4.1 Morphological dynamics of Brahmaputra River in recent decades
Investigation of the short-term dynamics of the Brahmaputra River shows its erosion-dominant nature. The river has eroded nearly 2686 sq. km of land in the last four decades (Table 1). Spatial distinctions have been noticed in the erosional intensity of the reaches of Brahmaputra. The lowermost reaches (viz., R1, R2, and R3) in Bangladesh have suffered high erosion (1240 sq. km). Bank accretion of the Brahmaputra River is significantly low (1226 sq. km) compared to its erosion.
The width of the Brahmaputra River varies from ~1 to 21 km along its entire reach, with minimal width at the constricted nodal segment of Guwahati. The river widens to ~21 km near its entry into Bangladesh. In 1976, the river had an average width of ~8.5 km (Table 2). Reach-wise analysis reveals that the average width of R3 was the highest followed by R2 and R10 in the year 1976, whereas the lowest width was in R6. In 1988, the river’s width increased substantially, with an average increase of ~1 km/reach, taking the width close to 9.5 km. The trend continued in the subsequent decade with an increase of ~0.5 km. However, the trend slowed down in the recent decades (2008 and 2018), and the present average width of the Brahmaputra River stands at 10.2 km (Table 2). Three classified reaches viz., R1, R3, and R5 experienced large expansions in their width with an average widening of ~4.1, 3.7, and 2.1 km, respectively. Figure 3 summarizes the progressive increase of Brahmaputra River width in recent decades.
Along the entire reach of Brahmaputra, the least bankline dynamics were observed at the nodal segments. There are six major nodes (N1-N6) identified in the images of 2018 (Fig. 1). Two of them are in Bangladesh and four in India. The nodes in Bangladesh are located near Sirajganj (N1) and Madarganj (N2). Similar corresponding nodes can be traced back to earlier work (Thorne et al., 1993; Sarker et al., 2014). Moreover, the shifting and migration of the nodes in Bangladesh are well documented (Thorne et al., 1993). In this study, N1 showed a relatively gradual widening in the initial decade (1976-1986), followed by a significant widening between 1987-1995; but bridge construction imposed restrictions in the subsequent period. The right bank distributary downstream to the bridge is virtually abandoned and is likely to extend the length of the nodal segment in the future. The transient node N2 is confined to its left bank and showed some intra-decadal alteration in width and small-scale migration during the study period. Out of four nodes in India, two (N3 and N4) are restricted by hard gneissic rocks (Sarma and Acharjee, 2018) near Goalpara and Guwahati. The third node (N5), near Tezpur, is again restricted by hard rock hills on its left bank. During the study period, restricted migration of the N5 is noticed, and the present position of the node is a result of the migration of the dominant channel towards the left bank, which is the hilly section and constricts any erosional activity. The fourth node (N6) is bounded by cohesive clay material near Salmora (Kotoky et al., 2003). The existence of N6 can be traced back nearly a century with very gradual erosion activity that underscores its continued existence.