1 Introduction
The extant literature on Brahmaputra River dynamics reflects the extensive effort of researchers to comprehend its nature. The lower reach of the Brahmaputra (Jamuna) within Bangladesh is a point of convergence for researchers captivated by its massive avulsion and channel metamorphosis (e.g., Coleman, 1969; Bristow, 1999; Sarker et al., 2014). The present course of Brahmaputra is locally known as Jamuna in Bangladesh and is referred to as the Brahmaputra-Jamuna for the section that lies in Bangladesh. The section within India is referred to as the Brahmaputra throughout this paper. Coleman (1969) considered a pioneer in the field, proposed increased flood discharge, faulting, or a combination of both, triggered the Brahmaputra-Jamuna channel from the old to the current course. Following this shift, the river exhibited a translatory modification from meandered to braided and erratic behavior in terms of bankline migration. Consequently, Bristow (1987) detailed the channel patterns and migration of Brahmaputra-Jamuna, by sourcing data from Landsat images and historical maps. He deduced that lateral migration with minor channel switching and the Brahmaputra-Jamuna avulsion in Bangladesh are dominant channel movements in the lower reach of the river. Thorne et al. (1993) also focused on erosion and accretion activities of this stretch of the river. They inferred that the severe bank erosion is related to location combined with the spacing of both first order islands and second order bars. Bristow (1999) suggested that avulsion of the Brahmaputra-Jamuna was a result of flow divergence around mid-channel islands, which reoriented the diverted channel into an existing floodplain channel. Khan and Islam (2003) found severe bank erosion and considerable channel widening of the avulsed stretch in Bangladesh. Best et al. (2007) critically examined the avulsion, erosion, structural control, and other aspects associated with floodplain, including the river morphology of Brahmaputra-Jamuna. Baki and Gan (2012) used Landsat imageries to conduct short-term and long-term analyses of the migration of the river’s course. Sarker et al. (2014) reinforced the earlier works with an in-depth morpho-dynamic study of the Brahmaputra-Jamuna River. They highlighted the historical westward migration, significant divergence in left and right (west) bank constituents, diagnosed causal links between drivers, besides morphological responses in time and space.
The Brahmaputra within India was also well studied by various researchers, especially in the last decade (Sarkar et al. , 2012; Sarma and Acharjee, 2018; Saikia et al., 2019). Sarkar et al. (2012) in an erosion and deposition assessment for eighteen years (1990-2008) of Brahmaputra River in Assam (India). Sarma and Acharjee (2018) estimated the braiding index and channel width of Brahmaputra at every 5-minute longitudinal interval within Assam. They found a positive correlation between channel width and braiding index, indicating an increase in braiding index with increasing channel width. In addition to the predominance of erosion, Saikia et al. (2019) credited the enlargement in the Brahmaputra area to the bifurcation of the streams without the loss of land. A good deal of the research has focused on specific stretches of the plains of the Brahmaputra in India, motivated by the ecological and cultural importance of the locale. Brahmaputra River along such specific stretches (e.g., Kaziranga national park, Majuli, and the upper reaches) emerged as stretches of principal interest and research (Kotoky et al., 2003; Sarma and Phukan, 2004; Kotoky et al. , 2005; Sarma, 2005; Sarma and Acharjee, 2012a; Lahiri and Sinha, 2012; Lahiri and Sinha, 2014; Basumatary et al., 2019). Apart from depicting the temporal variation, extent of erosion, and deposition along the stretches under examination, they investigated two major avulsions (Majuli and Lohit), structural and lithological controls, and the after-effects of the earthquake of 1950. Though extensive records support the assessment of Brahmaputra River dynamics, the chronological and regional literature along its length reveal researchers mainly focused either on stretches of interest or on related themes.
The chronic nature of the Brahmaputra River dynamics has engendered further exploration and research into the causative factors. Many researchers underlined structural controls, tectonic and seismic activities as factors that primarily triggered morphological adjustment of Brahmaputra River in the recent past (Morgan and McIntire, 1959; Coleman, 1969; Sarma, 2005; Das and Saraf, 2007; Best et al., 2007; Lahiri and Sinha, 2012; Sarkar et al. , 2012; Sarma and Acharjee, 2012a; Sarker et al., 2014; Lahiri and Sinha, 2014; Sarkar, 2017; Sarma and Acharjee, 2018). Researchers have tried to investigate links between structural controls, avulsion, and bankline dynamics of the river (Winkley et al., 1994; Lahiri and Sinha, 2014). The works of Lahiri and Sinha (2012, 2014) introduced seismic stratigraphy and the concept of basin evolution to explain the differential residence time of the Brahmaputra River at different locations. However, seismic activities are primarily linked to changing the rate of sediment supply and the transformation of the Brahmaputra River morphology (Goswami, 1985; Sarker et al., 2014). As far as the lithological aspect is concerned, the floodplains show longitudinal and bank-wise heterogeneity and are often related to the bankline dynamics of Brahmaputra (Kotoky et al., 2003; Sarker et al., 2014; Sarma and Acharjee, 2018). However, two types of nodes viz., permanent and transient are also marked out and noteworthy (Bristow, 1987; Kotoky et al., 2015; Sarma and Acharjee, 2018). The erratic nature of the Brahmaputra and its tributaries often display morphological and fluvial adjustments. Two such activities i.e., avulsion and thalweg migration enhanced morpho-dynamics of Brahmaputra (Sarma and Phukan, 2004; Sarker et al., 2014; Lahiri and Sinha, 2014; Basumatary et al., 2019). Attempts have been made to endorse the role of human interventions as critical factors in morphological changes of the river (Kotoky et al., 2003; Lahiri and Sinha, 2012; Sarker et al. , 2014; Sarma and Acharjee, 2018). The hydro-fluvial processes involved in the shaping and reshaping of Brahmaputra River morphology are widely discussed by the researchers and they are often complementary to each other. Subsidiary channel formation or abandonment, shear failure, bank displacement by liquefaction, cutting, slumping, and piping (activation of an old channel) are interpreted as major processes of the morpho-dynamical activities of Brahmaputra (Coleman, 1969; Kotoky et al., 2003; Sarma and Phukan, 2004; Sarker et al., 2014; Kotoky et al., 2015; Basumatary et al., 2019).
Previous literature indicates that regional tectonics and basin aggradation have largely decided the recent channel evolution of the Brahmaputra River (Goswami, 1985; Lahiri and Sinha, 2014; Sarker et al., 2014). These triggers control the residential time of the Brahmaputra River or its anabranches at different locations. The residential times of major anabranches of Brahmaputra and their angle of approach towards banklines determine the erosional magnitude of their adjacent banks (Coleman, 1969; Basumatary et al., 2019). The varied magnitude of erosional activities along different sections of Brahmaputra shows its preferred erosional sites. To document such spatial patterns, this work constructs an integrated picture of the entire Brahmaputra River dynamics and thereby such erosion-dominant sections of Brahmaputra River are identified. Major driving factors responsible for the erosion along those sections are explored. The residence time and patterns of channel movements of the Brahmaputra River are also discussed to ascertain their migrational behavior.