their sources and junctions help distinguish them within the Indus system.

The Satluj is called Langchen Khambab in Tibet and flows almost parallel to the Indus for about 400 km before entering India. It is an antecedent river, meaning its course predates uplift of the terrain it cuts across.

How does the Ganga develop from its headwaters and tributaries?

Where does the river acquire the name Ganga?

The Ganga begins as the Bhagirathi at the Gangotri glacier near Gaumukh, at 3,900 m, in Uttarkashi district, Uttarakhand. It cuts narrow gorges through the Central and Lesser Himalayas and meets the Alaknanda at Devprayag. After this confluence, or meeting of rivers, it is called Ganga.

The Alaknanda rises in the Satopanth glacier above Badrinath. Its Dhauli and Vishnu Ganga streams meet at Joshimath, or Vishnu Prayag. The Pindar joins at Karna Prayag; the Mandakini, also called Kali Ganga here, joins at Rudra Prayag.

The Ganga enters the plains at Haridwar, then flows south, southeast and east. It divides into the Bhagirathi and Padma distributaries, branches that separate from a river. Its water ultimately reaches the Bay of Bengal near Sagar Island.

The river is 2,525 km long: 110 km in Uttarakhand, 1,450 km in Uttar Pradesh, 445 km in Bihar and 520 km in West Bengal. Its basin covers about 8.6 lakh km² in India alone.

How do Himalayan and Peninsular tributaries join the system?

The Ganga system is India's largest river system. It includes perennial rivers from the Himalayas and non-perennial rivers from the Peninsula. Non-perennial rivers have very little water in the dry season. The Son is a major right-bank tributary.

Important left-bank tributaries are the Ramganga, Gomati, Ghaghara, Gandak, Kosi and Mahananda. The Yamuna is the westernmost and longest tributary of the Ganga. It rises at the Yamunotri glacier on the western slopes of the Banderpunch range and joins the Ganga at Prayag, or Allahabad.

The Yamuna receives the Chambal, Sind, Betwa and Ken from the Peninsular plateau on its right bank. The Hindan, Rind, Sengar and Varuna join its left bank. Much of its water supplies the western and eastern Yamuna canals and the Agra canal for irrigation, the supply of water to crops.

The Damodar was once known as the “sorrow of Bengal”. The Damodar Valley Corporation's multipurpose project brought it under control. A multipurpose project serves several uses of river water; this description should not be confused with the Kosi's association with Bihar.

Why do the Kosi and Brahmaputra frequently change their channels?

Case study: How does sediment change the Kosi's course?

The Kosi is an antecedent river. Its main stream, Arun, rises north of Mount Everest in Tibet. After crossing the Central Himalayas in Nepal, it receives the Son Kosi from the west and Tamur Kosi from the east, forming Sapt Kosi with the Arun.

Known as the “sorrow of Bihar”, the Kosi frequently changes course. Its course changes follow this sequence:

1. The Kosi brings a huge quantity of sediment from its upper reaches.
2. It deposits this material after reaching the plains.
3. The deposited material blocks the river's existing course.
4. Consequently, the river changes its course through the plains.

How does the Brahmaputra's name change along its route?

The Brahmaputra originates at the Chemayungdung glacier in the Kailash range near Mansarovar. It flows eastwards for nearly 1,200 km through dry, flat southern Tibet, where it is called Tsangpo, meaning “the purifier”. Rango Tsangpo is its major right-bank tributary there.

After cutting a deep gorge near Namcha Barwa, it emerges as Siang or Dihang. It enters India west of Sadiya in Arunachal Pradesh. Flowing southwest, it receives the Dibang, or Sikang, and Lohit on its left bank, after which it is called Brahmaputra.

During its 750 km journey through Assam valley, it receives the Burhi Dihing and Dhansari (South) on the left, and the Subansiri, Kameng, Manas and Sankosh on the right. The Subansiri originates in Tibet and is an antecedent river.

It enters Bangladesh near Dhubri and flows southwards. After the Tista joins on the right, it is known as the Jamuna. It merges with the Padma, which falls into the Bay of Bengal.

The Brahmaputra is known for floods, channel shifting and bank erosion, the wearing away of river banks. Most of its tributaries are large and bring large quantities of sediment because of heavy rainfall in their catchments.

The map labels the Indus and Punjab rivers in the northwest, the Ganga and Yamuna farther east, and the Tsangpo and Brahmaputra across the northern and northeastern part of the map.

See Fig. 3.2 in your NCERT textbook

How did Peninsular drainage evolve, and how does it differ from Himalayan drainage?

What features indicate the Peninsular system's age?

The Peninsular drainage system is older than Himalayan drainage. Its broad, largely-graded shallow valleys and mature rivers indicate this. A graded river profile is one in which the river has adjusted its slope towards a balance between erosion and deposition.

The Western Ghats, close to the western coast, divide the major rivers flowing into the Bay of Bengal from small streams entering the Arabian Sea. Most major Peninsular rivers, except the Narmada and Tapi, flow from west to east.

The Chambal, Sind, Betwa, Ken and Son originate in the northern Peninsula but belong to the Ganga system. Major Peninsular systems also include the Mahanadi, Godavari, Krishna and Kaveri.

Which three geological events shaped the system?

First, the western flank of the Peninsula underwent subsidence, meaning downward movement, and submerged below the sea during the early Tertiary, a geological interval. Generally, this disturbed the symmetrical arrangement of rivers on either side of the original watershed.

Second, Himalayan uplift was accompanied by subsidence and trough faulting along the northern Peninsular flank. Faulting means movement along fractures in the Earth's crust; trough faults create elongated depressions. The Narmada and Tapi flow through these depressions.

These rivers fill the original cracks with detritus, or fragments of rock and other worn material. This explains the lack of alluvial and deltaic deposits in these rivers.

Third, slight tilting of the Peninsular block from northwest towards southeast oriented the drainage towards the Bay of Bengal during the same period.

Note: Peninsular rivers are characterised by fixed courses, absence of meanders and non-perennial flow. The Narmada and Tapi, flowing through rift valleys, are exceptions to the general description. The Kaveri's rainfall supply also requires separate attention.

What distinguishes the major east-flowing Peninsular rivers?

Where do the Mahanadi, Godavari and Krishna flow?

The Mahanadi rises near Sihawa in Raipur district, Chhattisgarh, and crosses Odisha to reach the Bay of Bengal. Some navigation, or travel by water, occurs in its lower course. Madhya Pradesh and Chhattisgarh contain 53 per cent of its basin; Odisha contains 47 per cent.

The Godavari, the largest Peninsular river system, is also called Dakshin Ganga. It rises in Nasik district, Maharashtra, and reaches the Bay of Bengal. Its tributaries run through Maharashtra, Madhya Pradesh, Chhattisgarh, Odisha and Andhra Pradesh.

Its principal tributaries are the Penganga, Indravati, Pranhita and Manjra. Of its catchment, 49 per cent lies in Maharashtra, 20 per cent in Madhya Pradesh and Chhattisgarh, and the rest in Andhra Pradesh.

The Godavari has heavy floods in its lower reaches south of Polavaram, where it forms a gorge. It is navigable only in the deltaic stretch. After Rajamundri, it divides into several branches and forms a large delta.

The Krishna is the second largest east-flowing Peninsular river. It rises near Mahabaleshwar in the Sahyadris. The Koyna, Tungbhadra and Bhima are its major tributaries. Its catchment is shared by Maharashtra, Karnataka, Andhra Pradesh and Telangana.

The shares are 27 per cent in Maharashtra, 44 per cent in Karnataka, and 29 per cent in Andhra Pradesh and Telangana together. These shares describe catchment area, not shares of the river's length or water volume.

Case study: Why does the Kaveri have comparatively less fluctuation?

The Kaveri rises in the Brahmagiri hills at 1,341 m in Karnataka. Its important tributaries are the Kabini, Bhavani and Amravati. About 3 per cent of its basin is in Kerala, 41 per cent in Karnataka and 56 per cent in Tamil Nadu.

The upper catchment receives rain during the southwest monsoon season in summer. The lower catchment receives rain during the northeast monsoon season in winter. These are the seasonally changing monsoon wind systems associated with the rainfall described here.

This seasonal distribution allows the Kaveri to carry water throughout the year with comparatively less fluctuation than other Peninsular rivers. Fluctuation means variation in flow. Comparatively less variation does not mean an identical flow in every season.

How do the Narmada, Tapi and Luni differ from east-flowing rivers?

What are the courses of the Narmada and Tapi?

The Narmada rises on the western flank of the Amarkantak plateau at about 1,057 m. It flows through a rift valley between the Satpura range to the south and Vindhyan range to the north.

Near Jabalpur, it forms a gorge in marble rocks and the Dhuandhar waterfall. After about 1,312 km, it reaches the Arabian Sea south of Bharuch, forming a broad, 27 km long estuary, a river mouth where river water meets the sea in a broad opening.

Its catchment covers about 98,796 km². The Sardar Sarovar Project is built on the Narmada. Unlike the large delta formed by the Godavari's branches, the Narmada's mouth is described as an estuary.

The Tapi starts at Multai in Betul district, Madhya Pradesh. It flows westwards for 724 km and drains 65,145 km². Nearly 79 per cent of its basin lies in Maharashtra, 15 per cent in Madhya Pradesh, and the remaining 6 per cent in Gujarat.

Where does the Luni end?

The Luni is Rajasthan's largest river system west of the Aravali. It originates near Pushkar in two branches, the Saraswati and Sabarmati. They unite at Govindgarh, after which the river emerges from the Aravali as the Luni.

It flows westwards as far as Telwara, then turns southwest towards the Rann of Kuchchh. Its entire river system is ephemeral. This distinguishes its seasonal flow from the perennial Himalayan rivers and the Kaveri's year-round flow.

These rivers prevent a blanket eastward-flow rule for Peninsular India. The Narmada and Tapi are major west-flowing rivers reaching the Arabian Sea; the Luni follows a westward and then southwestward course towards the Rann of Kuchchh.

What limits river-water use, and how are pollution and conservation connected?

Why does abundant river water not mean uniform availability?

Indian rivers carry huge annual volumes of water, but availability is uneven in both time and space. Perennial rivers carry water throughout the year; non-perennial rivers have very little water in the dry season. During rains, much water flows away in floods and reaches the sea.

One part of the country may experience flooding while another faces drought, or prolonged water shortage. Inter-basin transfer means moving water from one river basin to another. It raises the possibility of transferring surplus water to water-deficit basins.

Examples for considering such links include the Periyar Diversion Scheme, Indira Gandhi Canal Project, Kurnool-Cuddapah Canal, Beas-Satluj Link Canal and Ganga-Kaveri Link Canal. Naming a scheme or proposal does not establish that every link is completed or that a regular surplus is available.

Uneven terrain creates difficulties: moving water from a plain to a plateau requires lifting it. Another question is whether northern rivers have sufficient surplus for regular transfers. A canal by itself does not settle terrain, supply or management problems.

Which problems affect river-water usability?

• Insufficient quantity: water may not be available in sufficient amounts.
• Pollution: contamination reduces the usability of river water.
• Silt load: river water carries fine sediment.
• Uneven seasonal flow: availability changes between seasons.
• Interstate disputes: states may disagree over river water.
• Shrinking channels: settlements extend towards the thalweg, the line following the deepest part of a river channel.

Pollution sources include dirty urban water, industrial effluents, or discharged liquid wastes, and other wastes. Most cremation grounds are on river banks, and bodies are sometimes thrown into rivers. Flowers and statues are immersed during some festivals; large-scale bathing and clothes washing also pollute river water.

What are the objectives and pillars of Namami Gange?

Namami Gange is an integrated conservation mission approved as a Union Government flagship programme in June 2014. Its objectives combine effective pollution abatement, meaning reduction, with conservation and rejuvenation, or restoration, of the Ganga.

Its eight main pillars are sewerage treatment infrastructure, river-front development, river-surface cleaning, biodiversity, afforestation, public awareness, industrial effluent monitoring and Ganga Gram. Sewerage treatment processes wastewater; biodiversity means the variety of living organisms; afforestation means establishing tree cover.

Ganga Gram names the village-focused pillar. Industrial effluent monitoring means checking industrial wastewater discharges. Together, the listed pillars address treatment, cleaning, living organisms, vegetation and public participation within the mission's conservation objectives.

Glossary

• Drainage — The flow of water through well-defined channels across an area.
• Drainage system — The network of channels through which an area's water flows.
• Catchment area — The specific area from which a river collects its water.
• Drainage basin — An area drained by a river and its tributaries together.
• Watershed — A boundary separating drainage basins; also often the catchment of small streams.
• Dendritic pattern — A drainage arrangement resembling tree branches, illustrated by northern plain rivers.
• Radial pattern — A pattern in which rivers flow outwards in all directions from a hill.
• Trellis pattern — Parallel primary tributaries joined at right angles by secondary tributaries.
• Centripetal pattern — Rivers converging from all directions into a lake or depression.
• Perennial river — A river carrying water throughout the year, including the dry season.
• Ephemeral river — A river with water during the rainy season and a dry course otherwise.
• Panjnad — The five Punjab rivers: Satluj, Beas, Ravi, Chenab and Jhelum.
• Distributary — A branch that separates from the main river and carries water away.
• Antecedent river — A river whose course existed before uplift of the terrain it traverses.
• Inter-basin transfer — Movement of water from one river basin to another basin.

Common errors and misconceptions

• Misconception: A watershed can mean only a dividing line. Correct: It also often means the catchment of small streams, while river basins cover larger areas.
• Misconception: Radial and centripetal drainage both spread outwards. Correct: Radial drainage spreads from a hill; centripetal drainage converges into a lake or depression.
• Misconception: Every river belonging to the Ganga system originates in the Himalayas. Correct: The Chambal, Sind, Betwa, Ken and Son originate in the northern Peninsula.
• Misconception: All major Peninsular rivers flow eastwards. Correct: Most do, but the Narmada and Tapi flow westwards into the Arabian Sea.
• Misconception: The Kaveri must become dry because Peninsular rivers are characterised by non-perennial flow. Correct: Rainfall in different seasons across its catchment supports year-round water with comparatively less fluctuation.
• Misconception: The Indo-Brahma explanation is an undisputed account. Correct: Opinions differ; geologists believe in the former river, and its division was probably related to Himalayan upheaval.
• Misconception: The Kosi was the “sorrow of Bengal”. Correct: That name belonged to the Damodar; the Kosi is associated with the “sorrow of Bihar”.
• Misconception: Listing a river-link scheme proves that surplus water can be transferred regularly. Correct: Terrain, lifting water and the availability of a sufficient regular surplus remain questions to examine.

Exam-style questions with model answers

Q1. Radial drainage consists of rivers flowing outwards from a hill; centripetal drainage consists of rivers flowing from all directions into a lake or depression. Identify and explain the pattern in each situation: A, rivers spread outwards from the Amarkantak range; B, rivers converge into a depression. [2 marks]

1. Situation A is radial drainage because the rivers spread outwards in different directions from the elevated Amarkantak range.
2. Situation B is centripetal drainage because the rivers converge from different directions towards a common depression.

Q2. Major basins exceed 20,000 km², medium basins cover 2,000 to 20,000 km², and minor basins cover less than 2,000 km². The Narmada basin covers about 98,796 km², the Tapi 65,145 km² and the Kaveri 81,155 km². Classify each basin and justify each classification using the supplied threshold. [3 marks]

1. The Narmada is a major river basin because its catchment covers about 98,796 km², which is greater than the major-basin threshold of 20,000 km².
2. The Tapi is a major river basin because its catchment covers 65,145 km², also exceeding the specified major-basin threshold of 20,000 km².
3. The Kaveri is a major river basin because its catchment covers 81,155 km², placing it above the same 20,000 km² threshold.

Q3. The Kosi carries huge quantities of sediment from its upper reaches, deposits this material in the plains, experiences blockage of its course and consequently changes course. Explain this mechanism in four successive points, using the information given. [4 marks]

1. The Kosi first transports a huge quantity of sediment from its upper reaches towards the plains.
2. On reaching the plains, it deposits the material that it has carried from those upper reaches.
3. The deposition blocks the existing river course, so sediment accumulation affects the route available to the flowing water.
4. Consequently, the Kosi changes its course in the plains, linking its shifting route to the transport and deposition of sediment.

Q4. Use these five paired facts to compare Himalayan and Peninsular drainage: the Peninsular system is older; Himalayan rivers receive snowmelt and precipitation, while Peninsular rivers are characterised by non-perennial flow; Himalayan mountain valleys include deep gorges, whereas Peninsular valleys are broad, largely-graded and shallow; Himalayan rivers meander and shift in plains, whereas Peninsular courses are characterised as fixed with exceptions; the Kaveri receives summer rain in its upper catchment and winter rain in its lower catchment and carries water throughout the year with comparatively less fluctuation. Make five separate points. [5 marks]

1. The Peninsular drainage system is older than the Himalayan system. Their contrasting relative ages form one basis for comparing the two drainage systems.
2. Himalayan rivers receive both snowmelt and precipitation, supporting perennial flow. Peninsular rivers are characterised by non-perennial flow, so their general water-supply pattern differs.
3. Himalayan rivers cut deep gorges in the mountains. Peninsular rivers occupy broad, largely-graded shallow valleys, giving their valley forms a different character.
4. In the plains, Himalayan rivers meander and frequently shift their courses. Peninsular rivers are characterised by fixed courses, although exceptions must be recognised.
5. The Kaveri qualifies the general seasonal comparison: summer rain upstream and winter rain downstream support year-round water with comparatively less fluctuation than other Peninsular rivers.

Q5. Trace the Ganga's development using these facts: it rises as the Bhagirathi at the Gangotri glacier near Gaumukh; the Alaknanda rises at the Satopanth glacier above Badrinath; they meet at Devprayag, after which the river is called Ganga; the Ganga enters the plains at Haridwar, then flows south, southeast and east; it divides into Bhagirathi and Padma distributaries and discharges into the Bay of Bengal near Sagar Island. Organise the answer into five stages. [5 marks]

1. The headwater called Bhagirathi rises at the Gangotri glacier near Gaumukh. This is the starting point for tracing the river's development into the Ganga.
2. The Alaknanda begins separately at the Satopanth glacier above Badrinath. It is the other river involved in the confluence where the Ganga acquires its name.
3. The Bhagirathi and Alaknanda meet at Devprayag. From this confluence onwards, their united river is known as the Ganga.
4. The Ganga enters the plains at Haridwar. Its subsequent course runs first towards the south, then towards the southeast and east.
5. Farther along its course, it divides into the Bhagirathi and Padma distributaries. The river finally discharges into the Bay of Bengal near Sagar Island.

Q6. The listed problems in using river water are insufficient quantity, pollution, silt load, uneven seasonal flow, interstate water disputes and shrinking channels as settlements extend towards the thalweg, the line of greatest channel depth. Explain these six problems separately. Use these supplied meanings: pollution is contamination, silt is fine sediment, seasonal flow varies through the year, and interstate disputes are disagreements between states over river water. [6 marks]

1. Insufficient quantity means river water is not available in enough volume. The issue concerns the amount available for use in the affected area.
2. Pollution means contamination of river water. It is a problem of the condition of the water, alongside the separate question of its quantity.
3. Silt is fine sediment carried in river water. Its deposition can clog channels and reduce reservoir storage capacity, making water storage and supply more difficult.
4. Uneven seasonal flow means water availability changes through the year. River-water use therefore faces a supply that varies between different seasons.
5. Interstate disputes are disagreements between states over river water. They constitute a problem concerning the use of water shared across state boundaries.
6. Channels shrink as settlements extend towards the thalweg, the line following the deepest part of the channel. Settlement expansion therefore reduces the space occupied by the channel.

Key takeaways

• Drainage is channelled water flow; a drainage system is the network, while a drainage basin includes a river and its tributaries.
• Nearly 77 per cent of India's drainage area is oriented towards the Bay of Bengal, compared with 23 per cent towards the Arabian Sea.
• Himalayan rivers receive snowmelt and precipitation, cut mountain gorges, and form meandering courses and depositional features in the plains.
• The Ganga acquires its name at Devprayag, where the Bhagirathi meets the Alaknanda, and enters the plains at Haridwar.
• The Kosi's sediment deposits can block its course in the plains, explaining its frequent changes of course.
• Most major Peninsular rivers flow eastwards, while the Narmada and Tapi flow westwards through rift valleys.
• The Kaveri receives rainfall in different seasons across its catchment, supporting water throughout the year with comparatively less fluctuation.
• River-water use faces quantity, pollution, silt, seasonal-flow, interstate-dispute and channel-shrinkage problems; river linking also raises terrain and surplus-water questions.

Test yourself

What is the difference between a catchment area and a drainage basin?

A catchment area supplies water to a river; a drainage basin is the area drained by a river and its tributaries.

Which tributary arrangement identifies trellis drainage?

Primary tributaries flow parallel to one another, and secondary tributaries join them at right angles.

Which five rivers constitute the Panjnad?

The five rivers are the Satluj, Beas, Ravi, Chenab and Jhelum.

Where does the Bhagirathi meet the Alaknanda, and what is the united river called?

They meet at Devprayag, after which the united river is called the Ganga.

What names identify the Brahmaputra in Tibet and after the Tista joins it in Bangladesh?

It is called Tsangpo in Tibet and Jamuna after the Tista joins it in Bangladesh.

Why is “all Peninsular rivers become dry” an incorrect generalisation?

The Kaveri carries water throughout the year because its upper and lower catchments receive rainfall in different monsoon seasons.

What is the Narmada's river-mouth feature, and where does it reach the sea?

It forms a broad, 27 km long estuary and enters the Arabian Sea south of Bharuch.

What are the objectives of Namami Gange?

Its objectives combine effective pollution abatement with conservation and rejuvenation of the National River Ganga.