The flux and composition of river suspended sediments contain valuable information on processes including erosion rates, the composition of the eroded bed rocks, chemical weathering and sediment storage and sorting within river channels. In this thesis, samples from the Kosi River in Nepal (a major tributary of the Ganges), are used to improve current methods of investigating the processes controlling river suspended loads.

Samples were collected as depth-profiles from all major tributaries and progressively along the mainstem to explore the mechanisms controlling the downstream changes in the Kosi River suspended sediments. Time series samples are used to investigate if suspended sediment compositions vary within the hydrological cycle and between successive years. Initial sampling commenced to quantify the impact of the 2015 Gorka earthquake. The April 2015, Mw 7.8 mainshock occured ~80 km west of the Kosi basin and the May Mw 7.3 aftershock within the basin. This thesis is one of the first assessments of the impact of the Nepal earthquakes on Himalayan river sediments.

This thesis builds on work by Lupker et. al, 2012 and Bouchez et al., 2012 who sought to separate the effects of hydrodynamic sorting from the true downstream evolution of suspended sediment compositions in the Amazon and Ganges Rivers. From the different settling velocities of the mineral phases, hydrodynamic sorting results in depth variations in the composition and concentration of river sediments. These published works (ibid) focus on downstream variations in major element sediment chemistry; however, little work has been conducted into the effect of mineral phases on sediment chemistry. Kosi River sediments are, therefore, considered as mineral mixtures in this theis by modelling mineral modes from silicate major element concentrations and mineral compositions.

Kosi River suspended sediments were found to be poorly sorted with depth, likely from the the highly turbulent secondary currents. Sediments have statistically higher Na/Al ratios and marginally lower K/Al ratios downstream at the Himalayan Front compared to those in the Himalayan Range. This downstream variation occurs from a greater abundance of plagioclase within Himalayan Front sediments, with the chemical composition of mineral phases being statistically indistinguishable at both sites. It is concluded that this downstream modal increase in plagioclase arises from the substantial flux of the plagioclase (and Na) rich sediments from the Arun Nadi River.

The composition of Kosi River sediments were found to be in “steady state” across the annual hydrological cycle from late 2015 to 2018, but with a clear seasonal switching of the dominant mineral phases. Early monsoon sediments (May-July) are dominated by micaceous phases, switching to high abundances of quartz and plagioclase from August onwards. Variation in sediment provenance were ruled out, using Nd and Sr isotopes, as the cause of the seasonal or downstream sediment changes. It is concluded that the seasonality of mineral phases likely arises from the preferential sorting and easy transfer of micaceous sediments in the early monsoon, such that they are significantly depleted by the mid-monsoon

From annual sediment fluxes, it is shown in this thesis that coseismic landsliding following the 2015 Gorka earthquakes resulted in an increase in the erosion and sediment fluxes above “baseline” levels in the Kosi River. Sediments collected within the Himalayan Range immediately following the earthquake (June to September 2015) were found to be more strongly dominated by contributions from the High Himalayan Crystalline sequence than within the “steady state” conditions. Whilst coseismic landsliding immediately following the shocks was recorded mainly in the west of the Kosi basin, results presented in this thesis suggest that the low peak ground accelerations across the entire basin seismically weakened slopes leading to enhanced erosion during the 2015 monsoon.