Fluvial Morphodynamics of the River Brahmaputra and its Implications on the Majuli Island, India

The Himalayan rivers of India exhibit certain unique characteristics as they undergo large seasonal fluctuations in their water regime. During the monsoon months, these rivers experience a soaring flow of water due to excessive rainfall and the melting of glaciers, often resulting in recurring floods in the plains. In addition to high flows, these rivers carry heavy sediment loads, which make the riverbanks vulnerable to erosion. The Brahmaputra, one of the largest rivers in India, is often associated with devastating floods in the state of Assam. Since the great earthquake of 1950, which struck the Upper Brahmaputra Valley in Assam, riverbank erosion has become a scourge for the land and its people. After the earthquake, the problems of flooding and riverbank erosion have intensified in the valley. Majuli, the largest and one of the most populous freshwater riverine islands in the world, as well as a proposed UNESCO cultural heritage site, has experienced significant morphological changes due to the continuous shifting of the river channels of the Brahmaputra and its tributaries. Thus, the study aims to understand the morphological dynamics of the Brahmaputra River over the last five decades by employing statistical indices such as the Plan Form Index (PFI), Braiding Index (B.I.), and Migration Index (M.I.). The PFI values indicate the degree of braiding in a river, with values below 4 indicating a highly braided channel, between 4 and 19 indicating a moderately braided channel, and above 19 indicating a low braided channel. The study shows that the PFI value for the Brahmaputra near Majuli decreased from 7.73 in 1975 to 4.29 in 2000, and further declined to 3.54 in 2024, indicating an increasingly braided nature. Similarly, Brice’s Braiding Index (B.I.) reflects a similar trend, rising from 4.31 in 1975 to 5.22 in 2020, and further to 5.49 in 2024. The Migration Index (M.I.) of the river increased from 0.885 for the period 1975–2000 to 0.909 for the period 2000–2024, highlighting a highly unstable river with frequent bank failures. It is important to note that as per the Census of India, the total area of Majuli Island was 1,246 km² in 1951. However, the present study indicates a significant reduction in the island's area, measuring 629 km² in 1975, 601 km² in 2000, and 487 km² in 2024 respectively. This indicates a loss of nearly two-thirds of the island's original area, with 107 out of 210 cadastral villages being engulfed by the river over the last 70 years. Thus, the study highlights the urgent need for both structural and non-structural measures to protect Majuli Island from further erosion by the Brahmaputra and its tributaries.

Keywords: River Brahmaputra, Morphology, Majuli Island, Erosion, Plan Form Index, Braiding Index, Migration Index