4.2 Major eroding sections of Brahmaputra River
Five major sections of the Brahmaputra River have been identified which have suffered high erosion in the last four decades. Those active sections lie in the Brahmaputra River’s lower, middle, and upper divisions. Researchers focusing on different sections of Brahmaputra have also depicted similar sides of active erosion (Sarkar et al., 2012; Sarker et al., 2014; Lahiri and Sinha, 2014).
The lower active section lies in Bangladesh. Brahmaputra-Jamuna, along this section, has gone through both erosion and accretion during the study period. This section has shown the dominance of bank erosion and channel widening. This shift is evident in the entire lower section, though the magnitude at different reaches varies. Interestingly, the intense land loss is noticed only during the study period’s initial decades (1976-1998), which contributed to a net land loss of around 678 sq. km (Table 1). As far as the dominant process of land loss is concerned, progressive bank cutting predominates along the right bank, while both progressive and alternate bank dynamics characterize the left bank (R1 and R2). The relatively random nature of left bank dynamics is related to the rapid activation and abandonment of braided channels triggered by flowing-channel repositioning (Fig. 4). It is important to note that the life span of eroding bends on the right bank is higher compared to the left bank (Sarker et al., 2014), which promotes the random nature of the left bank dynamics.
Two active sections in the middle division of the Brahmaputra River lie near the confluence of Manas and the upstream of Guwahati. The bankline of Brahmaputra has migrated as high as nearly 5 km near the confluence of Manas. Long-term assessment based on older toposheets clearly shows a sustained erosion along the section for almost a century. Extensive erosion near Manas confluence has engulfed several villages (e.g., Mainbari, Kubanbari, and Chenglidiya) and threatened the existence of Baghbor hill. The hill may become an island with continued erosion along the section (Fig. 5). The active section upstream to Guwahati lies on the left bank of Brahmaputra. The aggressive nature of the river along the section has eroded 181 sq. km of land from 1976 to 2018. It has swallowed several villages (e.g., Salmari, Sialmari, and Leruamukh). The rate of land loss along the section is nearly 4.3 sq. km/year, which is significant as the floodplain dwellers largely depend on the meager assets.
The other two active sections along the upper Brahmaputra division lies near the confluences of Subansiri and Lohit. Massive erosion near the confluence of Subansiri has significantly reduced the landmass of the Majuli river island. The western end of Majuli is sandwiched between Subansiri and Brahmaputra rivers, and therefore any oscillation of the rivers’ banklines have a significant effect on ever reducing landmass of the island. The left bank of the Brahmaputra near the present confluence of Lohit is continuously drifting towards southern floodplains. The shift of the river bankline has resulted in a land loss of around 99 sq. km in the last four decades. Progressive bank cutting is the dominant process of erosion along the section.