Drainage System

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (India: Physical Environment), Chapter 3.

Why do most of India's rivers end in the Bay of Bengal?

Look at a map of India and follow the big rivers. The Ganga, Brahmaputra, Mahanadi, Godavari, Krishna and Kaveri all finish in the east. Yet the Himalayas lie in the north and the Western Ghats are almost on the west coast. The answer lies in slope and structure: the Peninsular plateau tilts from west to east, and the Himalayan rivers flow down into the great plain, which tilts east as well. This chapter asks you to read a river map the way a geographer does: where it starts, what it carries, which way it slopes and why.

The vocabulary of rivers

The flow of water through well-defined channels is called drainage, and the network of channels is a drainage system. A river drains water from a specific area, its catchment area. An area drained by a river and its tributaries is a drainage basin, and the boundary line that separates one basin from another is the watershed.

Basins and watersheds are units: what happens in one part affects the rest. That is why they are accepted as the most suitable micro, meso and macro regions for planning.

The pattern a river forms depends on the rock structure, slope, geological time, the amount of water and how often it flows.

Three ways to classify Indian drainage

By the sea they reach. The Delhi ridge, the Aravali and the Sahyadris separate the Bay of Bengal drainage from the Arabian Sea drainage. About 77 per cent of the drainage area (Ganga, Brahmaputra, Mahanadi, Krishna and others) goes to the Bay of Bengal, and about 23 per cent (Indus, Narmada, Tapi, Mahi, Periyar) goes to the Arabian Sea.

By basin size. Three classes, based on catchment area.

By origin and nature. Himalayan drainage and Peninsular drainage. This is the most accepted basis, and the rest of the chapter follows it, though the Chambal, Betwa and Son are much older than other rivers that rise in the Himalayas.

The Himalayan drainage

The Himalayan system has evolved over a long geological history and includes the Ganga, Indus and Brahmaputra basins. Because snowmelt and rain both feed them, the rivers are perennial. As the Himalayas rose, the rivers kept cutting, forming giant gorges and V-shaped valleys. Rapids and waterfalls are common in the mountains.

On entering the plains they build flat valleys, ox-bow lakes, flood plains, braided channels and deltas near the mouth. Their course is highly tortuous in the mountains, but in the plains they meander and shift course often.

The Kosi. Known as the sorrow of Bihar, the Kosi has repeatedly changed its course. It carries a huge load of sediment from its upper reaches and drops it in the plains. The channel gets blocked with its own deposit, so the river jumps to a new path.

Pause and predict: when would the Kosi carry the most water, and what does that do to its load and course?

Check your answer: during the rainy season, when snow-melt and monsoon rain peak. A swollen river carries more sediment, deposits it in the flat plains and chokes its own channel, which makes it overflow and shift. Flooding does damage, but it also leaves fertile silt.

Beyond the textbook. For the exam: the Kosi is the sorrow of Bihar and the Damodar the sorrow of Bengal. In reality: over the last two centuries the Kosi has moved westward by more than 100 km across the plain, and in 2008 it broke an embankment and flowed into an old channel, flooding large areas. Why it matters: it shows that rivers on a flat, sediment-rich plain behave very differently from river courses drawn on a map.

How the Himalayan rivers evolved

There is a difference of opinion among scholars. Geologists believe that a mighty river, called the Shiwalik or Indo-Brahma river, once flowed along the whole length of the Himalaya from Assam to Punjab and on to Sind, and emptied into the Gulf of Sind near lower Punjab during the Miocene period, some 5 to 24 million years ago. The continuous Shiwalik deposits, which are of lake origin and contain sand, silt, clay, boulders and conglomerates, support this.

Over time this river was cut into three systems: the Indus and its five tributaries in the west, the Ganga and its Himalayan tributaries in the centre, and the Brahmaputra in Assam with its Himalayan tributaries in the east. The break-up was probably caused by the Pleistocene upheaval in the western Himalayas, including the uplift of the Potwar Plateau (the Delhi ridge), which became the water divide between the Indus and the Ganga. The down-thrusting of the Malda gap between the Rajmahal Hills and the Meghalaya plateau in the mid-Pleistocene sent the Ganga and Brahmaputra towards the Bay of Bengal.

Two views. For the exam: a single Indo-Brahma river was broken into three systems by upheaval and down-thrusting. Another reading: several large Himalayan rivers such as the Satluj, the Kosi and the Subansiri are described as antecedent rivers, meaning they existed before the mountains and kept cutting their gorges as the land rose. Both ideas are about how rivers crossed a rising Himalaya, and the textbook itself notes that opinions differ.

The Indus system

The Indus, also called the Sindhu, is the westernmost Himalayan river in India and one of the largest basins in the world.

Its Himalayan tributaries include the Shyok, Gilgit, Zaskar, Hunza, Nubra, Shigar, Gasting and Dras. A little above Mithankot it receives the Panjnad, the name for the five rivers of Punjab: Satluj, Beas, Ravi, Chenab and Jhelum. The Indus itself is not one of the five.

The Ganga system

The Ganga is the most important river of India for both its basin and its cultural importance. Its headstream, the Bhagirathi, rises in the Gangotri glacier near Gaumukh (3,900 m) in Uttarkashi district, Uttarakhand, and cuts narrow gorges through the Central and Lesser Himalayas. At Devprayag it meets the Alaknanda, and from there the river is called the Ganga.

The Alaknanda rises in the Satopanth glacier above Badrinath. The confluences along it follow a clear order.

The Ganga enters the plains at Haridwar, flows south, then south-east and east, and splits into two distributaries, the Bhagirathi and the Padma. Its length is 2,525 km, shared by Uttarakhand (110 km), Uttar Pradesh (1,450 km), Bihar (445 km) and West Bengal (520 km). Its basin covers about 8.6 lakh sq. km in India alone, and this is the largest river system in India. It finally reaches the Bay of Bengal near Sagar Island.

The Yamuna. The western-most and longest tributary of the Ganga, it rises in the Yamunotri glacier on the western slopes of the Banderpunch range (6,316 m) and joins the Ganga at Prayag (Allahabad). From the right bank it receives the Chambal, Sind, Betwa and Ken, which come from the Peninsular plateau, and from the left bank the Hindan, Rind, Sengar and Varuna. Much of its water feeds the western and eastern Yamuna canals and the Agra canal.

Namami Gange. Approved as a flagship programme of the Union Government in June 2014, this integrated conservation mission has the twin objectives of reducing pollution and conserving and rejuvenating the Ganga. Its pillars include sewage treatment infrastructure, river-front development, river-surface cleaning, biodiversity, afforestation, public awareness, industrial effluent monitoring and Ganga Gram.

The Brahmaputra system

One of the largest rivers in the world, the Brahmaputra rises in the Chemayungdung glacier of the Kailash range near Mansarovar. It runs east for nearly 1,200 km through the dry, flat country of southern Tibet, where it is the Tsangpo ("the purifier"). Its major right bank tributary in Tibet is the Rango Tsangpo. It then carves a deep gorge in the Central Himalayas near Namcha Barwa (7,755 m) and emerges at the foothills as the Siang or Dihang. It enters India west of Sadiya in Arunachal Pradesh. After it is joined by the Dibang and Lohit from the left, it is called the Brahmaputra.

The Brahmaputra is well known for floods, channel shifting and bank erosion. Most of its tributaries are large and bring a huge load of sediment from a catchment with heavy rainfall. The Subansiri, which rises in Tibet, is an antecedent river.

The Peninsular drainage

The Peninsular system is older than the Himalayan one, as seen from its broad, graded, shallow valleys and the maturity of its rivers. The Western Ghats, running close to the west coast, act as the water divide between the major rivers that flow to the Bay of Bengal and the short streams that flow into the Arabian Sea. Most major Peninsular rivers except the Narmada and Tapi flow from west to east. The Chambal, Sind, Betwa, Ken and Son, which rise in the northern part of the Peninsula, belong to the Ganga system.

Three events shaped it. The western flank of the Peninsula sank beneath the sea in the early Tertiary period, which broke the symmetry of the rivers on either side of the original watershed. The upheaval of the Himalayas bent the northern flank of the block, creating trough faults along which the Narmada and Tapi flow. And a slight tilt of the block from north-west to south-east gave the whole drainage its eastward orientation.

Pause and predict: the Narmada and Tapi flow in rift valleys. What does that suggest about their deltas?

Check your answer: they have no deltas. They flow in trough faults and fill the original cracks with their own sediment, so there are few alluvial and deltaic deposits, and they reach the sea through estuaries.

Why the Kaveri is different. Its upper catchment gets rain from the south-west (summer) monsoon, and its lower part gets rain from the north-east (winter) monsoon. Water therefore arrives in both seasons, and the river flows throughout the year with less fluctuation than other Peninsular rivers.

Beyond the textbook. For the exam: the Godavari is the largest Peninsular system and the Narmada and Tapi are the exceptions that flow west. In reality: the Narmada, Tapi and Mahi all reach the Arabian Sea, and each ends in a wide estuary, a funnel-shaped mouth where tides push far upstream, rather than a delta. Why it matters: the delta/estuary difference is one of the textbook's own exercise questions, and it is also the most common way these two rivers appear in options.

Using river water

India's rivers carry huge volumes of water, but unevenly in time and space. Much runs to the sea in floods while another part of the country is in drought. The textbook discusses ways to deal with this, including inter-basin transfers.

A canal is not enough to move water from the Ganga basin to a Peninsular river, because the terrain is uneven and water would have to be lifted from the plain to the plateau. The textbook lists problems that limit water use: not enough water in some places, pollution, silt load, uneven seasonal flow, disputes between states, and shrinking channels as settlements move onto riverbeds. River pollution comes from city sewage, industrial effluents, immersion of offerings and large-scale bathing and washing. Schemes such as the Ganga Action Plan and the cleaning of the Yamuna at Delhi, along with Namami Devi Narmade for the Narmada, aim to deal with it.

Exam traps

60-second revision

Where this chapter connects

• Previous: Structure and Physiography explains the plateau tilt, the rift valleys and the Himalayan uplift that decide where rivers flow.
• Next: Climate explains the monsoon rains that feed these rivers and why the Himalayan rivers have two sources of water.
• Related: Water (Oceans) and Landforms and Their Evolution cover deltas, gorges and river landforms in general.