The Brahmaputra system

Detail

1. Where the system came from: the Indo-Brahma idea

• Indo-Brahma hypothesis: the idea that one very large river (the Indo-Brahma or Shiwalik river) once flowed along the whole length of the Himalaya. It ran from Assam in the east to Punjab in the west.
• How the big river broke up:
• The western Himalaya and the Potwar Plateau (Delhi Ridge region) were pushed up.
• This split the single river into separate systems.

• The result was today's Indus, the Ganga and the Brahmaputra.

• So the Brahmaputra is one of the three great Himalayan systems. Like the others, it is perennial (it flows all year round), because it gets both snowmelt and monsoon rain.

2. Basin at a glance

• The Brahmaputra is one of the world's largest rivers. Its name means "son of Brahma".
• Total length: about 2,900 km, through Tibet (China), India and Bangladesh [2].
• Total basin area: about 5,80,000 sq km, spread over Tibet, Bhutan, India and Bangladesh [2]. A basin (or catchment) is all the land that drains into one river and its tributaries.
• Area inside India: 1,94,413 sq km, which is about 5.9% of India's area [2].

State (Indian part of the sub-basin)	Drainage area (sq km) [2]
Arunachal Pradesh	81,424 (largest)
Assam	70,634
West Bengal	12,585
Meghalaya	11,667
Nagaland	10,803
Sikkim	7,300

• Hydropower potential: 31,012 MW (assessed at 60% load factor), which is about 37% of India's total hydropower potential [2]. Load factor is how much of the year a plant actually runs at full output.
• Monitoring: the CWC (Central Water Commission) runs 137 gauge-discharge sites and 28 flood-forecasting stations in the sub-basin [2]. A gauge-discharge site measures the water level and the volume of flow.

3. Source and the Tibetan stretch: the Yarlung Tsangpo

• Origin: the Chemayungdung glacier of the Kailash range, near Lake Mansarovar in south-west Tibet (about 30–31°N). The source lies at about 5,150 m [2].
• Yarlung Tsangpo: the river's name in Tibet. "Tsangpo" means "the purifier".
• Course:
• It flows east for about 1,200 km.

• It runs parallel to the Himalaya, through dry, flat southern Tibet. This is a rain-shadow area: the Himalaya blocks the monsoon winds, so little rain falls here.

• Main right-bank tributary in Tibet: the Rango Tsangpo.

• The Great Bend:
• Near Namcha Barwa (7,755 m), the eastern anchor of the Himalaya, the calm river becomes turbulent (fast and rough).
• It turns sharply around the peak and cuts a very deep gorge.

• This U-turn is called the Great Bend. It lies close to Medog in Tibet.

• Why it can cut the gorge:

• The river is older than the rising ranges and keeps its course.
• It drops steeply from the Tibetan Plateau towards the plains.
• This gives it great power to cut down, so it digs a deep, narrow gorge.

4. Entry into India: the Siang (Dihang)

• Siang river (also called the Dihang): the name of the river where it comes out of the foothills in Arunachal Pradesh.
• Point of entry: it enters the plains west of Sadiya in Arunachal Pradesh.
• Birth of the "Brahmaputra":
• Two left-bank tributaries join the Siang: the Dibang (Sikang) and the Lohit.
• From this meeting point onwards, the river is called the Brahmaputra.

Stretch	Name of the river	Direction
Tibet (Chemayungdung glacier to the Great Bend)	Tsangpo / Yarlung Tsangpo	East
Arunachal Pradesh foothills	Siang / Dihang	South (through the gorge)
After the Dibang and Lohit join, through Assam	Brahmaputra	South-west, then west
Bangladesh, after the Tista joins	Jamuna	South
After meeting the Padma	Padma	To the Bay of Bengal

5. The Assam valley stretch (about 750 km)

• In Assam the river flows through a long, narrow valley. The Himalaya lie to the north and the Meghalaya plateau to the south.
• It is fed by many tributaries:

Bank	Tributaries	Where they come from
Left (south) bank	Burhi Dihing, Dhansari (South)	Patkai–Naga hills to the south
Right (north) bank	Subansiri, Kameng, Manas, Sankosh	Himalaya (Arunachal Pradesh, Bhutan, Tibet)

• The CWC also lists the Dibang, Lohit, Subansiri, Manas, Tista, Dhansiri and Champamati as the main tributaries inside India [2].
• Subansiri river:
• It is an antecedent river: a river that existed before the mountains rose. As the land was slowly pushed up, the river kept cutting down, so it kept its old path. This makes deep gorges across the ranges.

• It rises in Tibet and joins the Brahmaputra from the right bank.

• Subansiri Lower Hydroelectric Project:

• It lies on the Subansiri river, on the Arunachal Pradesh–Assam border [3].
• Capacity: 2,000 MW, made up of 8 units of 250 MW each. It is built by NHPC Limited [3].
• It is described as the country's largest hydropower project [4].
• Unit-2 (250 MW) started commercial operation on 23 December 2025 [4]. By 2026, 2 of the 8 units were running. The rest are planned in phases until December 2026 [3].
• Expected output: about 7,422 million units (MU) a year [3].
• It also aims to give flood moderation in the Brahmaputra valley, which means lowering flood peaks by holding back water. It will help grid stability by balancing solar and wind power, and supply peaking power (extra power at times of highest demand) [3].

6. Into Bangladesh: Tista, Jamuna, Padma

• Exit from India: the Brahmaputra enters Bangladesh near Dhubri (Assam). From there it flows south.
• Tista river:
• It rises in Sikkim and flows through north Bengal (West Bengal).
• It joins the Brahmaputra on the right bank inside Bangladesh.

• Sharing its water is a pending India–Bangladesh issue.

• After the Tista joins, the river is called the Jamuna.

• The Jamuna then merges with the Padma (the Ganga's name in Bangladesh), and the combined flow reaches the Bay of Bengal. The CWC describes the river as joining the Ganga after its 2,900 km course [2].

7. Seasonal flow: why it swells in summer

• Snowmelt: in summer, snow and glaciers in the Himalaya and Tibet melt, so the river swells in summer.
• Monsoon rain: at the same time (June–September), the south-west monsoon brings very heavy rain over Arunachal, Assam and Meghalaya.
• Result: the snowmelt peak and the rain peak come together. This makes the summer flow very high.

8. Floods, channel shifting and bank erosion

• Why the Brahmaputra floods:
• It has many large tributaries.
• It drains a very wet catchment, among the rainiest places on Earth.

• Its young Himalayan slopes are steep and loose, so it carries a very heavy load of silt.

• What happens next:

• When the river reaches the gentle slope of the Assam plain, it slows down.
• It drops its silt on its own bed, so the bed rises.
• The river splits into many channels around sandbars. This is called a braided channel.

• The channels keep moving. This is channel shifting.

• Result:

• The raised bed holds less water, so the river spills over its banks and floods.

• The moving channels eat into the banks. This is bank erosion, which destroys farmland and villages in Assam.

• Riverine islands such as Majuli form in this setting. They are covered under physiography. Flood management is covered under natural hazards.

9. Trans-border dams: Medog and the Siang Upper project

• China's project:
• China approved a mega dam on the lower Yarlung Tsangpo, which is the upper Brahmaputra [5].

• It is near the Great Bend (Medog) and is about 60 GW in size. Construction began in July 2025.

• India's response:

• India registered its concerns with China on 30 December 2024. It asked for transparency and consultation with downstream countries [5].
• The issue was raised again at the Foreign Secretary–Vice Foreign Minister meeting in Beijing on 26–27 January 2025 [5].

• Both sides agreed to hold an early meeting of the Expert Level Mechanism. This would discuss restarting the sharing of hydrological data (river level and flow data) and other cooperation on trans-border rivers [5].

• Why India worries:

• A dam upstream lets China control the timing of water flow.
• It could cut the dry-season flow.

• A sudden release of water, or a dam failure, could flood Arunachal and Assam. The region also has high earthquake risk.

• India's planned counter:

• The Siang Upper Multipurpose Project in Arunachal Pradesh, of about 11,000 MW. "Multipurpose" means it serves several uses: power, water storage and flood control.
• The Upper Siang Project appears in official lists of water projects at the pre-feasibility and project-development stage [6]. A pre-feasibility study is an early check of whether a project is worth building.
• Its large reservoir would act as a buffer (a store of water) against sudden releases from upstream.
• It faces local opposition, mainly from the Adi community, over flooding of land and villages.

Prelims Hooks

• Origin: Brahmaputra → Chemayungdung glacier, Kailash range, near Mansarovar, at about 5,150 m.
• Names in order, from source to sea: Tsangpo/Yarlung Tsangpo (Tibet) → Siang/Dihang (Arunachal) → Brahmaputra (after the Dibang and Lohit join, near Sadiya) → Jamuna (Bangladesh, after the Tista joins) → Padma → Bay of Bengal.
• "Tsangpo" = "the purifier". "Brahmaputra" = "son of Brahma".
• Rango Tsangpo is the main right-bank tributary in Tibet. The Dibang (Sikang) and Lohit are left-bank tributaries in Arunachal.
• Trap: the Burhi Dihing and Dhansari (South) are left-bank tributaries. The Subansiri, Kameng, Manas and Sankosh are right-bank tributaries.
• Subansiri is an antecedent river that rises in Tibet. The Subansiri Lower HEP (2,000 MW, 8 × 250 MW, NHPC) is on the Arunachal–Assam border [3].
• The river cuts a gorge round Namcha Barwa (7,755 m) at the Great Bend, near Medog.
• The Brahmaputra enters Bangladesh near Dhubri. The Tista (from Sikkim) joins it on the right bank in Bangladesh.
• Largest share of the Indian basin: Arunachal Pradesh (81,424 sq km), then Assam (70,634 sq km). The basin holds about 37% of India's hydropower potential [2].

Mains Points

• Flood–erosion cycle (GS-I/GS-III):
• A very wet catchment and heavy silt from young mountains raise the river bed.
• This causes braiding, channel shifting and bank erosion in Assam, with yearly floods and loss of land.

• Responses include flood forecasting (28 CWC stations [2]), flood moderation by storage dams like Subansiri Lower [3], and bank protection.

• Water security and trans-border rivers (GS-II/GS-III):

• China's ~60 GW Medog project on the upstream river creates risks for India: less water in the dry season, sudden releases and earthquake hazard.

• India is responding with diplomacy (concerns raised on 30 December 2024, Expert Level Mechanism for data sharing [5]) and with the Siang Upper storage project as a buffer.

• Hydropower and development of the North-East:

• The North-East has 31,012 MW of hydropower potential [2], which can provide clean energy and peaking power.

• This must be balanced against displacement of tribal people, earthquake risk, and harm to river ecology and to downstream farmers and fishers.

• India–Bangladesh river relations: the unsettled Tista water-sharing issue shows how upstream use in Sikkim and north Bengal affects dry-season flow in the lower basin.