by Rohan Nath
The mighty Brahmaputra River in Assam is home to the largest river island of the world, Majuli Island which spans an area of 1255 km2(Fig 1, 2). The island is a subdivision of the Jorhat district of Assam and is inhabited by around 153,000 people. “Vaisnavite” shrines, popularly known as “Sattras” is present on this island which led it to become a principal pilgrimage site for the last 400 years. However, flooding and severe bank erosion harm the island at this current time.
Image Source: Sarma, J. N., & Phukan, M. K. (2004). Origin and some geomorphological changes of Majuli Island of the Brahmaputra River in Assam, India. Geomorphology, 60(1-2), 1-19.
Three important rivers bound the island, namely, the Subansiri River, Kherkutia Suti in the north, and the Brahmaputra River in the south.
The Majuli island is characterized by a spindle shape lying in NE-SW direction with an altitude of 85.9m above sea level. Numerous lowlands and swamps of various shapes and sizes occupy the plain topography with low relief. Few big rivers and several small streams flow through the island. Alluvial fine loamy soils dominate the Majuli. The subtropical monsoon climate brings an annual rainfall between 1494 to 2552 mm.
Image Source: https://timesofindia.indiatimes.com/travel/destinations/majulithe-worlds largest-river-island-might-just-disappear-in-the-future/as64012861.cms
There is a high mean annual flow and suspended sediment load of the Brahmaputra and Subansiri (Table 1).
Table 1. The mean annual flow and suspended sediment load of the Brahmaputra and Subansiri.
|Year||River||Mean Annual Flow||Annual Suspended Sediment Load|
|1975-1990||Brahmaputra||8829.5 m3/s||402 million metric tons during 1955- 1979|
|1956-1982||Subansiri||1671 m3/s||35.48 million tons annually|
The mighty Brahmaputra leads to s severe bank erosion, which threatens the existence of Majuli.
The origin of Majuli
According to historical surveys and reports, Majuli stretched between a location named Banfang or Lakhu in the west and Bengmora in the east. In ancient histories of Assam, it had been referred to as ‘Majali’ or a piece of land located in between two adjacent and parallel rivers because the land was situated between the Luhit (Brahmaputra) in the north and its tributary, the Dihing (Burhi Dihing) in the south.
The Brahmaputra River (formerly known as the Luhit or Luit) flowed to the north of the land area of Majuli. The Dihing (one of the tributaries of the Brahmaputra) flowed south of Majuli and met the Brahmaputra at Lakhu (Fig. 3). The land area of Majuli was converted into an island when the Brahmaputra shifted its course southward and united with the Dihing near the confluence point at Dihingmukh which is located around 190 km east of Lakhu, the former confluence point. The southward shift of the Brahmaputra is believed to occur between 1661- 1696 due to a series of frequent floods and earthquakes.
Following the formation of the Majuli island, the Brahmaputra was divided into two separate branches.
i. Luhit Suti or Kherkutia Suti – Brahmaputra (Luhit) flow to the north of Majuli. ii. Burhi Suti – The flow of Brahmaputra was directed to the south of Majuli through the channel of the Dihing tributary.
There was a natural event of channel shift when the major flow of the Brahmaputra was diverted from the Kherkutia Suti into the Dihing leading to the enlargement of the channel (Fig. 4).
Currently, the Brahmaputra River resulted in an erosion in the southern boundary of the Majuli island with a faster erosion in the southwestern part of the island. There has been a noticeable increase in erosion during 1917-2001. The channel of Brahmaputra has widened due to the erosion of both of its banks. The great Assam earthquake of 1950, further leads to landslides in the eastern Himalayas which resulted in an increased flow of sediments into the river. The high amount of sediment descended the plains, choked the channel of the river which caused channel widening due to bank erosion. It has been reported that the width of the Brahmaputra channel has increased up to 300% since 1917.
The thick sand layer beneath the topsoil is undermined by the water which caused an increase in erosion. The erosion is slower in locations where there is a presence of cohesive silty-clay beds at the base of the banks.
The Government of Assam is actively involved in assessing damage to agricultural land due to sand deposition, the loss of property and life due to floods, and land area depletion due to erosion, leading to the displacement of settlements and families from their location. Therefore, it is an urgent need for the government to look upon it due to socioeconomic issues associated with it.
1. Sarma, J. N., & Phukan, M. K. (2004). Origin and some geomorphological changes of Majuli Island of the Brahmaputra River in Assam, India. Geomorphology, 60(1-2), 1- 19.