Geomorphological responses of rivers to active tectonics along the Siwalik Hills, Midwestern Nepalese Himalaya

The Nepalese Himalaya is well known for ongoing collisional tectonics, witnessed by major historical and recent earthquakes. The Siwalik Hills in Midwestern Nepalese Himalaya are bounded by eastwest trending Main Frontal Thrust (MFT) to the south and the Main Boundary Thrust (MBT) to the north. The area is dissected by numerous southwest to south-flowing bedrock rivers. This study investigates geomorphic metrics of these rivers to unravel landscape evolution and active tectonics of the Siwalik Hills. Digital Elevation Model (DEM) analysis was conducted to extract structural lineaments and longitudinal river profile and their metrics (knickpoints, Normalized Steepness Index (ksn), concavity index, and chi integral) using steam power-law approaches. Most of the lineaments trend eastwest like MFT. River profiles exhibit convex to double-concave shapes with upstream-propagating tectonic knickpoints that separate upstream and downstream reaches, indicating different phases of river incision. The spatial distribution of ksn shows high values along with low concavity values at the eastern part of the study area, reflecting disequilibrium conditions that are likely responding to a high uplift rate. Chi integral distribution shows a variation in drainage divide migration between the eastern and western parts of the study area. This study suggests that the rivers in the Siwalik Hills are undergoing active incision likely related to the ongoing uplift and active deformation associated with the Himalayan tectonics. The above findings can bring fresh perspectives to comprehend the neotectonic deformation and lateral variability along the Siwalik Hills landscapes within the Himalaya.