Morphometric response to channel avulsion in the Mahananda River Basin: A comparative analysis of pre- and post-abandonment watersheds across Nepal, India, and Bangladesh

Channel avulsion plays a critical role in shaping alluvial river systems, yet its morphometric and tectonic consequences remain underexplored in transboundary basins. This study assesses the morphometric response to a historical avulsion event in the lower Mahananda River Basin, spanning Nepal, India, and Bangladesh. Using georeferenced historical maps (1928, 1974), multi-resolution DEMs (SRTM and ALOS PALSAR), and hydrological modeling in ArcGIS Pro, we delineated three watershed units: the Old Mahananda Basin (OMB), the Current Mahananda Watershed (CMW), and the Abandoned Mahananda Watershed (AMW). A detailed morphometric comparison across linear, areal, relief, and tectonic indices reveals significant divergence post-avulsion. AMW exhibits an approximately 45% reduction in total stream length, lower drainage density (0.34 km/km²), and diminished relief energy, indicating hydromorphological degradation. In contrast, CMW maintains high fluvial activity, with a greater dissection index and slope variability. Tectonic asymmetry indicators—Asymmetry Factor (AF) and Transverse Topographic Symmetry Factor (TTSF)—suggest lateral tilting toward the east, likely influencing the channel’s relocation. Statistical analyses—including Principal Component Analysis (PCA), hierarchical clustering, and correlation heatmaps—identified drainage density, stream frequency, and hypsometric integral as key drivers of post-avulsion differentiation. Custom visualizations further confirmed distinct morphometric regimes across the three sub-watersheds. The integrated approach—merging GIS-based delineation with Python-assisted statistical modeling— offers a replicable framework for assessing avulsion-driven watershed transformation in tectonically sensitive alluvial plains. These findings contribute to a deeper understanding of geomorphic divergence and provide actionable insights for transboundary water resource planning and climate-adaptive land management.