Himalayan drainage system

Geography glossary

Also called: Himalayan rivers · Topic: Drainage System of India: Rivers, Basins and Patterns · NCERT: Class 11, Ch 3 "Drainage System"

Meaning

The Himalayan drainage system is the group of rivers that rise in or behind the Himalayas. They belong to three main basins: the Indus, the Ganga and the Brahmaputra. These rivers are perennial (they flow all year), because melting snow and rain both feed them. They water the great northern plains, and their floods, changes of course and shrinking glaciers make them a big issue for both Prelims and Mains.

Explanation

Parts and water supply

Three main basins, from west to east:
Indus: flows west and reaches the Arabian Sea.
Ganga: flows through the centre and reaches the Bay of Bengal.
Brahmaputra: flows east, joins the Ganga system and reaches the Bay of Bengal.
Why the rivers are perennial:
Summer: snow and glaciers melt and feed the rivers.
Monsoon (June–September): heavy rain adds a second peak of water.
Result: the rivers keep flowing even in the dry months.
Water Tower of Asia: the Himalayas, and the wider Hindu Kush Himalaya (HKH), store huge amounts of water as snow and ice. They release it slowly into Asia's big rivers. The HKH holds the largest mass of ice outside the North and South Poles.

Work in the mountains and on the plains

In the mountains, the rivers cut downwards:
The Himalayas kept rising while the rivers kept cutting into the rock.
Fast water on steep slopes carries rocks along and wears down its bed. This is vertical erosion (cutting down more than sideways).
Landforms it makes: giant gorges, V-shaped valleys, rapids and waterfalls (NCERT Figure 3.3).
On the plains, the rivers build land:
When the rivers leave the mountains, the slope drops sharply and the water slows down.
Slow water cannot carry its heavy silt, so it drops it. This is deposition.
Landforms it makes: floodplains, meanders (big snake-like bends), oxbow lakes (U-shaped lakes left when a meander is cut off), braided channels (one river split into many channels around sand bars) and deltas.
Himalayan rivers on the plains meander strongly and often shift their course.
Antecedent rivers: rivers that are older than the mountains they cross.
The river was already flowing before the land rose.
The land rose slowly, and the river cut down at the same speed.
So the river kept its old path and cut deep transverse gorges (gorges that run across the ranges, not along them).
NCERT Class 11 names the Satluj, Kosi and Subansiri. The Indus and Brahmaputra are also standard examples.

Evolution: the Indo-Brahma river

Indo-Brahma river (also called the Shiwalik river): one huge river that is believed to have flowed along the whole length of the Himalayas.
When: the Miocene period, about 5–24 million years ago.
Course: from Assam westwards to Punjab and Sind. It drained into the Gulf of Sind, in the Arabian Sea.
Evidence: the Shiwalik hills run in an unbroken line along the foot of the Himalayas. They are made of lacustrine (laid down in lakes) and alluvial (laid down by rivers) material: sands, silts, clays, boulders and conglomerates (rounded pebbles cemented together). A belt this long and unbroken points to one single river.
How it broke up:
West: in the Pleistocene, the western Himalayas, including the Potwar plateau, were pushed up. This raised land became the water divide between the Indus and the Ganga.
East: in the mid-Pleistocene, the Malda gap (between the Rajmahal hills and the Meghalaya plateau) sank. The Ganga and Brahmaputra took this lower path and turned south into the Bay of Bengal.
The result was three systems: the Indus and its five tributaries (Jhelum, Chenab, Ravi, Beas, Satluj), the Ganga and its Himalayan tributaries, and the Brahmaputra in Assam and its Himalayan tributaries.

Where it occurs

Source area near Mount Kailash, western Tibet (China):
Satluj: flows out of Rakas Tal at about 4,555 m. In Tibet it is called the Langchen Khambab.
Indus: rises near the Bokhar Chu glacier, close to Mansarovar.
Brahmaputra: rises in the Chemayungdung glacier, near Mansarovar.
Karnali (Ghaghara): rises nearby and flows through Nepal into India.
So one small area sends rivers to both the Arabian Sea and the Bay of Bengal.
Antecedent gorges:
Indus: deep gorge near Gilgit (Pakistan-occupied Kashmir).
Satluj: enters India through a gorge at Shipki La (Himachal Pradesh).
Kosi (Arun): cuts across the Great Himalaya in Nepal.
Subansiri: gorge in Arunachal Pradesh.
Brahmaputra (Tsangpo): makes a U-turn around Namcha Barwa, then enters Arunachal as the Siang/Dihang.
Tsangpo Gorge (Yarlung Zangpo Grand Canyon), Tibet: it lies between Namcha Barwa (7,782 m) and Gyala Peri (7,234 m). It is up to 6,009 m deep, the deepest canyon in the world [5].
Plains landforms: floodplains on the Ganga plains of UP and Bihar. Meanders on the middle and lower Ganga. Oxbow lakes on the Ganga and Kosi plains. Braided channels on the Brahmaputra in Assam and the Kosi. The Ganga–Brahmaputra (Sundarbans) delta.
Kosi, the "sorrow of Bihar": it has moved about 120 km westwards over about two centuries. In August 2008 the embankment broke at Kusaha (Nepal), and the river flowed into a channel it had left more than 200 years ago [2].
Indus–Ganga water divide: Haryana and Delhi, a very low divide linked to the Delhi ridge (the northern end of the Aravallis). The dried Ghaggar–Hakra channel lies on it.
HKH region: it stretches from Afghanistan to Myanmar across eight countries, and more than 1.3 billion people share these mountains [3].

Don't confuse with

Peninsular drainage system: most Peninsular rivers depend only on rain and flow mostly in the monsoon. Himalayan rivers get both snowmelt and rain, so they are perennial. The Ken and Betwa (Ken–Betwa Link, 2021) are Yamuna tributaries that rise on the Peninsular (Bundelkhand) plateau, so they are not Himalayan rivers.
Consequent river: it follows the slope of the land after the land has formed. An antecedent river is older than the mountains and cuts across them in transverse gorges (Satluj, Kosi, Subansiri).
Indo-Brahma (Shiwalik) river vs today's Brahmaputra: the old Miocene river flowed west from Assam into the Gulf of Sind. Today's Brahmaputra flows to the Bay of Bengal.
Mansarovar vs Rakas Tal: the Satluj rises in Rakas Tal. The Indus and Brahmaputra rise from glaciers near Mansarovar.

Prelims Hooks

Order of Himalayan basins from west to east: Indus → Ganga → Brahmaputra.
Antecedent rivers in NCERT Class 11: Satluj, Kosi, Subansiri. Trap: the Yamuna and Gandak are not on this list.
Indo-Brahma river: Miocene. It flowed from Assam to Punjab and Sind and drained into the Gulf of Sind, not the Bay of Bengal.
Break-up causes: Pleistocene uplift of the Potwar plateau (the Indus–Ganga divide), and mid-Pleistocene sinking of the Malda gap (between the Rajmahal hills and the Meghalaya plateau).
Kosi = "sorrow of Bihar". The Arun is its main stream. The 2008 breach was at Kusaha, Nepal [2].
Tsangpo Gorge around Namcha Barwa is the world's deepest canyon, up to 6,009 m deep [5].

Mains Points

Why Himalayan rivers flood and shift (GS-I/III):
Young, rising mountains wear away fast, so the rivers carry a huge silt load.
On the flat plains the silt settles, the channels choke, and the rivers shift course (the Kosi moved about 120 km in two centuries).
Embankments trap silt and raise the riverbed above the land around it. So when one breaks, as at Kusaha in 2008, the damage is worse: more than a million people were forced out of their homes [2]. India needs silt management, river-basin planning and a review of its treaty with Nepal.
Drainage as a record of geological history (GS-I): the unbroken Shiwalik deposits, the antecedent gorges (Satluj, Tsangpo) and the Malda gap show that today's three basins came from one Miocene river. Uplift and sinking of land in the Pleistocene then split it apart.
Water security and climate change (GS-III):
The HKH ranges could lose up to two-thirds of their ice by 2100. This could cause water and food shortages for over two billion people downstream in South and East Asia [4].
Chain of effects: warming → glaciers melt faster → more water and more glacial-lake floods at first → later, less meltwater in the dry season → the rivers may stop being reliably perennial.
Response: glacier monitoring, GLOF (glacial lake outburst flood) early warning, and water agreements with China, Nepal, Bhutan and Bangladesh.

Related concepts

The Himalayan drainage: character and evolution

Sources

1Class 11, Ch 3 "Drainage System" (primary)
2Monsoon Floods Reshape Indian River — NASA Earth Observatoryscience.nasa.gov · tier 2
3Adaptation Outlook for the Hindu Kush Himalaya in the Making — UNEPunep.org · tier 2
4Climate Change Melts Ice on the World's Highest Mountains and Puts Two Billion People Downstream at Risk — UNDPundp.org · tier 2
5Yarlung Zangpo Grand Canyon, Tibet — NASA Photojournalscience.nasa.gov · tier 2