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Abstract During a flood, the geometry of a river channel constrains the flows of water and sediment, however, over many floods, bankfull channel geometry evolves to reflect the longer‐term fluxes of water and sediment supplied by the catchment. Physics‐based models predict the average relationship between bankfull geometry and discharge to within an order of magnitude, however, observed variability about the prediction remains unaccounted for. We used high‐resolution topography to extract continuous measurements of bankfull width from 67 sites spanning the continental United States, yielding a reach‐scale probabilistic description of river width for each site. Within an individual reach, bankfull river width is well‐described by a lognormal distribution. Rivers that spend a greater proportion of time above bankfull are wider for the same bankfull discharge, revealing an unrecognized pathway through which climatic or engineered changes in flow frequency could alter river geometry and therefore impact aquatic habitat and flooding risk.