2 Study area
The Brahmaputra is a trans-Himalayan River originating in Angsi Glaciers
in western Tibet (Ray et al., 2015). The parent stream from its source
flows along southern Tibet as Yarlung Tsangpo. After breaking through
the great Himalayan gorge, it enters India and finally descends as
Dihang from the mountainous terrain to the Assam valley. During 1915,
Dihang was converging with two of its major tributaries Dibang and
Lohit, near Kobo (in Assam), to form the Brahmaputra. This confluence
point shifted by ~16 km downstream to Laikaghat in
Dhemaji by 1975 and later by 2005, a further downstream shift of 19 km
took place (Lahiri and Sinha, 2012). Presently the confluence is located
near Dinjan in Dibrugarh, Assam (Fig. 1). The Brahmaputra flows
downstream for about 610 km in the westerly direction and turns south
near Dhubri, Assam, and enters Bangladesh. It continues further south
for about 230 km as the Jamuna before meeting the Ganga (the Padma in
Bangladesh) near Goalanda. Das and Saraf (2007) advocated that the
sudden southern turn of Brahmaputra near Dhubri is mainly governed by
the high land present along the right bank.
The channel reach of Brahmaputra can be divided into three categories:
anabranching-cum-braided channel reaches, braided reaches, and narrow
single channel reaches (Sarma and Acharjee, 2018). Thorne et al. (1993)
described the island reaches as sediment storage zones and the nodal
reaches as sediment transmission zones. Morphological adjustment of the
river near nodes is explained by Coleman (1969). He mentioned that
exaggerated lateral movements of banks above and below a node are
common. The nodes along the Brahmaputra River can be deciphered from
satellite images (Fig. 1). In India, the nodes are permanent and
controlled by hillocks, except at the eastern node where cohesive clay
forms the predominant bank material, whereas in Bangladesh, the nodes
are transient (Bristow, 1987; Kotoky et al., 2003, Sarma and Acharjee,
2018). Notably, the river has experienced two major re-organizational
activities in the form of avulsion near Majuli (Assam) and Gaibandha
(Bangladesh) in the 17th and 18thcenturies, respectively (Bristow, 1999; Sarma and Phukan, 2004). It is
proved that tectonic activities have major roles in reshaping the
channel of Brahmaputra (Goswami, 1985; Winkley et al., 1994; Das and
Saraf, 2007; Lahiri and Sinha, 2014).