Bank Pull or Bar Push: Who Leads the Dance of Meander Migration?

Abstract. 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 long-standing 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, empirical research addressing which of these mechanisms dominates in real-world environments has predominantly focused on a small number of bends and at limited temporal scales. In this paper, we identify analyse the occurrence of bank pull versus bar push across 4,923 river bends worldwide. We estimated outer-bank 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. Of the 4,923 bends analyzed, 20 % were classified as bank pull dominated, 16 % as bar push dominated, whilst the remaining 63 % showed no clear migration signal. We also identified river characteristics that control the relative frequency of push versus pull migrating bends. Our findings indicate that vegetated bends, with higher soil compactness surrounded by sandy substrates and situated in rivers with slower flow velocities, are more likely to exhibit bend migration via bar push, where sediment deposition along the inner bank plays a dominant role. In contrast, less densely vegetated rivers with higher flow velocities and lower soil compactness 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 quasi-global empirical framework capable of distinguishing the temporal sequencing of erosion and sedimentation across thousands of river bends using long-term satellite observations, providing valuable insights that could be used to improve river dynamics modelling and inform more effective river management strategies.