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.