Drainage Systems of India

Rivers are the lifelines of India. They water the fields, carry silt that builds fertile plains, supply drinking water, generate power and provide routes for transport. Indian rivers fall into two great families with very different characters: the Himalayan rivers, which flow all year round, and the Peninsular rivers, which depend on the rains. Telling the two apart, and knowing which river drains into which sea, is essential exam knowledge.

Drainage means the river system of an area, that is, the way water flows over the land. The area drained by a single river together with all its tributaries is called its drainage basin. An upland that separates two neighbouring basins is called a water divide, for example a ridge of hills from which rivers flow down on either side.

Indian rivers drain into two main bodies of water. Most of the large rivers empty into the Bay of Bengal in the east, while a few important ones flow into the Arabian Sea in the west.

Sequence questions also test where rivers sit on the map from north to south. Two anchors help. The Kishenganga, a tributary of the Jhelum, flows in Kashmir, far to the north of the Ganga. In the Godavari system of the peninsula, the Wainganga lies to the north of the Penganga.

The three great Himalayan river systems are the Indus, the Ganga and the Brahmaputra. Their key feature is that they are perennial: they flow throughout the year. They are fed both by the summer monsoon rains and by the melting of Himalayan snow. In their long courses they carve out gorges in the mountains. They form large bends called meanders in the plains and build deltas at their mouths.

The Indus rises in Tibet and flows north-west, mostly through Pakistan. Its eastern tributaries, such as the Sutlej, the Beas and the Ravi, water the Indian Punjab. These tributaries do not all meet the Indus separately. They join in a clear hierarchy. The Jhelum and the Ravi flow into the Chenab. The Beas flows into the Sutlej. The Sutlej, carrying all their combined waters, is the one that joins the Indus directly.

Origins matter for the exam. The Sutlej rises at Rakas Tal in Tibet, so it does not originate in India. The Beas, the Ravi and the Chenab, by contrast, all rise within India. Five rivers pass through Himachal Pradesh: the Beas, the Chenab, the Ravi, the Satluj and the Yamuna. In the high north, the rivers of Ladakh and Himachal follow a north-to-south order: the Shyok, then the Zaskar, then the Spiti, and finally the Satluj.

The Brahmaputra also rises in Tibet, where it is called the Tsangpo. Near the peak of Namcha Barwa it takes a dramatic U-turn and cuts south into India. This hairpin bend follows the syntaxial bending of the Himalayas, the sharp knee where the geologically young mountain chain itself turns. The river enters India in Arunachal Pradesh and flows through Assam, carrying a huge load of silt. It then joins the Ganga in Bangladesh. It is famous for its floods and for shifting its channel.

In India the Brahmaputra gathers a set of tributaries worth memorising: the Dibang, the Kameng, the Lohit, the Subansiri and the Manas. The Lohit and the Subansiri flow through Arunachal Pradesh. Do not confuse them with the Barak, which rises in Manipur and flows through Assam and Mizoram, not Arunachal.

The Teesta is another tested river. It rises in the Sikkim Himalayas, not at the Brahmaputra's source. The Rangeet, which also rises in Sikkim, is its tributary. The Teesta does not reach the Bay of Bengal on its own. It joins the Brahmaputra in Bangladesh.

Matching each glacier to the river it feeds is a recurring exercise.

• Gangotri: feeds the Bhagirathi, the headstream of the Ganga.
• Bandarpunch: feeds the Yamuna.
• Bara Shigri: feeds the Chenab.
• Siachen: feeds the Nubra.
• Milam: feeds the Goriganga, not the Mandakini.
• Zemu: drains to the Teesta, not the Manas.

The Ganga is India's most important river. It rises in the Himalayas, where its headstream, the Bhagirathi, is fed by the Gangotri Glacier, and is joined at Devprayag by the Alaknanda to form the Ganga. It emerges onto the plains at Haridwar.

On the plains the Ganga is joined by many tributaries. From the Himalayas come the Yamuna, the Gomati, the Ghaghara, the Gandak and the Kosi, and from the peninsular uplands come the Chambal, the Betwa and the Son. The Yamuna is its longest and most important tributary, joining it at Allahabad (Prayagraj). Downstream of Prayagraj the remaining tributaries join in a fixed west-to-east order: the Gomati first, then the Ghaghara, then the Gandak, and finally the Kosi.

Finally the Ganga turns south and east into Bangladesh, where it joins the Brahmaputra. Together they form the world's largest delta, the Sundarban Delta, before pouring into the Bay of Bengal.

The Ganga Plain is India's most densely populated region. The reason is its fertile alluvial soil, which supports intensive farming, not the fact that the Ganga is the most harnessed of Indian rivers, though it is that too. The plain also floods often, and floods have grown more frequent in recent decades. Long-continued silt deposition has reduced the depth of the river valleys, so they hold less water and overflow more easily.

The rivers of the Peninsular Plateau are older than the Himalayan rivers but mostly seasonal, since they depend on rainfall and shrink greatly in the dry season. They flow in fixed, shallow valleys with little meandering, and most rise in the Western Ghats.

Most peninsular rivers flow eastward into the Bay of Bengal: the Mahanadi, the Godavari, the Krishna and the Kaveri. The Godavari is the largest of these and is often called the "Dakshin Ganga", the Ganga of the south.

Two important rivers are exceptions, flowing westward into the Arabian Sea: the Narmada and the Tapi. Instead of building deltas they form estuaries, and they flow through deep cracks in the land called rift valleys. The missing deltas have a precise cause. The rocky beds of these rift valleys yield very little alluvial sediment, so the rivers carry no silt load to dump at their mouths. A third west-flowing exception is the Kalinadi of Karnataka, which also drains into the Arabian Sea. Note that a rift valley does not force a river to flow west. The east-flowing Damodar also occupies a rift valley, one formed by the down-warping of the land.

Not every peninsular river starts in the Western Ghats. The sources sort into a few groups.

• Western Ghats: the Godavari, the Krishna and the Cauvery, the great east-flowing rivers.
• Eastern Ghats: the Nagavali and the Vamsadhara rise here.
• Chota Nagpur plateau (Jharkhand hills): the Brahmani and the Subarnarekha rise here.
• Aravallis: the Luni of Rajasthan rises here. It never reaches the sea. It is lost in the marshes of the Rann of Kachchh, the classic case of inland drainage.

Kerala adds a twist. Most of its rivers flow west, yet the state does have east-flowing rivers: the Kabani and the Bhavani, both tributaries of the Cauvery.

From north to south the east-flowing rivers run: Subarnarekha, Mahanadi, Godavari, Krishna, Pennar, Cauvery, Vaigai. The Pennar sits between the Krishna and the Cauvery, and the Vaigai of Tamil Nadu is the southernmost. By length the big four rank: the Godavari (about 1,465 km) first, then the Narmada (about 1,312 km), then the Mahanadi (about 851 km), then the Tapi (about 724 km).

The Vamsadhara and the Pennar are independent rivers. They flow directly into the Bay of Bengal and feed no larger river. The Pennar, also written Penna, carved the spectacular Gandikota canyon in Andhra Pradesh.

The Godavari earns its "Dakshin Ganga" title partly through its tributaries: the Penganga, the Wainganga, the Pranahita, the Indravati and the Manjra. The Nizam Sagar reservoir stands on the Manjra.

To the north-east, the Brahmani rises in Jharkhand, where it is called the Koel. The Baitarani rises in Odisha. The two merge near the coast at Bhitarkanika, a protected mangrove wildlife area.

Matching a town to its river is a standing exam exercise. Learn these pairs.

• Pandharpur: on the Chandrabhaga, the local name of the Bhima.
• Tiruchirappalli: on the Cauvery.
• Hampi: on the Tungabhadra, not the Malaprabha.
• Jabalpur: on the Narmada.
• Betul: near the Tapti.
• Jagdalpur: near the Indravati.
• Ujjain: on the Shipra.

One canal fact is tested alongside these. The Indira Gandhi Canal draws its water from the Harike Barrage, built at the confluence of the Sutlej and the Beas. It does not draw from the Ghaggar.

Where a river plunges over a steep edge of hard rock, it forms a waterfall. India's most famous falls lie on the plateau rivers, because the Peninsular Plateau is full of sudden breaks in slope. Examiners love to pair each waterfall with its river and region, so learn the matches exactly.

• Jog (Gersoppa) Falls: on the Sharavati river in the Western Ghats of Karnataka, among the highest waterfalls in India.
• Sivasamudram Falls: on the Cauvery river in Karnataka, the site of one of Asia's earliest hydroelectric stations.
• Dhuandhar Falls: on the Narmada near Jabalpur in Madhya Pradesh, where the river crashes through the Marble Rocks. Note that Jabalpur lies outside the Malwa region.
• Kapildhara Falls: also on the Narmada, near its source at Amarkantak. It is not on the Godavari.
• Hundru Falls: on the Subarnarekha river on the Chota Nagpur Plateau in Jharkhand.

Two confusions recur. Jog Falls is on the Sharavati, not the Netravati. Kapildhara and Dhuandhar both belong to the Narmada, not to any east-flowing river.

India has many lakes, formed in different ways. Some, like the lakes of Kashmir, were carved by glaciers. Some are formed by the action of rivers, such as the ox-bow lakes left by meanders. Some are formed by the sea along the coast, such as Chilika Lake in Odisha, the largest saltwater lake. Chilika is not the only coastal lagoon: Ashtamudi in Kerala and Pulicat on the coast north of Chennai belong to the same family. Periyar Lake in Kerala does not. It is a dam reservoir, not a lagoon. The largest freshwater lake, the Wular Lake in Kashmir, was formed by tectonic activity, and the Jhelum passes through it on its course.

Many lakes are man-made, built behind dams to store water, such as the lake behind the Bhakra Nangal dam. Lakes help to regulate the flow of rivers, prevent flooding during the rains, store water for the dry season, and support fishing and tourism.

• Sambhar: near Jaipur, India's largest inland saline lake. Rajasthan holds a whole cluster of saline lakes: Didwana, Kuchaman, Sargol and Khatu besides Sambhar.
• Kolleru: a natural lake in Andhra Pradesh. It is fed by seasonal streams and by floodwater of the Krishna and the Godavari. It does not lie directly on the Krishna.
• Kodaikanal: an artificial lake in Tamil Nadu, created by a dam in 1863. Kolleru, Nainital and Renuka, by contrast, are natural lakes.
• Hokera Wetland: in Jammu and Kashmir.
• Renuka Wetland: in Himachal Pradesh.
• Rudrasagar Lake: in Tripura.
• Sasthamkotta Lake: in Kerala.

• Govind Sagar: the reservoir on the Satluj behind the Bhakra Nangal dam.
• Ukai: a reservoir on the Tapi in Gujarat.
• Ghataprabha: a reservoir in Karnataka.
• Indira Sagar: in Madhya Pradesh.
• Maithon: in Jharkhand.
• Aliyar: in Tamil Nadu; Isapur in Maharashtra and Kangsabati in West Bengal are water reservoirs too.

Not all of India's water flows on the surface. Groundwater, the water stored in rocks and soil below the ground, irrigates more farmland in India than canals and tanks combined. In fact, India has the largest area under groundwater irrigation in the world, drawn up through millions of wells and tubewells.

This heavy use is regulated by the Central Ground Water Authority (CGWA), a body set up under the Environment (Protection) Act, 1986, a law that lets the central government act to protect the environment. The CGWA assesses groundwater blocks and grades them as safe, semi-critical, critical or over-exploited, depending on how much extraction exceeds natural recharge. Only a minority of assessment units fall in the over-exploited or critical grades, well under a third of the total, but their number is concentrated in the intensively farmed north-west, in Punjab, Haryana and western Uttar Pradesh, and in parts of the dry peninsular interior.

Water covers about three-quarters of the Earth's surface, but the great bulk of it is salty ocean. Only a tiny fraction is the fresh water that people, farms and industries can use. As India's population, farming and cities grow, the demand for this small share rises fast.

Water scarcity is a shortage of water in relation to its demand. It is not always caused by too little rain. It can occur even in areas of plentiful rainfall. The main causes are the rising demand from a growing population, the heavy use of water to irrigate fields, the thirst of expanding industries and cities, and the over-use of groundwater, which lowers the water table. Scarcity is also made worse by pollution. A river or well may hold water, but if it is polluted by sewage, chemicals or industrial waste it becomes unfit to use. Unequal access adds to the problem. Some groups get plenty while others go short.

If you look at a river and its tributaries from above, they make a definite shape on the land. This shape is the drainage pattern, and it is decided by the slope of the land and the kind of rock beneath. Four patterns are worth knowing.

A dendritic pattern looks like the branches of a tree, forming where the land is fairly uniform. Most of the rivers of the northern plains are dendritic. A trellis pattern forms where a river is joined by its tributaries at almost right angles, in regions of folded rock. A radial pattern forms when rivers flow outward in all directions from a central highland, as from the Amarkantak hills. A centripetal pattern is the reverse, where streams flow inward towards a central basin or lake.

The regime of a river is the pattern of its flow, or how much water it carries, through the year. This is one of the clearest differences between India's two river families.

The Himalayan rivers have a fairly even regime. They are fed by monsoon rain in summer and by melting snow in the hotter months. So they carry water all year, with their highest flow during the rains. The Peninsular rivers have a seasonal regime entirely dependent on the monsoon. They swell during the rains and shrink to a trickle in the dry season. They have no snow to feed them.

The rivers of the Indus system are shared between India and Pakistan, and their waters were divided by the Indus Waters Treaty of 1960, brokered with the help of the World Bank. Under the treaty, India was given control of the three eastern rivers: the Ravi, the Beas and the Sutlej. Pakistan received the three western rivers: the Indus, the Jhelum and the Chenab. The treaty has survived even through wars and remains an important example of the sharing of river water between nations.

A dam is a barrier built across a river to hold back water in a reservoir. After independence, India built large dams that serve many purposes at once: irrigation, hydroelectric power, flood control, drinking water, navigation and fisheries. These are called multipurpose river valley projects. Jawaharlal Nehru famously called such dams the "temples of modern India". He meant they would drive farming and industry forward together. The first of them was the Damodar Valley Corporation (DVC), set up in 1948 on the Damodar river in Jharkhand and West Bengal, modelled on the Tennessee Valley Authority of the United States. The DVC runs dams, hydroelectric stations and thermal power stations, but it has no gas power stations.

The exam repeatedly asks which dam stands on which river. Memorise these pairs.

• Bhakra Nangal: on the Sutlej in Himachal Pradesh and Punjab, one of the best-known multipurpose projects.
• Hirakud: on the Mahanadi in Odisha, another classic multipurpose project.
• Tehri Dam: on the Bhagirathi in Uttarakhand, one of the tallest dams in India, the core of the Tehri Hydropower Complex.
• Sardar Sarovar and Omkareshwar: both on the Narmada, in Gujarat and Madhya Pradesh respectively.
• Gandhi Sagar Dam: on the Chambal in Madhya Pradesh, the first and largest of the four dams of the Chambal Valley Project.
• Mettur Dam: on the Cauvery in Tamil Nadu.
• Alamatti Dam: on the Krishna in Karnataka, the key dam of the Upper Krishna Project.

The Farakka Barrage in West Bengal diverts Ganga water into the Hooghly river to keep the Kolkata port channel alive. Its feeder canal has a carrying capacity of 40,000 cusecs. India also plans to join rivers through its inter-linking programme. The first link taken up is the Ken-Betwa Link Project, connecting the Ken and Betwa rivers under an agreement between Uttar Pradesh and Madhya Pradesh.

Smaller irrigation projects are tagged to their states: the Damanganga project in Gujarat, the Girna project in Maharashtra and the Pamba project in Kerala.

Over time, large dams have drawn strong criticism, and knowing both sides is important for examinations.

Building a huge reservoir submerges vast areas of land and forest. It displaces thousands of people from their homes and farms. Those displaced are often the poorest, and they are not always properly resettled. Dams also disturb the natural flow of the river. This harms aquatic life and stops silt from reaching the downstream flood plains, making them less fertile. Dams can trigger conflicts between regions over the sharing of water. Ironically, they have sometimes worsened floods when too much water is released at once. Opposition to such projects gave rise to popular movements like the Narmada Bachao Andolan, the people's movement against large dams on the Narmada.

Because large dams have so many drawbacks, attention has turned to a simpler, local solution: rainwater harvesting. This means collecting rainwater where it falls and storing it for later use. It can also mean letting rainwater soak into the ground to recharge wells and the water table.

India has a long tradition of such methods. In the hills of the west, people built guls and kuls, channels that carry water to the fields. In Rajasthan, rooftop rainwater was stored in underground tanks called tankas. In many regions, fields were bunded to hold rainwater for farming. Today rooftop rainwater harvesting is being revived. In some places, such as parts of Shillong and Rajasthan, it has become a vital source of drinking water. It is cheap, local and community-run, so it avoids most of the harms of big dams.