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Description: The backwater reach of coastal rivers is associated with considerable spatial and temporal variability in water and sediment flux. Here we test the hypothesis that the spatial and temporal variability in water flux and particle sizes in transport result in systematic changes in the geometry of bank-attached bars across the backwater transition. Measured transverse slopes of bank-attached bars in the Mississippi and Trinity Rivers show a systematic increase where the river transitions from normal flow to the backwater. We propose a simple force balance relationship, in which the transverse slope of the bars constructed through traction transport varies in proportion to the square of depth-averaged flow velocity and is inversely proportional to the square of the median particle size of the supplied sediment, in bends with similar curvature. The observed trend is therefore explained by a downstream reduction in particle sizes coupled with a downstream increase in flow velocity across the backwater transition at high flow.