River response to active tectonics: a study of channel migration and land loss in the lower reaches of Bhagirathi in Murshidabad district, West Bengal, India

The Bhagirathi River is historically prone to river bank erosion and slope failure. In Murshidabad District of West Bengal, India, the Bhagirathi River has shown significant changes in its river course over time with particular changes to its meandering nature. Through this study the authors have investigated the geomorphic causes from satellite imagery for such changes and have also found evidences of neotectonic activity in the area through field observations and geophysical studies. The study area is within the Bengal Basin and outcrops are not visible on the surface due to thick alluvium cover and thus there is no direct structural control visible on the surface that might indicate active tectonism. Comparison of different sets of aerial photographs and satellite images for the past 50 years have been carried out to track changes to the Bhagirathi River course along with calculation of geomorphic indices such as Basin Asymmetry Factor, Stream Length-gradient Index and Transverse Topographic Symmetry Factor to identify any tectonic activity in the area. We have also calculated sinuosity index and radius of curvature for multiple bends of the Bhagirathi River in the study area to have a comprehensive overview of the changes in river course. Bathymetry data and current velocity/direction measurements along with field evidences show the presence of a NE-SW trending lineament near Basabari-Shujapur village. This lineament is found to be tectonically active based on geophysical data and observation of soft sediment deformation structures in field. Our study indicates that the changes in the course of the Bhagirathi River are tectonically controlled.