Bank pull or bar push: Who leads the dance ofmeander migration?

Despite significant advances in the study of meandering rivers, the precise mechanisms by which erosion and sedimentation interact to drive meander migration remain poorly understood. Two longstanding competing theories attempt to explain this interaction: one suggests that inner-bank sediment deposition precedes outer-bank erosion (bar push), while the other posits that outer-bank erosion initiates inner-bank sedimentation (bank pull). To date, research addressing which of these mechanisms dominates in real environments has predominantly focused on a small number of bends and at limited temporal scales, leading to inconclusive results. In this paper, we analyse the occurrence of bank pull versus bar push across 4,923 river bends worldwide. This was done by estimating outerbank erosion and inner-bank sedimentation rates using a 38-year time series of Landsat data, from which classifications of bank pull and bar push were derived through Dynamic Cross-Correlation analysis. We found that both bank pull and bar push occur frequently in a wide range of natural environments, but with a slight dominance (55.7% of bends) of bank pull over bar push (47.3%). Nevertheless, we also identify subtle patterns in the relative frequency of push versus pull migrating bends based on river characteristics. Vegetated rivers with slower, less variable, flow and finer boundary materials are more likely to exhibit migration via bar push, where sediment deposition along the inner bank plays a dominant role. In contrast, less densely vegetated rivers with faster, more variable flows and with coarser boundary materials, show a higher occurrence of bends migrating via bank pull. This means that most bar-push migrating bends are located in densely-vegetated tropical environments, with bar pull tending to dominate in other regions. This study represents the first largescale analysis of bank pull and bar push in real rivers, providing valuable insights that could be used to improve river dynamics modelling and inform more effective river management strategies.