EVENT-SCALE INTERACTIONS AMONG CLIMATE, TECTONICS, HILLSLOPE AND FLUVIAL PROCESSES IN THE HIMALAYA (Invited Presentation)

Interactions between climate and tectonics and between hillslope and fluvial processes affect long term landscape evolution through the accumulation of individual events. On the event scale, such interactions can substantially impact the relationship between climatic and/or tectonic forcing and the landscape response. Tectonic forcing in the form of earthquakes can drive co-seismic and post-seismic landsliding, but the further mobilization and export of that material is dependent on climatic forcing, while impacts of climatic forcing can be strongly influenced by river-hillslope interactions and the legacy of past earthquakes. This is exemplified by the behavior of two adjacent river systems in the Himalaya, the Bhote Koshi/Sun Koshi and the Melamchi Rivers in central Nepal. Both of these catchments experienced extensive landsliding during the Gorkha earthquake, followed by similar climatic forcing over the subsequent monsoon seasons, including a heavy rainfall event in June 2021. The June 2021 event caused abrupt floods in both catchments, which resulted in little impact in the Sun Koshi River but catastrophic aggradation in the Melamchi River, with meters of deposition along 10s of km of river channel, multiple fatalities and extensive damage to buildings and infrastructure. This difference resulted from channel hillslope interaction in the Melamchi River, where the initial flood destabilized an older landslide deposit, which then failed, dammed the river, and burst after several hours. The resulting aggradation was exacerbated by the large amount of colluvium stored in the catchment, including co-seismic landslide debris. In contrast, the response in the Bhote Koshi may have been dampened by a glacial lake outburst flood in 2016 that cleared out co-seismic landslide debris in the river corridor and may have influenced susceptibility to lateral erosion by the river. These contrasts highlight the complexity in catchment response to seismic and climatic forcing and the potential difficulties in linking short term and long observations of climate, tectonics, and topography.