Controls on sediment flux in active mountain ranges

• Main Author: Graf, Emma L. S
• Format: Dissertation
• Language: English
• Published: University of Edinburgh 2023
• Subjects: active mountain ranges; coseismic landslide; drainage divide migration; drainage divide mobility; drainage reorganisation; heterogeneous lithology; lithology; rock erodibility; sediment flux; sediment flux controls
• DOI: 10.7488/era/3085

The shape of landforms at the Earth's surface is determined by a balance between processes creating topography and relief (tectonic uplift) and processes removing relief (denudation by rivers or glaciers, earthquakes, etc.). Rivers are of particular importance in this context - in non-glaciated landscapes, which form the majority of the Earth's land surface, they set the pace of landscape evolution by setting the base level for hillslope processes, by incising into bedrock, and by transporting sediment from source to sink (e.g. from mountain ranges to basins and seas). Moreover, rivers are closely linked to human livelihoods by supplying water for drinking and irrigation, fertile soil for agriculture, and avenues for trade and transport. In addition, river networks are an important tool for reconstructing the past history of landscapes, as they record tectonic and geologic history through their channel gradient, form, and the shape of the channel network. In this thesis, I explore the factors governing the rate at which landscapes adjust to perturbations, both using a long-term approach by modelling the effect of lithology on the creation of relief, and focusing on shorter temporal scales by documenting the transport of coarse sediment pulses along mountain rivers. First, I investigate the control of heterogeneous lithology and drainage reorganisation events on the rate of drainage divide migration. I demonstrate a method to extract and optimise values of erodibility K to best match the present-day relief of a lithologically complex landscape. I then explore the influence of heterogeneous lithology and drainage reorganisation on the rates and patterns of drainage divide migration by simulating a range of base level fall scenarios for a study area in the eastern Swiss Jura Mountains. Here, I demonstrate that through extracting erodibility values from topography and subsequently optimising them through Monte Carlo sampling, it is possible to constrain a range of erodibility values which faithfully reproduces the relief in a given model landscape. Additionally, I show that heterogeneous lithology will result in continuous divide migration. In the next chapter, I move from more general observations on landscape evolution to a detailed study of perturbations to a river system by extreme events. Similar to the 2008 Wenchuan (China) earthquake, the 2015 Gorkha (Nepal) earthquake was associated with widespread landsliding, which was expected to result in significant channel aggradation. Due to the high sensitivity of Himalayan rivers to changes in sediment supply, constraining the timescales of coseismic landslide sediment export is crucial. Here, I track the gravel bedload fraction of coseismic landslide sediment by mapping changes in gravel area along two rivers in central Nepal from 2012 to 2021. While I find no evidence of influence of the Gorkha earthquake beyond small and localised increases in gravel area following the 2015 monsoon, one of the studied rivers displays an up to four-fold increase in gravel area along a 30 km reach following a catastrophic flood event in June 2021. This indicates that extreme hydrological events, rather than large earthquakes, govern the export of coarse sediment from these catchments. The third and final results chapter builds upon the work shown in the previous chapter by estimating the timescales of sediment transport from mountain catchments to the Himalayan mountain front. Using a time series of high-resolution channel cross-sections derived from an Acoustic Doppler Current Profiler as well as measurements of channel gradient and gravel grain size, I estimate the Koshi River's sediment transport capacity and compare this with the likely grain sizes of sediment inputs into the river in order to gain an understanding of the likely timescales on which coseismic Gorkha landslide sediment will reach the Indo-Gangetic plain. My results suggest that the sediment supplied to the Koshi basin through coseismic mass wasting is too coarse to be mobilised by regular monsoon floods and will require discharges with return periods of up to a hundred years to be removed from the catchment. While the work presented in this thesis spans a range of spatial and temporal scales, the overarching theme is the controls on landscape response to perturbations. Placed within this broader context, this thesis fits in well with the existing body of research and deepens our understanding of the crucial role river networks and lithology play in landscape evolution, particularly regarding the role of rock erodibility in controlling drainage divide mobility and the importance of valley and channel morphology in governing the rate of sediment export from river catchments.