Coarsening of fluvial sediments after the Gorkha Earthquake 2015, Nepal

Abstract. In April, 2015 the M_w 7.8 Gorkha Earthquake hit central Nepal, causing widespread co- and post-seismic landsliding. Such surface processes have the potential to increase erosion and perturb fluvial systems for years to decades. Although mass wasting is known to supply coarse material to channels, its impact on the grain size characteristics of suspended sediments remains poorly understood. Here, we present time-series grain size distribution data collected during 2015–2018 from four hydrologic stations using a nested catchment approach in the Koshi basin, where abundant landslides occurred after the Gorkha event. We analyse post-earthquake grain size distribution of riverine suspended sediment and landslide deposit using an end-member modelling analysis. Our results exhibit statistically distinctive signals. We find that one of the coarsest grain size end members (~400 mm) is particularly dominant in the two smallest catchments in the monsoon immediately following the earthquake in 2015. This grain size end member is also abundant in our co-seismic landslide deposit samples, indicating that the higher presence of this end member at our upstream stations likely relates to sediment supply from co- or post-seismic landslides. The earthquake signal in the suspended sediment is only present during the 2015 monsoon season, suggesting that the fine sediment transport in central Nepal was only perturbated by the earthquake for about one season. This provides novel and valuable process-based insight that elevated landslide activity leads to coarsening of suspended load in small catchments. However, it seems unlikely that this signal is recorded in far downstream depositional basins as it is lost during transport.