Controls on fluvial grain sizes in post-glacial landscapes

Abstract. The grain sizes of sediments in channels have been linked to landscape characteristics, such as flow distance from headwaters, topographic relief, lithology and climate, in landscapes with little past or present glacial influence. Few studies have explored the controls on sediment characteristics in formerly glaciated landscapes. In this study, we document river surface grain sizes at 279 localities across Scotland. We collect photographs of gravel bars through a citizen science survey, Scotland's Big Sediment Survey. Grain sizes distributions are extracted from the photographs using both manual and automated techniques. We 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. Specifically, we find no significant Spearman's relationships between d84 and environmental variables; the strongest relationship was found between d84 and average basin aridity with a weak r² value of 0.29. We also find that the predictability of our random forest model is poor and only captures 22 % of the variance of d84. We find no correlation between grain size and flow competence, which suggests that sediment is both transport-limited and supply-limited. We propose that Scotland's post-glacial legacy drives the lack of sedimentological trends documented in this study, and that changes in landscape morphology and sediment sources caused by glacial processes lead to a complete decoupling between fluvial sediment grain size and environmental variables. This interpretation aligns with other studies that have highlighted the ongoing role of the post-glacial legacy on landscape evolution in tectonically quiescent terrains, both in Scotland and globally. Our results suggest that fluvial sediment grain size cannot be predicted by a global model based on environmental variables in post-glacial landscapes.