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.