Post-glacial fluvial dynamics of the British Isles

Abstract

Throughout the Quaternary, ice sheets and glaciers episodically expanded to cover up to 30 percent of the Earth’s surface. Many of these areas are now ice-free due to climatic warming after approximately 14.5 ka. Rivers are the main erosional agents and drivers of landscape evolution in post-glacial landscapes; however, their response to past glaciations is notoriously complex. Key challenges associated with understanding fluvial processes in post-glacial landscapes originate from the glacial modification of hillslopes and channels, such as decoupling of hillslopes from channels due to the over-deepening and widening of valleys by glaciers, or extensive glacial sediment drapes (e.g., till, moraines, paraglacial terraces) which influence sediment supply and transport capacity. Additionally, Glacial Isostatic Adjustment (GIA) results in considerable spatial and temporal variations in relative sea-levels, which set the base-levels of rivers. Rivers communicate changes in base-level to the rest of the landscape by the upstream propagation of transient signals. Relatively little research has quantified geomorphic processes in post-glacial landscapes in post-orogenic regions. Much geomorphological research has focused on unglaciated landscapes and glaciated landscapes in tectonically active regions.

In the first part of this thesis, I explore the controls on erosion rates in the post-glacial Feshie basin, Scotland. Erosion rates are inferred from the concentration of in-situ cosmogenic radionuclides (CRN) measured in river sands. When erosion rates are calculated based on the common assumption of basin-wide homogeneity of erosion, I counter-intuitively find no correlation between erosion rates and topographic metrics (e.g. slope). To explain the concentrations, I suggest that sediment is sourced from both the ‘background’ hillslopes and paraglacial terraces. I test this hypothesis with a mixing model, which indicates that the observed distribution of CRN concentrations can be explained if terrace escarpments have cm-scale retreat rates during large flood events. These results highlight the on-going glacial legacy on landscape evolution.

In the second part of this thesis, I explore controls of fluvial grain sizes in Scotland. I document river surface grain sizes at 300 locations through a citizen science survey. I then investigate whether grain sizes can be correlated and predicted from environmental variables (e.g., basin slope, flow distance from headwaters) through Spearman’s correlation statistics and random forest regression modelling. In contrast to other studies that have primarily focused on non-glaciated landscapes, we find no apparent controls on surface grain sizes in channels across Scotland. I suggest that Scotland’s post-glacial legacy drives the lack of sedimentological trends, which aligns with the interpretations from the first research chapter.

In the third part of this thesis, I explore the response of rivers to base-level rise (which is largely driven by GIA) and coastal erosion in Southern England. Emerging research suggests that coastal erosion can initiate the formation of migrating knickpoints. Through topographic analysis, I find that some rivers have migrating knickpoints in Southern England. I then investigate the fluvial and coastal factors influencing these knickpoints at the regional scale, as outlined by previous research. I find a clear lithological control: channels underlain by more resistant rocks consistently incise at their outlets, compared to less resistant rocks, for a given drainage area (< 25km2). However, I find no drainage area or coastal erosion rate control.

Overall, this thesis contributes to the growing body of research quantifying landscape evolution in regions affected by glaciation. Importantly, I find that post-glacial landscapes in post-orogenic terrains are largely influenced by the glacial legacy more than 10 ka after deglaciation. Moreover, I find that past glaciations influence river processes in regions that have not been glaciated through base-level rise from GIA.