The Brahmaputra runs roughly 2,900 km from western Tibet to the Bay of Bengal, and it changes name twice on the way: the Yarlung Tsangpo in Tibet, the Siang or Dihang in Arunachal Pradesh, and the Brahmaputra in Assam. Between the first and second of those it makes one of the most dramatic turns in world geography, carving what is generally reckoned the deepest gorge on Earth.
Seventeen hundred kilometres in the wrong direction
The river rises near Mount Kailash in western Tibet, close to the sources of the Indus and the Sutlej, and then does something counterintuitive: it flows east, parallel to the Himalaya and on the far side of it, for something like 1,700 km. At an average elevation around 4,000 metres it is one of the highest major rivers in the world, running through a dry, cold, sparsely populated trough between the Himalaya to the south and the Tibetan plateau to the north.
For all that distance it stays north of the mountains, gathering the plateau's meltwater and getting no closer to the sea. Then, east of Lhasa, it runs out of trough.
The Great Bend, and a gorge deeper than any other
At the eastern end of its Tibetan run the river meets the Namcha Barwa massif, a 7,782-metre peak at the eastern anchor of the Himalaya, and swings around it in a tight hairpin — the Great Bend. In doing so it drops from the plateau to the plains of Assam, losing well over 2,000 metres in a relatively short distance.
The result is the Yarlung Tsangpo Grand Canyon. Measured from river to the surrounding summits, its depth exceeds 5,000 metres, which makes it deeper than the Grand Canyon by a factor of several — a comparison worth keeping in proportion, since the two are cut in very different rock and by very different processes. Our Grand Canyon guide covers the American one; the Tibetan gorge is narrower, wetter, forested at its lower elevations, and until the late twentieth century sections of it had never been traversed on foot.
Geologically the bend is not an accident. The Namcha Barwa syntaxis is a place where the Indian plate is driving into Asia at an angle, and the rock is rising extremely fast. The river has been cutting down at roughly the rate the mountains rise, which is why the gorge is so deep and why the process is sometimes described as the river sawing through a block being pushed up beneath it.
A braided river, not a channel
Emerging into Assam, the Brahmaputra abruptly becomes something else again. Freed of its gorge and heavily loaded with sediment, it spreads into a braided river — not one channel but dozens, weaving between temporary sand islands called chars, and in places more than 10 km wide.
The sediment load is among the highest of any river in the world, a direct consequence of the fast-eroding young mountains upstream. That sediment builds the chars, and the river then destroys them: a char that supports a village one year can be gone the next. Populations living on them are among the most frequently displaced in South Asia, and the land tenure problem this creates — property that physically ceases to exist — has no clean legal answer.
The largest of these formations is Majuli, historically among the biggest river islands in the world, and a centre of Assamese Vaishnavite monastic culture since the sixteenth century. It has lost a very large share of its area to erosion over the past century.
Floods that are the ecosystem, not an interruption to it
The Brahmaputra floods annually and heavily, fed by the summer monsoon and by Himalayan snowmelt arriving at the same time. In Assam this regularly displaces large numbers of people and drowns cropland, and the trend of severity has been upward.
But the flooding is also what makes the valley what it is. Kaziranga National Park, on the south bank, holds the great majority of the world's remaining greater one-horned rhinoceros population, along with elephants, swamp deer and wild water buffalo — and it is a grassland ecosystem maintained by inundation. The floods drown some animals each year and drive others onto the higher ground of the Karbi Anglong hills across the highway; they also flush out silt, suppress woody encroachment and renew the grasslands the grazers depend on. Managing the park means managing a flood regime, not preventing it.
The earthquake that rearranged the river
On 15 August 1950 an earthquake of roughly magnitude 8.6 struck the Assam–Tibet border region, one of the largest continental earthquakes ever recorded. Its effect on the Brahmaputra was immediate and lasting.
Landslides on an enormous scale stripped material off the surrounding mountains and dumped it into the tributaries. The riverbed in parts of Assam rose by several metres as that sediment worked downstream, which reduced the channel's capacity and made subsequent floods worse. Some tributaries were temporarily dammed by slides and then released catastrophically. Contemporary accounts describe the river running black with debris and the water level in places rising well after the shaking stopped.
The consequences were not a one-off. A river already carrying one of the world's heaviest sediment loads received a large additional pulse, and the aggradation of the bed through the Assam valley is a factor in the flooding pattern to this day. It is a case where a single seismic event permanently changed a major river's behaviour — and a reminder that the Himalayan front is tectonically active enough for this to happen again.
Where it ends: the largest delta in the world
In Bangladesh the Brahmaputra takes the name Jamuna, joins the Ganges to become the Padma, is joined in turn by the Meghna, and empties into the Bay of Bengal. The combined Ganges–Brahmaputra delta is the largest river delta on Earth, covering something over 100,000 km² and supporting a population well into the hundreds of millions.
It is also among the most precarious. The delta is low, flat and subsiding; it is exposed to cyclones tracking up the funnel-shaped Bay of Bengal, which amplifies storm surge; and it depends on continued sediment delivery to build upward fast enough to offset both subsidence and rising sea level. Upstream dams that trap sediment therefore threaten the delta in a way that is easy to overlook — the risk is not only less water, but less mud.
The seaward fringe is the Sundarbans, the largest mangrove forest in the world, shared between Bangladesh and India and home to a tiger population that has adapted to a saline, tidal environment. The mangroves absorb a meaningful share of cyclone energy before it reaches inhabited land, which makes their steady loss to shrimp farming, embankment and salinity change a defensive as well as an ecological problem.
Three countries and a dam question
The river's political geography is as complicated as its physical one. It rises in Chinese-administered Tibet, crosses into India through Arunachal Pradesh — territory China claims — and then flows into Bangladesh, where it is called the Jamuna and joins the Ganges to form the Padma, and finally the Meghna, before reaching the Bay of Bengal.
Three riparian parties, no comprehensive basin treaty, and a large hydroelectric potential concentrated at exactly the point where the river drops off the plateau. Chinese plans for major dam development at the Great Bend have been a persistent source of Indian and Bangladeshi concern, for the familiar downstream reasons: control over timing of flow, retention of sediment that the delta needs to keep pace with sea level, and the general problem of depending on infrastructure operated by another state. India's own hydro projects in Arunachal generate similar anxieties one step further downstream. Compare the arrangement on the Danube, where ten countries share a river under a formal basin commission, and the absence of a Brahmaputra equivalent stands out.
The son of Brahma
Among the great rivers of the Indian subcontinent, the Brahmaputra is unusual in being grammatically and mythologically male. Most are goddesses — the Ganges above all, along with the Yamuna, the Narmada and the Kaveri. Brahmaputra means son of Brahma, and the associated legend has the river born of the creator god and carried down from the mountains.
The practical reflection of this is a different ritual relationship. The Ganges is a river of pilgrimage cities and bathing ghats along much of its length; the Brahmaputra's sanctity is more concentrated, focused on sites like the Kamakhya temple above Guwahati and the satras of Majuli. Its Tibetan identity is different again: the Yarlung Tsangpo runs through the valley regarded as the cradle of Tibetan civilisation, where the earliest kings are said to have ruled.
One river, then, that is a high desert stream, the deepest gorge on the planet, a braided giant several kilometres wide, and a delta channel — with a different name, a different culture and a different set of problems in each of those roles.
