The Himalayan drainage: character and evolution

Detail

1. What the Himalayan drainage system is

• Himalayan drainage system: all the rivers that rise in or behind the Himalayas. It has three main basins:
• Indus: flows west. It reaches the Arabian Sea.
• Ganga: flows through the centre. It reaches the Bay of Bengal.

• Brahmaputra: flows east. It joins the Ganga system and reaches the Bay of Bengal.

• Perennial rivers (rivers that flow all year):

• Summer: snow and glaciers melt and feed the rivers.
• Monsoon (June–September): heavy rain adds a second peak of water.
• Result: the rivers never dry up, even in the dry months. Most Peninsular rivers depend only on rain and so flow mostly in the monsoon.

2. Work in the mountains: cutting downwards

• Cause: the Himalayas kept rising, and the rivers kept cutting down into the rock at the same time.
• Process: water moving fast down steep slopes carries rocks and cuts its bed. This is called vertical erosion (cutting downward more than sideways).
• Landforms produced (NCERT Figure 3.3):
• Giant gorges: very deep, narrow valleys with steep rock walls.
• V-shaped valleys: narrow at the bottom and wide at the top, because the river cuts down faster than the slopes wear back.
• Rapids: stretches of fast, rough water over rocky beds.
• Waterfalls: places where the river drops over a step of hard rock.

3. Work on the plains: building land and shifting course

• Cause: when the rivers leave the mountains, the slope drops sharply and the water slows down.
• Process: slow water cannot carry its heavy load of silt and sand, so it drops it (deposition).
• Landforms produced:

Landform	Simple meaning	Where seen
Flat valleys / floodplains	Wide, flat land built from silt dropped during floods	Ganga plains of UP and Bihar
Meanders	Big, snake-like bends in a slow river	Middle and lower Ganga
Oxbow lakes	U-shaped lakes left behind when a meander is cut off	Ganga and Kosi plains
Braided channels	One river split into many small channels around sand bars	Brahmaputra in Assam; Kosi
Deltas	Triangle-shaped land at the river mouth, built from silt	Ganga–Brahmaputra (Sundarbans) delta

• Himalayan rivers on the plains meander strongly and often shift their course.

4. The Kosi: the "sorrow of Bihar"

• Why the name: its floods and changes of course have destroyed farms and villages in north Bihar again and again.
• How the Kosi shifts (cause → process → result):
• Cause: its upper reaches in Nepal and Tibet are steep and made of loose, easily worn rock. The river brings down huge amounts of sediment (silt, sand and gravel).
• Process: on the flat Bihar plain the river slows and drops this sediment. Its bed rises and its own channel gets blocked.

• Result: the water breaks out and finds a new, lower path.

• Scale of shifting: the Kosi has moved about 120 km westwards over about two centuries.

• The 2008 Kusaha breach:
• In August 2008 the embankment broke at Kusaha, in Nepal, just upstream of the Kosi barrage.
• The river burst its banks and flowed into a channel it had left more than 200 years ago [2].
• NASA's Terra satellite (MODIS sensor) images show the change. On 8 August 2008 the river curved westwards in a "c" shape. By 24 August 2008 it had cut due south, missing its modern arc. The flooded channel was kilometres wide in places [2].

• More than a million people were forced out of their homes (2008) [2].

• Lesson: embankments trap silt inside the channel. The bed rises above the land around it, so when the embankment breaks the damage is worse.

5. The Himalayas as the "Water Tower of Asia"

• Water tower of Asia: a name for the Himalayas (and the wider Hindu Kush Himalaya, HKH). They store huge amounts of water as snow and ice and release it slowly into Asia's great rivers.
• The HKH holds the largest mass of ice outside the North and South Poles.

• The HKH region stretches from Afghanistan to Myanmar across eight countries. More than 1.3 billion people share these mountains [3].

• Climate risk (HKH Assessment, ICIMOD, 2019):

• The ranges could lose up to two-thirds of their ice by 2100 [4].
• This could cause water and food shortages for over two billion people in the river basins 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 no longer be reliably perennial.

6. Antecedent rivers

• Antecedent river: a river that is older than the mountains it flows across.
• Step 1: the river was already flowing before the land rose.
• Step 2: the land rose slowly, and the river kept cutting down at the same speed.

• Step 3: the river held its old path and cut deep transverse gorges (gorges that run across the mountain ranges, not along them).

• NCERT Class 11 names the Satluj, Kosi and Subansiri as antecedent. The Indus and Brahmaputra are also standard examples.

River	Where it rises	Gorge / crossing
Indus	Tibet, near Mansarovar	Deep gorge across the Himalayas near Gilgit (Pakistan-occupied Kashmir)
Satluj	Rakas Tal, Tibet	Enters India at Shipki La (Himachal Pradesh) through a gorge
Kosi (Arun)	Tibet, north of Mount Everest. The Arun is its main stream	Cuts across the Great Himalaya in Nepal
Subansiri	Tibet	Gorge in Arunachal Pradesh
Brahmaputra (Tsangpo)	Chemayungdung glacier, near Mansarovar	U-turn around Namcha Barwa, then enters Arunachal as the Siang/Dihang

• Tsangpo Gorge (Yarlung Zangpo Grand Canyon), Tibet:
• The river passes between Namcha Barwa (7,782 m) and Gyala Peri (7,234 m) (NCERT gives Namcha Barwa as 7,755 m) [5].
• The gorge is up to 6,009 m deep. This makes it the deepest canyon in the world [5].

7. Evolution: the Indo-Brahma (Shiwalik) 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 in the past.
• When: the Miocene period, about 5–24 million years ago.
• Course: from Assam in the east, westwards to Punjab and Sind. It drained into the Gulf of Sind, near the present Indus mouth, 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 (rock made of rounded pebbles cemented together).
• Such a long, continuous river-laid belt points to one single river flowing along it.

8. How the Indo-Brahma river broke up

Cause	When	Where	Result
Upheaval (uplift) in the western Himalayas, including the Potwar plateau	Pleistocene (about 2.6 million–11,700 years ago)	West: the Potwar plateau lies in Pakistan's Punjab, between the Indus and Jhelum (NCERT writes "Potwar Plateau (Delhi Ridge)")	Raised land became the water divide between the Indus and the Ganga
Down-throw (sinking) of the Malda gap	Mid-Pleistocene	East: between the Rajmahal hills (Jharkhand) and the Meghalaya plateau	The Ganga and Brahmaputra turned south into the Bay of Bengal

• The three systems that resulted:
• West: the Indus and its five tributaries (Jhelum, Chenab, Ravi, Beas and Satluj).
• Centre: the Ganga and its Himalayan tributaries.

• East: the stretch of the Brahmaputra in Assam and its Himalayan tributaries.

• Why the Malda gap matters: a block of land sank, opening a low gate between two old hard blocks. Water always takes the lowest path, so the eastern rivers left the old westward route and drained south-east.

9. The Indus–Ganga water divide today

• Water divide: higher ground that separates two river basins. Rain falling on each side flows to a different river.
• Haryana and Delhi form the divide between the Indus and Ganga systems today (NCERT Class 11, Ch 2).
• It is a very low divide, linked to the Delhi ridge, the northern end of the Aravallis.
• West of it: rivers drain to the Indus and the Arabian Sea.

• East of it: the Yamuna drains to the Ganga and the Bay of Bengal.

• Ghaggar link: the Ghaggar–Hakra dried channel lies on this divide. Its old course is central to the debate on the lost "Sarasvati" river. Scholars still do not agree on it.

10. Source lakes: Mansarovar and Rakas Tal

• Mansarovar and Rakas Tal: two high lakes next to each other near Mount Kailash in western Tibet (China).
• Mansarovar is a freshwater lake. It is a holy site on the Kailash–Mansarovar Yatra (a pilgrimage India runs with China through Lipulekh pass in Uttarakhand and Nathu La in Sikkim).

• Rakas Tal lies just west of Mansarovar.

• Rivers that rise here:

• 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 of the Kailash range, near Mansarovar.

• Karnali (Ghaghara): rises nearby and flows through Nepal into India.

• So: one small area of Tibet sends rivers to both the Arabian Sea (Indus, Satluj) and the Bay of Bengal (Brahmaputra, Karnali).

11. Present-day link: interlinking of rivers (Ken–Betwa)

• Interlinking of rivers: moving extra water from one basin to a basin that is short of water, using dams and canals.
• Ken–Betwa Link Project: the first major centrally driven river-interlinking project in India.
• 22 March 2021: the Union Jal Shakti Minister and the Chief Ministers of Madhya Pradesh and Uttar Pradesh signed an agreement to build it [6].
• 8 December 2021: the Union Cabinet approved its funding and building. Total cost: ₹44,605 crore (2020-21 prices). Central support: ₹39,317 crore [6].
• Works: the Daudhan dam and a canal from the Ken to the Betwa, plus the Lower Orr Project, Kotha barrage and Bina Complex Multipurpose Project [6].

• Benefits: irrigation for 10.62 lakh ha each year, drinking water for about 62 lakh people, 103 MW of hydropower and 27 MW of solar power [6].

• Note: the Ken and Betwa are Yamuna tributaries. They flow north from the Peninsular (Bundelkhand) plateau into the Ganga system, so they are not Himalayan rivers.

Prelims Hooks

• Indo-Brahma (Shiwalik) river: Miocene (about 5–24 million years ago). 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 (Indus–Ganga divide) and mid-Pleistocene down-throw of the Malda gap (Rajmahal hills–Meghalaya plateau).
• Antecedent rivers in NCERT Class 11: Satluj, Kosi, Subansiri. Trap: the Yamuna and Gandak are not on that list.
• Satluj rises in Rakas Tal, not Mansarovar. The Indus, Brahmaputra and Karnali rise nearby.
• Kosi = "sorrow of Bihar". The Arun is its main stream. In August 2008 it breached at Kusaha (Nepal) and moved into a channel it had left more than 200 years ago [2].
• Tsangpo Gorge (around Namcha Barwa) is the world's deepest canyon, up to 6,009 m deep [5].
• Indus–Ganga water divide = Haryana–Delhi (Delhi ridge).
• Ken–Betwa approved 8 December 2021. Daudhan dam. States: MP and UP. Cost ₹44,605 crore [6].
• Order of Himalayan basins from west to east: Indus → Ganga → Brahmaputra.

Mains Points

• Why Himalayan rivers flood and shift (GS-I/III): young, rising mountains → heavy erosion → huge silt load → silt dropped on flat plains → channels choke and rivers shift (the Kosi moved about 120 km in two centuries). Embankments trap silt and raise the riverbed, so a breach like Kusaha (2008), which drove more than a million people from their homes, becomes a disaster [2]. Silt management, river-basin planning and an India–Nepal treaty review are needed.
• Drainage tells us geological history (GS-I): unbroken Shiwalik deposits, antecedent gorges (Satluj, Tsangpo) and the Malda gap show that today's three basins come from one Miocene river. Pleistocene uplift and sinking of land then split it up.
• Water security and climate (GS-III): the Himalayas are the "Water Tower of Asia". Losing up to two-thirds of their ice by 2100 threatens over two billion people downstream [4]. India needs glacier monitoring, GLOF (glacial lake outburst flood) early warning and agreements with China, Nepal, Bhutan and Bangladesh.
• Interlinking as a response (GS-III): Ken–Betwa (2021) moves water to dry Bundelkhand, giving 10.62 lakh ha of irrigation and drinking water for 62 lakh people [6]. Key concerns are flooding inside Panna Tiger Reserve, people losing their homes, and the ecological cost of moving water between basins.