3 Materials and methods
This study uses Landsat imageries (MSS, TM, ETM+, and OLI) of the last
four decades (1976-2018), mostly at decadal intervals. The image
selection strategy is based mainly on the availability of cloud-free
data in the region. Post-monsoon satellite imageries are downloaded
(from the USGS website) to avoid cloud-content data. Several Landsat
images are required to cover the entire Brahmaputra River, which is
sometimes non-feasible due to lack of cloud-free data for a year under
evaluation. To overcome such situations, the images of contiguous months
of successive years are considered having similar hydrologic conditions
(lean phase period). The downloaded imageries are stacked in ERDAS
Imagine and finally mosaiced to get a composite image of the entire
Brahmaputra River. The composite image is further utilized to digitize
the bankline of the Brahmaputra for the study periods.
The ambiguous nature of true bankline identification of the Brahmaputra
River is well documented (Hassan et al., 1997; Sarker et al., 2014). Due
to complex anabranching, true bankline consideration is always a
challenge, where Brahmaputra is concerned. In this study, active
anabranches that rejoin the Brahmaputra are considered part of the
river, though flanking channels are mostly avoided. Crevasse splay and
overbank sediment spills are very common in the Brahmaputra. Such
geomorphic traces fade out during the driest periods and are relatively
easy to distinguish. Therefore, the satellite images of successive
months of the relatively dry period are used to differentiate between
such geomorphic impressions and true bankline. To reduce the abrupt over
or underestimation of bankline dynamics, the delineated banklines of
successive study year images are standardized with each other. Further,
the banklines of the Brahmaputra utilized for the study periods are
digitized (Fig. 2). To document the dynamics of the Brahmaputra River,
the entire digitized banklines in Bangladesh and India is divided into
equidistant segments. For this purpose, the downstream point where
Brahmaputra was confluenced with Ganga (Padma) in 1976 is taken as
reference. The river is divided into ten equal reaches, demarcated as
R1, R2… and R10, from reference point to the upstream point of
the Brahmaputra, which lies near Laikaghat, and was the area of the
concourse of the Dihang, Dibang, and Lohit dating back to 1976 (Fig. 1).
Subsequently, the reach-wise comparison is performed to evaluate the
land erosion and accretion.
The digitized banklines for study periods are also used to measure the
width of the Brahmaputra River. For this purpose, lateral lines at an
interval of ~1 km are drawn for the entire stretch of
the river. While drawing the cross-section lines, the orientation of the
Brahmaputra River is kept in mind. The cross-sections for river width
measurement are drawn perpendicular to the direction of Brahmaputra
channel.
This work evaluates the major eroding sections of Brahmaputra. Such
active sections are identified based on the severity of erosion along
the river. The natures of the erosion, i.e., progressive or random, are
interpreted from multi-temporal images. Planform characteristics of
Brahmaputra are also interpreted from satellite imageries. It includes
identifying the dominant channels, braided and nodal belts, and their
nature. The Second order channels, which represent the dominant
channels, of Brahmaputra are interpreted based on Bristow’s channel
hierarchy system (Bristow, 1987; Bristow and Best, 1993). Bristow (1987)
described the morphology of Brahmaputra as consisting of first, second,
and third order channels. The first order channel comprises the whole
river, which may include several second order channels. The second order
channel has low stage third order channels within them. This
morphological description is used by several researchers working in the
Brahmaputra (Thorne et al., 1993; Sarker et al., 2014).
Multiple approaches, including satellite image-based interpretations,
fieldwork, and meta-analysis, are performed to evaluate the major
drivers of erosion along the active sections of Brahmaputra River. It
has inferences from- (a) previous independent research work literature,
(b) major seismic activities, (c) floodplain lowlands, and (d)
trajectories and sustenance of second order channels near banklines.
Shillong and Assam earthquakes of 1897 and 1950 have largely affected
the recent channel evolution of Brahmaputra (Oldham, 1899; Goswami,
1985). The possible role of those well-known earthquakes in Brahmaputra
River dynamics is discussed. Corona photographs of 1961 are utilized to
interpret the effect of the 1950 earthquake along the upper Brahmaputra
reaches. It includes identification of sediment lobates ( associated with the earthquake )along the eastern end of Brahmaputra plains.
Elevation profiles are drawn using SRTM DEM to identify the lowlands
along the floodplains of Brahmaputra. Flow course and the sustenance of
the second order channels are identified in ArcGIS to assess their role
in erosional activities.