Comment on egusphere-2022-1347

Abstract

Large earthquakes can contribute to mountain growth by building topography, but also contribute to mass removal from mountain ranges through widespread mass wasting. On annual to decadal timescales, large earthquakes also have the potential to significantly alter fluvial sediment dynamics if a significant volume of the sediment generated reaches the fluvial network. In this contribution, we focus on the Melamchi-Indrawati and Bhote Koshi rivers in central Nepal, which have both experienced widespread landsliding associated with the 2015 Gorkha (Nepal) earthquake. Using a time series of high-resolution satellite imagery, we have mapped exposed gravel along the river from 2012–2021 to identify zones of active channel deposition and document changes over time. Counter to expectations, we show negligible increases in coarse sediment accumulation in both catchments since the Gorkha earthquake. However, an extremely high concentration flow event on 15 June 2021 caused an approximately four-fold increase in exposed gravel along a 30 km reach of the channel with up to 12 m of channel aggradation in the Melamchi-Indrawati rivers; this event was localised and did not impact the neighbouring Bhote Koshi catchment. Based on published reports, new helicopter based photography and satellite data, we demonstrate that this event was sourced from a localised rainfall event between 4500 and 4800 m, and that the majority of the sediment was supplied from sources that were unrelated to the landslides generated by the Gorkha earthquake.