4.3 Major drivers
The lower section of the Brahmaputra-Jamuna is identified as the most
dynamic stretch, which is mostly active in the initial decades
(1976-1998). The present course of Brahmaputra-Jamuna is a consequence
of the avulsion that started between 1828-1843 (Bristow, 1999). The
avulsion began diversion of flow to Brahmaputra-Jamuna channel and
gradual evolution of the new channel from meandering to a braided one.
The wide alteration in Brahmaputra-Jamuna reach is part of the
post-avulsion planform evolution of the river. It is also emphasized
that the efforts to stabilize the bankline through engineered structures
may have contributed to the decreasing trend of land loss in recent
decades (Sarker et al., 2014). Yet, the role of engineering structures
cannot be denied in the decreasing trend of land loss. But the quality
of embankments and their effectiveness along the lower reach have
remained questionable (Hossain et al., 2008; Hossain et al., 2011), and
therefore its greater role in bank erosion reduction is open to further
debate. In this regard, the dynamic equilibrium of Brahmaputra-Jamuna
and its channel evolution (Coleman, 1969; Sarker and Thorne, 2006;
Takagi et al., 2007) is more decisive. Takagi et al . (2007) noted
that the river had switched to dynamic equilibrium in the late 1990s.
The evolved reach of Brahmaputra-Jamuna is attaining equilibrium with
its contiguous upper reaches. It has resulted in a relatively efficient
widened channel that receded the morphological dynamics of the
Brahmaputra-Jamuna River.
Observation of the Brahmaputra bankline near the confluence of Manas
shows its active erosional nature. The positions of second order
channels have played a significant role in massive land loss near the
confluences. Along the stretch, Das and Saraf (2007) noted the
strengthening of the anabranching channel along the northern edge of the
Shillong plateau. They attributed the changes along the section to be
due to the tectonic activity of the Shillong plateau, most likely the
1987 earthquake. Sarkar et al. (2012) opined that developed lowlands
along fault affected plateau edge triggering the river to respond. We
observed that the Brahmaputra is eroding the northern bank near Manas
confluence along the active stretch. The massive erosion indicates a
possible depression formed along the north edge of the Brahmaputra near
Manas confluence. Bilham and England (2001) demonstrated the pop-up of
the Shillong plateau during the 1897 Assam earthquake. The post-effect
of the earthquake is always a concern for the researchers (Das and
Saraf, 2007; Lahiri and Sinha, 2014). Lahiri and Sinha (2014) explained
the continuation of the pop-up phenomenon and its effect on subsidence
near Majuli. The rapid erosion along the stretch is a reaction to the
1897 earthquake, which formed a depression along the northern edge of
the Brahmaputra River. It should be noted that the river channel along
the active stretch was narrow and mostly confined to the southern part
in the past.
The left bank of Brahmaputra upstream of Guwahati has inexorably
migrated towards floodplains in the last four decades. The flood map
shows that the floodplain adjacent to the eroded side is lowland and
among the region’s most frequently inundated areas. The lowland has
prompted migration and sustained second order channel along the left
bank. And the induced greater fluvial pressure eroded the bankline along
the stretch. Therefore, the position of the channel along the lowland
floodplain is consistently eroding the southern bank of this middle
reach. Shear failures are a common cause of erosion as the second order
channel continuously hugging the left bank of Brahmaputra along the
active section.
The massive land loss near the confluence of Subansiri is due to the
widening of the Brahmaputra as well as its tendency of switching towards
the north. The widening and northward shift of the channel near the tail
end of Majuli has captured the earlier course of Brahmaputra
(Luit/Subansiri). The Brahmaputra expanded along the floodplains of
Luit/Subansiri, which was naturally a lowland. A prominent elevation low
can be noticed in the northwest of Brahmaputra-Subansiri confluence
(Fig. 6a and b). It lies below the Dafla hill blocks (DHB), a Siwalik
fold belt. Das (2004) remarked that DHB is in the quiescence period and
less active (earthquake) relative to other areas of the region. However,
he insisted that the area may undergo brittle failures in the future.
DHB is also affecting the course of Brahmaputra (Das and Saraf 2007).
The river channel between Kaziranga and Brahmaputra-Subansiri confluence
follows the southward Siwalik fold of DHB. It demonstrates the dominance
of DHB in floodplain topography along the stretch and, therefore, looks
to be a leading factor in the development of elevation low along the
northwest of Brahmaputra-Subansiri confluence. This subsidence is
contributing to the orientation of the flow of Brahmaputra towards the
north bank resulting in the massive erosion near the Subansiri
confluence.
The Brahmaputra has widened drastically along the present confluence of
Lohit and progressively migrated towards the south. It is suggested that
the erosion is triggered by the avulsion of Lohit (Borgohain et al.,
2016). Sarma and Acharjee (2012b) interpreted that the subsided area on
the north of Oakland-Guijan scarp (fault) developed due to the great
earthquake of Assam is causing the bank erosion. The comparison with
older maps clearly shows the river has expanded considerably on both
sides to accommodate the large sediment released during the later
periods of the 1897 earthquakes and the Assam earthquake of 1950. Corona
photograph of 1961 shows the large sedimentation along the eastern front
of the Brahmaputra related to the well-known 1950 earthquake (Fig.
7). Similarly, Sah et al. (2022) identified sand splay deposits (associated with the earthquake) along the foothill region of Jiadhal-Dikari belt using the Corona photograph. The massive sedimentaion along the eastern front has affected the morphology of the Brahmaputra River. The
elevation profile along the cross-section of Lohit near its Brahmaputra
confluence shows a belt of low land along the corridor of the Dibru
river (Fig. 8a and b). The elevation might be affected by vegetation in
the northeastern corner, but overall it shows an elevation low belt
along the Dibru river corridor. Prominent migration of Brahmaputra left
bank along the corridor of Dibru river is discernable. A similar lowland
is identified by Sarma and Acharjee (2012b), which was inferred based on
the increase in the amount of lowland between 1916 to 1963. The lowland
encouraged the thalweg position of Brahmaputra near the left bank and
promoted progressive bank erosion along the stretch. The temporary
imbalance generated from the shifted flow of Lohit to the Brahmaputra is
supplemented to the bankline erosion.