Assessment of River Channel Dynamics and Flood Propagation of the Lower Subansiri River

The Lower Subansiri River, a significant northern tributary of the Brahmaputra, showcases considerable morphodynamic variability characterized by shifts in sinuosity, braiding intensity, curvature radius, and channel movement over the past forty years due to flood migration. This paper combines multi-temporal Landsat images (1986–2025), GIS-based morphometric analysis, and one-dimensional unsteady hydraulic modelling to evaluate long-term channel change and flood propagation between the Subansiri dam and Goriamari. Findings indicate periods of migration alternating between the left and right bank, with significant shifts (up to 8 km) observed from 1995 to 2005, a phase of partial stabilization from 2005 to 2015, followed by renewed local instability starting in 2015. The devastating flood of 2000 was a geomorphic turning point, as it caused massive avulsions, bar reworking, and straightening of the master channel alterations continuing until 2025. Land use/land cover analysis shows extensive conversion of natural vegetation to agriculture and built-up area expansion, especially after 2005, augmenting exposure to flood and erosion risks. The combined analyses of morphometry, hydrodynamics, and land use/land cover highlight the role of significant hydrological events, variations in sediment load, and human activities in driving changes in river channels. The significance of these findings is crucial for managing the floodplain in the Subansiri–Brahmaputra confluence area, as well as for hazard zoning and land use planning aimed at sustainable development.