Geomorphic response of bedrock landslides induced landscape evolution across the Teesta catchment, Eastern Himalaya

Bedrock landslides are the primary agent of hillslope erosion, and mass wasting, and an essential source of sediment flux to the fluvial network in the mountainous terrain, in particular, in the Himalayan mountain belt. To understand the characteristics of the landscape, we calculate geomorphic matrices including the topographic variables, longitudinal and topographic swath profile, channel steepness index, and stream length gradient index to analyze the spatial distribution of landslide occurrences over landscape evolution. The intensity of rainfall gradient and topographic variables were spatially correlated with the erosion and exhumation rates of the studied catchment. Our analysis suggests that the zones with slope ranges of 24°–28°, relief ranges of 800–1000 m, and elevation ranges of 1500–1700 m, which coincide with the rainfall intensity range of 2500–2700 mm/year in the Teesta river catchment, have the highest probability of frequently occurring landslides. Higher tectonic activity is principally responsible for the landslide over the Higher Himalaya to the north of the Main Central Thrust (MCT)–Main Boundary Thrust (MBT) along the orographic barrier. In contrast, litho-tectonics regulates and mostly triggers landslides adjacent to the MCT–MBT structural affinity dominated by rainfall intensity. Our observation suggests that erosion rates frequently exceed long-term exhumation rates and are spatially more variable. Moreover, they exhibit significantly divergent spatial patterns, which suggests that the processes governing these rates are independent. Exhumation rates have been shown to decrease from south to north over geological periods, rising in the southwest region at ~ 1.2 mm/year and decreasing to ~ 0.5 mm/year in the northernmost region of the Teesta catchment. Long-term exhumation rates are not correlated with geomorphic or climatic variables. The highest apparent erosion rates (5 mm/year) are seen in the catchment that crosses the MCT Zone, however, these rates appear to have been severely impacted by recent landslides. Conversely, changes in rainfall rate do not appear to significantly impact either rate of long-term exhumation or erosion in the Teesta catchment.