Contrasting characteristics of a Himalaya- and a Shillong Plateau-fed basin in the Brahmaputra catchment, Bhutan and India: A study in tectonic geomorphology

Rivers leave distinct imprints on the landscape as they respond to tectonic, structural, and climatic changes over time. A river basin is likely to be shaped by active tectonic movements, which can be unravelled with the help of geomorphic analyses of historical maps, images and elevation models, employing statistical and cartographic techniques. The area selected for this work includes the 2404-km² Dhansiri (North) Basin on the northern bank and the 1440-km² Digaru Basin on the diagonally opposite southern bank of the Brahmaputra River. They drain the southern Himalaya and the northern fringes of the Shilong Plateau, respectively. This study correlates the role of neotectonics on the river basin with channel planform changes. Besides this, seven morphotectonic indices – asymmetry factor, hypsometric integral, transverse topographic symmetry factor, valley floor height to valley width ratio, river profile analysis, stream-length gradient index, and sinuosity index – are explored for insights on the imprints of tectonic activity. Field verification of selected channel reaches are also made. The findings indicate that the northern Himalayan foreland basin exhibits greater tectonic activity than the southern plateau-fringe basin. Furthermore, they reveal that the two basins are different from each other in terms of their tectonic oddity and dynamic channel character. The basins hold regional significance, influencing environmental processes through tectonic interactions, sediment fluxes, and associated disaster risks. Understanding their dynamics provides insights for regional hazard mitigation, climate adaptation, and sustainable development planning within the broader context of changing environmental conditions.