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).