Why Does the Brahmaputra Change Its Course? The Science of River Erosion

Every monsoon, the same kinds of stories appear in Assam’s newspapers. Another school building has slipped into the river. A family has had to move house for the third time in ten years. A road that once ran along the bank now ends abruptly at the water’s edge.

The Brahmaputra does not just overflow. It shifts its course, splits into new channels and joins up again, leaving old channels to dry out, and can move hundreds of metres in a single season. According to the Brahmaputra Board, since 1950 the Brahmaputra and its tributaries have eroded more than 4.27 lakh hectares of Assam, almost 7.4 per cent of the land of the entire state, or nearly 8,000 hectares a year on average.

Why does the Brahmaputra behave like this? The answer lies in geology, climate and the enormous amount of sediment the river carries.

A river born in the mountains

The Brahmaputra rises in Tibet, where it is called the Yarlung Tsangpo. It cuts through the eastern Himalaya and enters Arunachal Pradesh as the Siang. On reaching the plains of Assam it is joined by the Dibang and the Lohit, flows west through the Assam valley, and then turns south into Bangladesh, where it is called the Jamuna.

Two features of this journey shape the river’s behaviour. First, the Brahmaputra drops steeply from high mountains to low plains, so it reaches the Assam valley with great energy and huge loads of rock and silt. Second, it flows through one of the most geologically active regions on Earth, where the Indian plate pushes against the Asian plate and the ground beneath the river is constantly rising, sinking and shaking.

Reason 1: The river carries an enormous sediment load

Sediment is the primary factor behind the Brahmaputra’s shifting course.

The Brahmaputra ranks among the rivers with the highest sediment load in the world. Research cited by Sarma and Acharjee (2018) in the journal Geosciences puts the annual suspended sediment load of the Brahmaputra at approximately 402 million tonnes. The mean daily discharge of suspended sediment at Pandu, near Guwahati, during the monsoon is roughly 2 million tonnes.

This sediment comes from the young, steep Himalaya, where heavy rain, landslides and weathering constantly break rock loose. When the river reaches the plains of Assam, it slows down and drops part of its load.

Over time these deposits build up and raise the riverbed, a process called aggradation. As the channels fill, they can hold less water, and the river is forced to find new paths.

Reason 2: It is a braided river

Most rivers flow in a single channel between two banks. In Assam, the Brahmaputra looks completely different. From above, it is a web of channels that split and rejoin around sandbanks and islands, known locally as chars or chaporis. This pattern is called a braided river.

Rivers usually braid when four conditions are present, and the Brahmaputra has all of them:

• A high volume of coarse sediment, including sand, deposited by the river in the form of bars.
• Easily eroded banks consisting of loose sand and silt, rather than solid rock or firm clay.
• Tremendous fluctuation of water volume between the dry and monsoon seasons.
• A broad and shallow riverbed.

With respect to the last condition, Sarma and Acharjee (2018) found that the width-to-depth ratio of braided stretches of the Brahmaputra is about 800, meaning the river is about 800 times wider than it is deep. They also recorded a change in discharge of about 17,000 cubic metres per second within 24 hours in June 1990.

In a braided river, the channels are never stable. A sandbar that forms in one flood can push the water towards the opposite bank in the next, and a minor channel can become the main one within months.

Reason 3: The banks are soft

For most of its course in Assam, the Brahmaputra flows over soft alluvium that the river itself has laid down over thousands of years. This is mostly sand and silt, which does not hold together well when wet.

During a flood, the current eats away the lower part of the bank. When the water falls, the soaked upper part is left without support and collapses in large chunks. This process, called bank failure or slumping, is how land falls into the river.

The river’s width speaks for itself. According to Sarma and Acharjee (2018), the mean channel width of the Brahmaputra in Assam was around 5.9 km in 1912–1928, and about 9.0 km in 2009. Channel widening took place in 55 of the 64 river reaches examined.

Tellingly, the narrowest and most stable stretches of the river are where it flows past hard gneissic rock, as at Pandu, Silghat and Jogighopa. Where the banks are hard, the river stays put; where they are soft, it wanders.

Reason 4: Earthquakes and plate movements

A powerful earthquake struck the Assam-Tibet border region on 15 August 1950. The 1950 Assam earthquake had a magnitude of about 8.5 to 8.6, and it caused extensive landslides in the eastern Himalaya.

These landslides poured a huge amount of sediment into the rivers. According to Sarma and Acharjee (2018), the earthquake was linked to about 45 billion cubic metres of sediment input. As this sediment moved downstream, the riverbed rose. In particular, the riverbed was raised by about 3 metres at Dibrugarh.

But there are slower tectonic influences too. Lahiri and Sinha (2012), in a study published in the journal Geomorphology covering the Brahmaputra in the upper Assam valley from 1915 to 2005, found that the channel belt expanded from approximately 9.7 km to approximately 14.0 km wide, a 44 per cent increase overall, with some stretches widening by as much as 250 per cent. The authors attribute variations in the river’s behaviour to tectonic factors such as the underlying geology and slope. They also found that tributaries from the Himalaya in the north behave very differently from those from the Naga-Patkai hills in the south.

In short, the ground beneath the Brahmaputra is itself on the move, and it constantly shapes how the river flows.

Reason 5: Seasonal floods and heavy rain

The Brahmaputra basin gets some of the heaviest monsoon rain in the world. The river’s flow rises sharply in summer and falls sharply in winter, and each rise and fall reshapes the river.

When the flow is high, the river carries more sediment, moves sandbanks and attacks its banks harder. As the flow falls, sediment settles in new places and the channels change again.

Reason 6: Human activity

Natural forces explain most of the Brahmaputra’s behaviour, but people also play a part.

Deforestation and land use in the upper reaches may increase erosion and the amount of sediment entering the river. According to Sarma and Acharjee (2018), the widening of the river between 1996 and 2009 was correlated with deforestation due to roads and logging in the Arunachal Himalayas; however, the cause-and-effect relationship could not be established with certainty.

Embankments built to shield towns and farms from flooding restrict the river to a narrower path. This may raise water levels in certain areas and increase pressure on unprotected banks, and a breach can cause sudden, severe flooding.

Encroachment on floodplains and riverbanks places more houses and agricultural land within the direct reach of the river, leading to higher human losses from erosion.

Majuli: the island at risk

Nowhere is the power of the Brahmaputra clearer than at Majuli, the river island in upper Assam that is home to many of the state’s Vaishnavite satras.

Majuli is situated between channels of the Brahmaputra and has been undergoing severe erosion for decades. The Central Water and Power Research Station (CWPRS), Pune, carried out a study on behalf of the Ministry of Jal Shakti, analysing about 120 km of the Brahmaputra around Majuli using satellite imagery. The study found that from 1986 to 2023, about 75 sq km of land had been eroded and about 58 sq km deposited along the right bank of the Brahmaputra.

The same research also shows how the river’s pattern of erosion can reverse. At Kamalabari, the bank eroded between 1986 and 2009, after which the river moved southwards and deposition followed. In a braided river, land lost in one place often builds up somewhere else, but the families who lose their land rarely gain any of the new land.

A river that has always moved

Changing course is not new for the Brahmaputra. Geological and historical evidence shows that in Bangladesh, the river shifted its main channel from the course now called the Old Brahmaputra to the present Jamuna channel between the late eighteenth and early nineteenth centuries. River scientists describe a sudden shift of this kind as an avulsion.

What has changed is the scale of human settlement along its banks. A shift that once moved through forest and grassland now moves through villages, towns, roads and farms.

Effects on people and the economy

The effects of erosion in Assam include:

• Land loss and displacement. People who lose land to erosion lose their main source of livelihood, and many are displaced repeatedly.
• Damage to infrastructure. Roads, embankments, schools and towns such as Dibrugarh have faced repeated threats from erosion.
• Loss of farmland. Riverbank land is some of the most fertile in the state, so erosion reduces food production and local livelihoods.
• Loss of culture. In Majuli, erosion threatens satras and villages which have existed for centuries.

How erosion is managed

Because the Brahmaputra is driven by natural forces on a huge scale, it cannot be fully controlled. Efforts focus on protecting the most vulnerable stretches and helping people adapt.

Some of the measures adopted include:

• Bank protection works, such as stone or concrete revetments and geobags (large synthetic bags filled with sand) placed along vulnerable sections of the bank.
• Spurs and porcupines built into the river to slow the current near the bank and encourage sediment to settle.
• Embankments to protect towns and fields from floods.
• Satellite monitoring and modelling to pinpoint eroding stretches. IIT Guwahati is carrying out a project with the North Eastern Hydraulic and Allied Research Institute (NEHARI) to study erosion along the Brahmaputra between Arunachal Pradesh and the Bangladesh border, to identify vulnerable stretches and mitigation measures.
• Nature-based measures, such as planting deep-rooted vegetation on banks and protecting catchment forests to reduce the sediment load.

Researchers increasingly argue that the approach should work with the river’s natural behaviour, not against it, including better land-use planning, avoiding settlement in hazardous areas, and support for families who have been forced to relocate.

Key facts at a glance

Factor	What the research shows
Land lost to erosion in Assam since 1950	Over 4.27 lakh hectares (about 7.4% of the state)
Average annual erosion	Nearly 8,000 hectares
Annual suspended sediment load	About 402 million tonnes
Mean channel width in Assam	About 5.9 km (1912–28) to about 9.0 km (2009)
Channel belt widening, upper Assam (1915–2005)	About 44% on average, up to 250% in some reaches
Bed rise at Dibrugarh after 1950 earthquake	About 3 metres
Majuli right bank, 1986–2023	About 75 sq km eroded, 58 sq km deposited

Conclusion

The Brahmaputra changes its course because of the way it was formed and the land it flows through. It carries one of the largest sediment loads of any river, passes through an area of soft, easily eroded alluvium, swells tremendously during the monsoon, and flows through an earthquake-prone region where tectonic activity still continues. Together, these forces make it a braided river whose channels are always shifting.

Understanding the science behind erosion in Assam matters, because it shows that the problem cannot be solved by engineering alone. It needs long-term planning and a careful approach.