Chapter 4: Drainage
In the 1880s, the Survey of India faced a puzzle it could not settle by looking. A great river, the Tsangpo, ran eastward across southern Tibet for more than a thousand kilometres. Far to the south, in Assam, the mighty Brahmaputra swung westward through a broad valley. The two looked like one river split in half — but between them stood the Himalaya, pierced by a gorge so deep and so wild that no surveyor could follow the water through it. Were they the same river, or two different ones?
To find out, the surveyors turned to the water itself. They employed a Sikkimese explorer, Kinthup, and gave him an instruction that sounds like a riddle: cut five hundred logs, mark each one, and float them down the Tsangpo. Downstream in Assam, watchers would wait on the Brahmaputra. If the marked logs drifted past, one river fed the other, and both names belonged to a single stream.
The test was defeated by delay and distance, and the logs were never counted. Yet the idea behind it was exactly right. The Tsangpo and the Brahmaputra are one river. It rises near Lake Mansarovar, crosses Tibet, wheels around the peak of Namcha Barwa in a hairpin bend, and plunges south into India — carving, as it goes, one of the deepest gorges on Earth.
Map
Map 4.1 Caption: The Tsangpo of Tibet and the Brahmaputra of Assam are one river that loops around Namcha Barwa to enter India. Brief: A route map tracing the Tsangpo–Brahmaputra from source to plains. Show the Tibetan Plateau with the river rising near Lake Mansarovar and flowing east as the Tsangpo; mark the hairpin Great Bend around the Namcha Barwa peak; show the river descending south into Arunachal Pradesh as the Dihang (Siang), turning west across the Assam valley as the Brahmaputra, then bending south towards Bangladesh. Label Lake Mansarovar, Namcha Barwa, the Great Bend, the Dihang/Siang, and the Assam valley. Mark international boundaries, and show the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh; use boundaries as officially recognised by the Government of India. This is a course map only — do not shade relief or discharge.
4.1 The Anatomy of a River System
You have met rivers before as blue lines on a map. This chapter treats them instead as moving machines that carve and build the land, and to describe a machine you first need the names of its parts.
Drainage is the movement of water, chiefly from rainfall, off a land surface and along its channels towards the sea or an inland basin. The network of channels that a region’s water follows is its drainage system. Every river in this chapter is one such system at work.
A river does not choose its course freely. It takes the line of steepest available descent, so the layout of a drainage system is set by the relief and the rock beneath it. Where the land tilts, the water organises itself into channels; where the land lies flat, it spreads and struggles to gather.
The Basin and Its Divide
The whole area drained by a river and its tributaries is its drainage basin, also called its catchment. Every drop of rain that falls inside a basin eventually reaches that one river — the basin is the river’s collecting ground.
Basins are separated from one another by a water divide, the higher ground or ridge along which rain parts and runs off to different rivers. The divide is also known as the watershed. Stand on it, and a raindrop landing at your left foot may reach one sea while a drop at your right foot reaches another.
In hills a divide is an obvious crest. In the flat northern plains it can be so low as to be almost invisible, yet it still does its work: on one side of this gentle rise the water heads towards the Arabian Sea, and on the other towards the Bay of Bengal. Basins also nest inside one another — each tributary drains its own sub-basin within the larger basin of the trunk river.
The Parts of a River
A river begins at its source — a glacier snout, a spring, a lake, or simply seepage in high ground — and ends at its mouth, where it empties into a sea, a lake or a larger river.
Along the way, smaller streams called tributaries join it, each meeting point being a confluence. Near the sea the flow may split the other way into distributaries — channels that branch away from the main river and carry water off separately, most often across a delta.
The distinction matters. A tributary brings water in and adds to the river; a distributary leads water out and divides it. The trunk river together with all its tributaries and distributaries makes up a river system.
Diagram
Fig. 4.1-1 Caption: Every drop falling inside the divide drains to the one river that leaves at the mouth. Brief: A plan diagram of a single drainage basin. Draw the outer water-divide boundary as a closed loop enclosing the basin. Inside, show a source in the high ground, several tributaries joining a central trunk river at confluences, and one or two distributary channels splitting near the mouth where the river meets the sea. Label: water divide (watershed), source, tributary, confluence, trunk river, distributary, mouth. Use arrows on every channel pointing downstream, from source to mouth.
The Regime of a River
How much water a river carries is its discharge — the volume passing a point in the channel each second, measured in cubic metres per second, or cumecs (one cumec is a thousand litres of water sweeping past every second). Discharge is not steady; it rises and falls through the year.
The pattern of this rise and fall across the seasons is the river’s regime. And a river’s regime is decided above all by where its water comes from.
A river fed by melting snow and glaciers as well as by rain carries water in every season and is perennial — it never dries up. A river fed by rain alone runs full during the monsoon and can shrink to a chain of pools before the rains return; such a river is seasonal. This single fact — the source of a river’s water — will shortly divide all of India’s rivers into two great families.
4.2 The Evolution of Indian Drainage
India’s rivers fall into two great families: the Himalayan rivers of the north, and the Peninsular rivers of the centre and south. The split is not merely one of place. It is a difference of age and of origin, and it decides how each family behaves.
The two families were born on utterly different ground. The Himalaya are among the youngest mountains on Earth and are still rising. The Peninsular plateau is the opposite — a rigid, worn block that ranks among the oldest and most stable land surfaces on the planet. A river’s story begins with the land it was handed, so we take each family in turn.
4.2.1 The Himalayan Rivers: Antecedent Drainage
Here is a problem worth pausing on. The Himalaya form a wall of rock several kilometres high, and it is still growing. Yet great rivers such as the Indus and the Sutlej slice clean through that wall in gorges thousands of metres deep. How can a river saw through a mountain taller than itself?
The answer is that the rivers are older than the mountains. They were already flowing across this land before the Himalaya were pushed up along their path. As the ranges rose — slowly, over millions of years — each river held its course and cut downward at the same pace, wearing its channel deeper as fast as the land lifted beneath it.
A river that keeps its original course while the land across it rises, sawing a gorge straight through the rising ground, shows antecedent drainage — the drainage that came before (ante-) the uplift. The word records the sequence: first the river, then the mountain.
Diagram
Fig. 4.2-1 Caption: The river was there first; it cut downward as fast as the land rose, leaving a gorge straight across the range. Brief: A two-stage block diagram of antecedent drainage. Stage 1 (left): a river flowing across a low, nearly level surface. Stage 2 (right): the same ground uplifted into a mountain range, with the river now running through a deep, narrow gorge cut straight across the crest, its line unchanged. Use large upward arrows on both flanks of the range to show uplift, and a downward arrow in the channel to show the river’s downcutting. Label: original river course, uplift, gorge cut by downcutting. Emphasise that the river’s course is unchanged while the range has risen around it.
The gorges are the evidence. A river that began after the mountains stood could never have carved a path directly across them — it would have run around the barrier, along the easier ground. Only a river already in place could hold its line and cut down through a rising wall. The gorge itself is the proof that the water came first.
Because many of these rivers rise in or beyond the high ranges, in country buried under snow and ice, they are fed by melt-water as well as rain. That is why they carry water in every season and never run dry — a point we return to when the two families are compared.
4.2.2 The Indobrahm Theory
The lowest, outermost Himalayan range, the Shiwaliks, is built of thick alluvial deposits — sands, clays and boulder conglomerates. Material like this is dropped by a large river. So the deposits pose a question: which river laid them down?
One well-known answer is the Indobrahm theory. Geologists proposed that a single mighty river once ran the length of the Himalayan foreland, from Assam in the east westward to Punjab and beyond into Sind, carrying the combined waters of what are now the Indus, the Ganga and the Brahmaputra. This ancestral stream — the Indobrahm river, also called the Shiwalik river — is imagined to have spread the vast Shiwalik alluvium along its path.
The theory holds that this great river was later dismembered — broken apart — by two events. First, upheavals in the western Himalaya, including the Potwar plateau, during the Pleistocene raised a barrier across its course. Second, headward erosion by tributaries — the cutting-back of a stream into the land at its own head, lengthening it upstream — ate into the old channel and captured stretches of it. Out of this breakup emerged the three modern systems. It is even supposed that the Yamuna once flowed to the Indus and was later captured by the Ganga.
Flowchart
Fig. 4.2-2 Caption: One theory has a single ancestral river breaking apart into the three Himalayan systems of today. Nodes: A single ancestral river, the Indobrahm, flows along the Himalayan foreland from Assam to Sind, carrying the combined Indus, Ganga and Brahmaputra waters → it spreads the thick Shiwalik alluvium across the foreland → two events dismember it: (a) Pleistocene upheavals in the western Himalaya and the Potwar plateau raise a barrier; (b) headward erosion by tributaries cuts back and captures stretches of the old channel → the river breaks into three separate systems → branch to: the Indus with its Punjab tributaries; the Ganga with its Himalayan tributaries (the Yamuna captured from the Indus); the Brahmaputra of Assam with its tributaries
The Indobrahm is now largely set aside, on two principal grounds. First, one need not imagine so enormous a single river to explain the Shiwalik deposits at all — the ordinary rivers pouring down the Himalayan slopes could have brought that material down between them. Second, the depositional record does not fit the story: the immense thickness of alluvium filling the gap between the Rajmahal hills and the Shillong plateau would have taken far longer to accumulate than the theory allows.
So the Indobrahm survives as a hypothesis of historical interest rather than settled fact. It is a useful reminder that even a tidy, sweeping story must answer to the evidence in the ground.
4.2.3 The Peninsular Rivers: Consequent Drainage
The Peninsular plateau offers its rivers the opposite kind of ground: an ancient, rigid block, long stable and long worn down, close to sea level in places. Its rivers carry the marks of that great age.
They run in broad, shallow, graded valleys and do mostly sideways cutting rather than deep downcutting. These are the signs of old rivers that reached maturity long ago and have little vertical work left to do — a sharp contrast with the young, gorge-cutting Himalayan rivers still biting downward into rising land.
Most Peninsular rivers simply obey the plateau’s gentle eastward tilt, flowing west to east into the Bay of Bengal. A river whose course is fixed by the original slope of the land, following the lie of the ground as it first emerged, shows consequent drainage — the course is a direct consequence of the slope. The distinction with the Himalayan family is clean: an antecedent river is older than the relief and cuts across it, while a consequent river is a product of the relief and follows it.
Two large Peninsular rivers break the eastward rule and run west to the Arabian Sea: the Narmada and the Tapi. They flow not down the general slope but along great trough-faults — long structural cracks where a strip of ground dropped between two parallel faults to form a rift valley, or trough. These faults opened when the northern flank of the Peninsular block subsided, at the time the Himalaya and the northern plains were taking shape. The two rivers occupy ready-made troughs they never carved, their courses set by the lines of the faults.
4.3 The Himalayan River Systems
The rivers born of the Himalaya form three great systems — the Indus, the Ganga and the Brahmaputra. Each rises in or beyond the high ranges, in snow and glacier country, so each carries melt-water as well as monsoon rain and flows the whole year round. Map 4.2 shows how the three share the plains between them, along with the Peninsular rivers to their south.
Map
Map 4.2 Caption: The Himalayan and Peninsular rivers share the surface of India between them. Brief: A drainage map of India showing the major rivers named in this chapter. Label the Himalayan systems — the Indus with the Jhelum, Chenab, Ravi, Beas and Sutlej; the Ganga with the Yamuna, Ghaghara, Gandak and Kosi; the Brahmaputra — and the Peninsular rivers Mahanadi, Godavari, Krishna, Kaveri, Narmada, Tapi, Chambal, Betwa, Son and Damodar. Mark the Bay of Bengal, the Arabian Sea, the Ganga–Brahmaputra delta and the Farakka divergence. Show the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh, with international boundaries as officially recognised by the Government of India. A locational reference map only — do not shade relief or discharge.
4.3.1 The Indus System
The Indus and its five great tributaries — the Jhelum, the Chenab, the Ravi, the Beas and the Sutlej — drain the western Himalaya and the plains of the Punjab (the name means ‘land of five rivers’).
The Indus rises in Tibet near Lake Mansarovar, at about 5,180 m, and enters India in the Union Territory of Ladakh. Cutting a spectacular gorge, it pierces the Kailash range and flows north-west through Ladakh before passing into Pakistan, emerging from the hills at Attock and running south-west to the Arabian Sea east of Karachi. At about 2,880 km it is one of the longest rivers in the world.
The waters of the system are shared with Pakistan under the Indus Waters Treaty of 1960. Broadly, the three western rivers — the Indus, the Jhelum and the Chenab — were allotted largely to Pakistan, and the three eastern rivers — the Ravi, the Beas and the Sutlej — to India, with India permitted only limited use of the western rivers.
The five tributaries each begin high in the mountains. The Jhelum rises at the spring of Verinag in the south-eastern Vale of Kashmir, passes through Wular Lake, and joins the Chenab near Jhang in Pakistan. The Chenab, the largest of the five, is formed in Himachal Pradesh by the union of the Chandra and the Bhaga — hence its older name, the Chandrabhaga — and enters the plains near Akhnoor.
The Ravi rises in the Kullu hills near the Rohtang, flows past Chamba and enters the plains near Madhopur; it is about 720 km long. The Beas rises at Beas Kund near the Rohtang Pass, runs through the Kullu valley, and joins the Sutlej at Harike after about 470 km.
The Sutlej is the longest of the tributaries and an antecedent river. It rises near Lake Rakshastal (Rakas), close to Mansarovar in Tibet, at about 4,600 m, and enters India through the Shipki La, cutting deep gorges across the ranges. The Bhakra dam stands on it. Running about 1,450 km in all, it collects the Beas at Harike; the five united rivers, called the Panjnad, then join the Indus in Pakistan.
Flowchart
Fig. 4.3-1 Caption: The five rivers of the Punjab gather into one before joining the Indus. Nodes: Indus (source near Mansarovar; enters India in Ladakh) → flows down to the plains → tributaries gather: Jhelum → Chenab; Ravi → Chenab; Beas → Sutlej; then Sutlej → Chenab → the Chenab, now carrying all five, is called the Panjnad → Panjnad → Indus → Arabian Sea
4.3.2 The Ganga System
The Ganga is the most important river of India and drains the country’s largest basin, about 8.6 lakh sq km within India alone.
It is born of two headstreams. The Bhagirathi comes from the Gangotri glacier, at its snout at Gaumukh, about 3,900 m up; the Alaknanda comes from the Satopanth and Bhagirath Kharak glaciers further east. The two unite at Devprayag, and below that meeting the river is called the Ganga. It leaves the hills onto the plains at Haridwar. Of its roughly 2,525 km, about 2,415 km lie in India and the remaining 110 km in Bangladesh.
Across the plains the Ganga is fed from two directions, and this is what makes it so large. From the Himalaya on its left bank come the Ghaghara, the Gandak and the Kosi; from the Peninsula on its right bank come the Yamuna — with its own tributaries the Chambal and others — and the Son.
The Yamuna is the largest tributary. It rises from the Yamunotri glacier on the Bandarpunch massif and runs roughly parallel to the Ganga before joining it at Prayagraj, some 1,376 km from its source. Among the left-bank rivers, the Ghaghara begins beyond the mountains as the Karnali and cuts deep gorges across Nepal; the Gandak drains central Nepal and joins the Ganga at Sonpur near Patna; and the Kosi enters Bihar in a spread of braided channels.
A braided channel is a river split into a web of shifting strands weaving around banks and islands of its own sand and silt. The Kosi’s habit of braiding and wandering has made it notorious.
Beyond Farakka, in West Bengal, the river divides. The main flow crosses into Bangladesh as the Padma, while a distributary, the Bhagirathi–Hooghly, runs south through West Bengal. In Bangladesh the Padma is joined by the Brahmaputra (known there as the Jamuna) and then the Meghna. Together they build the largest delta in the world — the low, fan-shaped spread of sediment and branching channels a river lays down where it meets a calm sea. Its seaward fringe is the Sundarbans.
Flowchart
Fig. 4.3-2 Caption: The Ganga gathers water from both the Himalaya and the Peninsula, then splits at Farakka. Nodes: Bhagirathi (Gangotri glacier) + Alaknanda (Satopanth glacier) → unite at Devprayag to form the Ganga → onto the plains at Haridwar → joined from the Himalaya (left bank) by the Ghaghara, Gandak and Kosi, and from the Peninsula (right bank) by the Yamuna (with the Chambal) and the Son → reaches Farakka → splits into two: main flow → Padma into Bangladesh → joined by the Brahmaputra (Jamuna) and the Meghna → Ganga–Brahmaputra delta → Bay of Bengal; and distributary → Bhagirathi–Hooghly south through West Bengal → Bay of Bengal
4.3.3 The Brahmaputra System
The Brahmaputra rises from the Chemayungdung glacier near Lake Mansarovar in Tibet and flows east across the southern edge of the plateau for about 1,250 km as the Tsangpo. Rounding the peak of Namcha Barwa in a great bend, it drops south into Arunachal Pradesh as the Dihang (also called the Siang). Near Sadiya, in eastern Assam, it is joined by the Dibang and the Lohit and takes the name Brahmaputra.
From there it flows west across the wide, flat Assam valley. It carries one of the heaviest silt loads of any Indian river, braids across a bed several kilometres wide in flood, and holds Majuli, one of the largest river islands in the world. This great load and the sudden monsoon rise together make the Brahmaputra one of India’s most flood-prone rivers. Like the Indus and the Sutlej, it is an antecedent river, cutting its gorge across the rising ranges. In all it runs about 2,900 km.
Its tributaries pour in from both sides of the valley — the Subansiri, the Kameng and the Manas from the north, the Dibang, the Lohit and the Buri Dihing from the south. At the eastern edge of Bangladesh the river turns south, joins the Padma, and adds its enormous flow to the delta and the Bay of Bengal.
4.4 The Peninsular River Systems
The rivers of the Peninsula fall into two groups by the direction they flow. Most run west to east down the plateau’s gentle tilt and empty into the Bay of Bengal. A few run the other way, west into the Arabian Sea. All are fed by rain alone, so most shrink in the dry months, and all flow in the broad, graded valleys of an old and settled land.
4.4.1 Rivers Draining to the Bay of Bengal
Four large rivers carry most of the Peninsula’s water eastward: the Mahanadi, the Godavari, the Krishna and the Kaveri. Table 4.1 sets their lengths side by side.
The Mahanadi rises near Sihawa in the northern hills of Chhattisgarh and flows across Odisha to the Bay of Bengal, about 851 km in all. The Hirakud dam, one of the longest dams in the world, stands across it. Its basin is shared by Chhattisgarh, Odisha and neighbouring states.
The Godavari is the largest of the Peninsular rivers, so large that it is called the Dakshin Ganga, the Ganga of the south. It rises near Nashik in the Western Ghats of Maharashtra and runs south-east across the Deccan to reach the Bay of Bengal in Andhra Pradesh, about 1,465 km away. Its basin, some 3.13 lakh sq km, is the biggest in the Peninsula, and about half of it lies in Maharashtra. Its main tributaries include the Pravara, the Manjra, the Penganga, the Wardha, the Wainganga, the Pranhita and the Indravati.
The Krishna rises near Mahabaleshwar in the Western Ghats and flows south-east to the Bay of Bengal, about 1,400 km long, draining the second largest Peninsular basin. Its tributaries include the Koyna, the Bhima and, largest of all, the Tungabhadra.
The Kaveri rises in the Brahmagiri hills of Kodagu in Karnataka and crosses Karnataka and Tamil Nadu to the sea, about 800 km. Alone among the great Peninsular rivers, it barely dries in the lean season.
This steadier flow is why the Kaveri delta became one of the oldest and most productive rice regions of southern India. A river that keeps running through the dry season can water a second crop, and it is dependable water, as much as fertile soil, that makes the delta a rice bowl.
Two other kinds of Peninsular river never reach the eastern sea directly. The Damodar rises on the Chota Nagpur plateau in Jharkhand, cuts through the coalfields, and joins the Hooghly in West Bengal; once so destructive in flood that it was called the ‘Sorrow of Bengal’, it is now controlled by the dams of the Damodar Valley Corporation. And from the northern edge of the plateau, the Chambal, the Betwa, the Ken and the Son flow north to join the Ganga system.
The Chambal is the best known of these. Rising near Mhow in the Vindhya range, it flows north through soft, easily eroded rock, cutting a maze of gullies known as badland — barren, deeply ravined country where the land is dissected into a tangle of ridges and channels. The Chambal ravines are the classic example in India.
4.4.2 Rivers Draining to the Arabian Sea
Only two large rivers cross the Peninsula the other way, running west to the Arabian Sea, and both owe their course to the structure of the block rather than its slope.
The Narmada rises at Amarkantak in Madhya Pradesh and flows west for about 1,312 km through a rift valley pressed between the Vindhya range to the north and the Satpura range to the south. Near Jabalpur it carves a famous gorge through white marble rock. It reaches the Gulf of Khambhat below Bharuch not through a delta but through a wide, tidal mouth. The Tapi rises in the Satpura range near Multai, in Betul district, and runs west for about 724 km in a second trough parallel to the Narmada, reaching the sea near Surat. Further north-west, the Sabarmati, rising in the Aravalli hills, and the Mahi drain to the same gulf.
Diagram
Fig. 4.4-1 Caption: The Narmada and the Tapi run west in fault-troughs, dropped between the ranges rather than cut by the rivers themselves. Brief: A north-to-south cross-section across central India. From north to south show: the Vindhya range as an upland; a down-dropped block (rift trough) holding the Narmada; the Satpura range as an upland; and a second parallel trough holding the Tapi. Draw the bounding faults as steep lines where the ground has dropped between them. Label: Vindhya range, Narmada (in its rift), Satpura range, Tapi (in its rift), bounding faults. Emphasise that each river sits in a ready-made trough, not a valley it carved.
Neither river builds a delta. Where a river drops its load into a calm sea it spreads a delta, but the mouths of the Narmada and the Tapi open onto the Gulf of Khambhat, where strong tides sweep the sediment away as fast as it arrives. So each ends in an estuary — a wide, tidal river-mouth that opens straight to the sea, where fresh and salt water mix, without a delta.
The rest of the west coast has no great deltas at all. The Western Ghats rise close to the shore, so the many rivers of the Konkan and Malabar coasts are short and swift, tumbling down steep slopes across a narrow coastal plain and carrying their sediment straight out to sea before it can settle.
4.5 Himalayan and Peninsular Rivers Compared
We have met the two families separately. Set side by side, they differ in almost every way — length, water supply, valley shape, behaviour, floods and deltas. Yet all these differences grow from a single root, and once you see the root the rest follows.
The Root Difference
That root is the age and origin of the land each family drains. The Himalayan rivers are young and antecedent, flowing off high, rising mountains built of soft, easily worn rock and mantled in snow. The Peninsular rivers are old and consequent, crossing a rigid, long-worn block of hard rock that catches no permanent snow. Hold this one contrast in mind, and every other difference reads as its consequence.
Source and Regime
Because the Himalayan rivers rise in glaciers and permanent snow, they are fed by melt-water as well as monsoon rain. They carry water in every season and are perennial. Because the Peninsular rivers rise in modest hills and depend on rain alone, they run full during the monsoon and can fall to a chain of pools before it returns; they are seasonal. Figure 4.5-1 sets the two regimes against each other.
Graph
Fig. 4.5-1 Caption: A perennial Himalayan river carries water all year; a seasonal Peninsular river swells in the monsoon and falls away. Brief: A line graph of river discharge through the year. X-axis: months, January to December. Y-axis: discharge in cumecs (cubic metres per second). Plot two lines. (1) A Himalayan, perennial river that stays well above zero all year, rising to a broad summer-and-monsoon peak fed by spring snow-melt plus rain, and never dropping to zero in the dry season. (2) A Peninsular, seasonal river that runs very low or near zero from January to May, rises sharply to a tall peak around July to September, then falls steeply back towards zero by December. Label the lines ‘Himalayan (perennial)’ and ‘Peninsular (seasonal)’. The curves are illustrative regime shapes, not a gauged record of any one river.
Read the two lines across the months. The Himalayan line never touches the bottom, because even in the rainless months snow is melting somewhere upstream. The Peninsular line rides along the bottom for half the year and then leaps, because it has only one source — the rain — and only one season of it. The graph shows the shape of each regime; it does not show how much total water each river carries, since the two are drawn to illustrate the pattern, not to compare volumes.
Valley, Course and Load
The two families have also reached different stages of their working life. The Himalayan rivers are still young and vigorous, cutting downward into rising land and running through deep, narrow gorges. The Peninsular rivers reached maturity long ago; they flow in broad, shallow, graded valleys and do more sideways cutting than downcutting, for there is little height left to lose.
Their courses behave differently too. On the flat, soft alluvium of the northern plains the Himalayan rivers swing in great looping meander bends, cut off old loops as ox-bow lakes, and shift their channels from year to year. The Peninsular rivers, set in hard rock, hold fixed and nearly straight courses that scarcely change.
The loads they carry follow the same logic. The young, crumbling Himalaya yield enormous quantities of silt, so their rivers build large deltas at the Bay of Bengal and flood violently when the monsoon and the snow-melt arrive together. The hard old Peninsula yields far less sediment, so its eastern rivers build smaller deltas, and its two western rivers, the Narmada and the Tapi, end in estuaries rather than deltas.
Length and basin size line up the same way. The Himalayan rivers are generally longer and drain far larger basins — the Ganga basin is the largest in India — while the Peninsular rivers are shorter, with smaller catchments, the Godavari being the largest among them.
These differences shape how people use the two families, though neither is simply the more useful. The perennial Himalayan rivers, crossing wide flat plains, allow irrigation and some navigation the whole year round. The seasonal Peninsular rivers, running in rock valleys, are harder to navigate but well suited to dams and reservoirs that store the monsoon flow for hydel power and dry-season irrigation.
4.6 Summary
A river is a system, not just a line on a map. The area it drains is its basin, bounded by a water divide; tributaries add water at confluences, distributaries carry it off near the mouth. How its discharge rises and falls through the year is its regime, and that regime is set above all by where its water comes from.
India’s rivers fall into two families defined by origin. The Himalayan rivers are young; the Peninsular rivers are old. Almost every difference between them traces back to the age of the land they cross and the source of the water they carry.
The great Himalayan rivers are antecedent — older than the mountains. They were flowing before the ranges rose across their path, and they held their courses and cut downward as fast as the land lifted. Their deep gorges are the proof that the water came first.
The Indobrahm theory imagined a single ancestral river running the length of the Himalayan foreland and later dismembering into the Indus, Ganga and Brahmaputra. It is now largely set aside: ordinary Himalayan streams could have laid the Shiwalik deposits, and the alluvial record does not fit so short a story.
The Peninsular rivers are consequent — their courses follow the plateau’s slope. Most run east down its gentle tilt to the Bay of Bengal; the Narmada and the Tapi run west along rift troughs they never carved.
The Indus rises near Lake Mansarovar in Tibet and gathers five main tributaries in the Punjab — the Jhelum, Chenab, Ravi, Beas and Sutlej. Its waters are shared under the Indus Waters Treaty of 1960, the western rivers largely to Pakistan and the eastern to India.
The Ganga forms at Devprayag, where the Bhagirathi from the Gangotri glacier meets the Alaknanda. It drains India’s largest basin. Its longest tributary is the Yamuna; from the Himalaya come the Ghaghara, Gandak and Kosi — the last, the ‘sorrow of Bihar’, shifting course as it drops its silt on the plain. Below Farakka the main flow becomes the Padma.
The Brahmaputra rises from the Chemayungdung glacier, crosses Tibet as the Tsangpo, loops round Namcha Barwa in the Great Bend, and enters India as the Dihang. Braided and heavily silted, it floods the Assam valley almost yearly before joining the Ganga to build the world’s largest delta.
Four large rivers carry the Peninsula’s water east: the Mahanadi, the Godavari (the Dakshin Ganga, largest of the Peninsular rivers), the Krishna, and the near-perennial Kaveri, which draws rain in two seasons. On the plateau’s northern edge, the Chambal has cut the badlands of India.
The west-flowing Narmada and Tapi reach the Arabian Sea through troughs but build no deltas: their short, swift courses carry little silt to a coast that has sunk, so they form estuaries instead. The Luni, the one large desert river, ends in the Rann of Kachchh.
Set side by side, the two families differ in supply (perennial versus seasonal), in valley form (gorges versus shallow graded valleys), in course (shifting versus stable), and in size and load. Yet both are organised by slope into basins, both draw their greatest flow from the monsoon, and both are lifelines for the millions in their basins.
Flowchart
Fig. 4.6-1 Caption: Two kinds of ground gave India two families of rivers — nearly everything else about each one follows from that. Nodes: The land India’s rivers flow over → splits into two kinds of ground → branch A, the young, still-rising Himalaya: rivers older than the mountains (antecedent) → cut gorges straight across the rising ranges → rise in snow and ice, so snow-fed and perennial → long courses, huge catchments, heavy sediment loads → meander, braid and shift course on the soft plains → build the world’s largest delta → three systems: the Indus, the Ganga, the Brahmaputra; and branch B, the old, stable Peninsular block: rivers born of the slope (consequent) → follow the eastward tilt or structural troughs → rain-fed only, so seasonal → shorter courses, smaller basins, lighter loads, mature shallow valleys → hold fixed, near-straight courses on hard rock → east-flowing rivers build deltas while the Narmada and Tapi form estuaries → east: Mahanadi, Godavari, Krishna, Kaveri; west: Narmada, Tapi
What This Chapter Cannot Tell You
A source and an outfall fix where a river begins and ends, but say nothing about how much water it carries, or how that flow swings from a flood year to a drought year. The regime graph shows a typical year and smooths those swings away. And the neat two-family model is a simplification: real rivers blend the traits, and a warming climate — shrinking glaciers, a shifting monsoon — could alter the very water supply that defines each family.
4.7 Key Terms
antecedent drainage — a river that keeps its original course while the land rises across it, cutting a gorge straight through the rising ground; the drainage is older than the relief.
badland — a landscape of soft rock or alluvium cut by run-off into a dense maze of ravines and gullies, as in the Chambal valley.
catchment — another name for a drainage basin: the whole area whose rainfall drains to one river.
confluence — the point where a tributary joins a larger river.
consequent drainage — a river whose course is fixed by the original slope of the land, following the lie of the ground; the drainage is a consequence of the relief.
delta — a triangular tract of new land built where a river slows at a still sea and drops its load, split by distributaries.
discharge — the volume of water passing a point in a channel each second, measured in cubic metres per second (cumecs).
distributary — a channel that branches away from the main river and carries water off separately, usually across a delta.
drainage — the movement of water, chiefly from rainfall, off a land surface and along its channels towards the sea or an inland basin.
drainage basin — the whole area drained by a river and its tributaries; every drop of rain falling inside it reaches that one river.
drainage system — the network of channels that a region’s water follows.
estuary — a river mouth drowned by the sea, forming a tidal channel that widens seawards, as at the Narmada and Tapi.
headward erosion — the cutting-back of a stream into the land at its own head, lengthening it upstream.
Indobrahm river — the single ancestral river of one theory, imagined to have flowed the length of the Himalayan foreland before dismembering into the Indus, Ganga and Brahmaputra.
mouth — the point where a river ends, emptying into a sea, lake or larger river.
perennial — describing a river that carries water in every season and never dries up, because it is fed by snow-melt as well as rain.
regime — the pattern of a river’s discharge through the year — how its flow rises and falls with the seasons.
rift valley — a trough where a strip of ground has dropped between two parallel faults; the Narmada and the Tapi occupy such troughs.
river system — a trunk river together with all its tributaries and distributaries.
seasonal — describing a river that runs full in the monsoon and shrinks in the dry season, because it is fed by rain alone.
source — the point where a river begins — a glacier snout, a spring, a lake, or seepage in high ground.
tributary — a smaller stream that joins a larger river, adding its water.
water divide — the higher ground or ridge along which rainfall parts and runs off to different rivers; also called the watershed.
4.8 Points to Ponder
An antecedent river cuts a gorge straight through a mountain that rose in its path. So which one is really shaping the other — is the river carving the mountain down, or the rising mountain forcing the river to work? Can both be true at once?
The Himalayan rivers stay full through the dry season because glaciers keep feeding them. If those glaciers shrink over the coming decades, will these rivers begin to behave more like Peninsular ones — and what would that shift mean for the millions who trust them to flow all year?
A single river tears down rock in the mountains and builds fresh land at the sea. Is such a river best called a destroyer or a creator — and does your answer change depending on where along its course you are standing?
This chapter sorts every Indian river into one of two families, Himalayan or Peninsular. Is a two-family scheme a discovery about the rivers themselves, or a convenience for the people describing them? Where might a real river refuse to fit the boxes?
The Kosi has wandered across the Bihar plain for centuries, and people have long tried to pin it in place with embankments. Is a river that ‘wants’ to shift its course something to be controlled, or something to be lived with — and what does the behaviour of the land itself suggest?
4.9 Objective Questions
- Which of the following is a west-flowing Peninsular river?
- Godavari
- Mahanadi
- Tapi
- Krishna [1 mark]
- At Devprayag the river takes the name Ganga after the Bhagirathi unites with the —
- Yamuna
- Alaknanda
- Ghaghara
- Gomati [1 mark]
- A river that keeps its course and cuts a gorge through a mountain range rising across its path shows —
- consequent drainage
- antecedent drainage
- headward erosion
- seasonal flow [1 mark]
- Which tributary of the Indus lies wholly within India?
- Chenab
- Jhelum
- Beas
- Sutlej [1 mark]
- Assertion (A): The Narmada and the Tapi reach the Arabian Sea but build no deltas. Reason (R): They run short, swift courses down rift troughs to a coast that has subsided, so the sea invades their mouths.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true. [1 mark]
- Assertion (A): The Peninsular rivers flow in broad, shallow valleys. Reason (R): They are young rivers still cutting downward into a rising block of land.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true. [1 mark]
- Match each feature in Column A with the correct river in Column B. [1 mark]
| Column A | Column B |
|---|---|
| a. Dakshin Ganga | i. Kosi |
| b. Sorrow of Bihar | ii. Brahmaputra |
| c. The Great Bend | iii. Sutlej |
| d. Held back by the Bhakra dam | iv. Godavari |
- Fill in the blank: The higher ground or ridge along which rainfall parts and drains to different rivers is called the ______. [1 mark]
4.10 Check Your Concepts
Define a drainage basin and a water divide. [2 marks]
Name the two headstreams that unite at Devprayag to form the Ganga, and give the source of one of them. [2 marks]
Differentiate between antecedent and consequent drainage, giving one Indian example of each. [3 marks]
Give reasons why the Kosi is called the ‘sorrow of Bihar’. [3 marks]
Explain, with an example, how the deep gorges of the Himalayan rivers are evidence that these rivers are older than the mountains they cross. [4 marks]
The Kaveri is described as the most nearly perennial of the Peninsular rivers. Explain why, with reference to the two seasons in which its catchment receives rain. [4 marks]
Compare the Himalayan and Peninsular rivers under the headings: origin, water supply and regime, valley form, and the land they build at the sea. Support your answer with Indian examples. [6 marks]
Explain the Indobrahm theory of the evolution of the Himalayan rivers, and state two grounds on which it has been set aside. [6 marks]
4.11 Map, Diagram & Data Interpretation
Study Fig. 4.5-1, the graph of two river regimes. [4 marks]
- Which curve represents a Peninsular river? Give one reason for your choice.
- In which part of the year is the difference between the two rivers greatest?
- State one thing this graph does not tell you about the two rivers.
Look at Fig. 4.4-1, the cross-section across central India. [3 marks]
- Name the two ranges that hem in the Narmada and the Tapi.
- Explain why these two rivers run west when most Peninsular rivers run east.
On an outline map of India, mark and label the following: (a) the Indus, (b) the Ganga, (c) the Godavari, (d) the Narmada, and (e) the Ganga–Brahmaputra delta. [5 marks]
At a gauging station, a river’s channel has a cross-section area of 120 m² and the water moves at an average velocity of 1.5 m/s. Using the relation that discharge equals cross-section area multiplied by average velocity, calculate the river’s discharge in cubic metres per second (cumecs), showing your working. [3 marks]
Using Table 4.1: (a) name two Himalayan rivers that rise in Tibet, and (b) explain what their common origin in high snow country means for how they flow through the year. [3 marks]