The last decades have shown an increase in the frequency and magnitude of floods (Blöschl et al. 2020). The intensification of flood regimes will likely enhance rivers’ capacity to erode and transport sediment. Sedimentary properties, such as texture, porosity and geochemistry are crucial factors that affect the economic and ecologic values of overbank zones. Moreover, understanding of sediment fluxes and pathways is also key in predicting the distribution of adhesive heavy metal contaminants, microplastics and forever PFAS chemicals. Yet, modelled estimates of sediment transport and overbank deposition are notoriously inaccurate. A recent experimental study of bed load transport suggested up to a factor 5 attributable to the shape of particles (Deal et al., 2023). Here we show that the shape of sand grains is probably also a critically overlooked factor in suspended sediment transport. Its effects are observed from lateral-distance sorting in overbank deposits from different catchments. We used hydraulic modelling to further test our initial observations and to explore the magnitude of changes in sediment transport that are potentially caused by grain shape.

Distinct bankline patterns appeared after the removal of protection works along a navigable reach of the Meuse River. A series of oblique embayments now dominate the riverine landscape after ten years of bank erosion, but their location and asymmetry cannot be explained yet. This work analyses and integrates field measurements of flow, ship waves, bank composition, bed topography and historical maps to explain the observed patterns along two reaches of the river. An extraordinary low-water-level event generated by a ship accident provided the unique opportunity to also analyse the subaqueous bank topography. The results indicate that the formation of oblique embayments arises from the combination of floodplain heterogeneity, structured by scroll-bar deposits, and the regulation of water levels, resulting in ship-wave attack at a narrow range of bank elevation for 70% of the time. Substrate erodibility acts on the effectiveness of trees to slow down local bank erosion rates, which is possibly enhanced by a positive feedback between woody roots and cohesive soil. The strong regulation of water levels and the waves generated by the intense ship traffic produce an increasingly long mildly-sloping terrace at the bank toe and progressively dominate the bank erosion process. This study demonstrates the important role of floodplain and scroll bar formation in shaping later bank erosion, which has implications for predictive numerical models, restoration strategies, and understanding the role of vegetation in bank erosion processes.