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.