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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. 

Rivers are dynamic systems that change shape due to natural events like floods, landslides, and wildfires, as well as human-made structures like bridges. These changes create unique and complex river patterns. However, measuring these changes accurately is challenging because traditional field surveys are time-consuming and often focus on easily accessible areas, which may not represent the whole river. Thanks to new high-resolution mapping technology and detailed river flow records, we can now see these changes in greater detail than ever before. In this study, we focused on the Logan River, which flows through a canyon with a mix of rocky and sediment-filled areas. Using advanced mapping tools, we measured the river's width along a 45 km stretch and checked these measurements against field surveys to ensure accuracy. This approach allows us to identify sections of the river that show consistent patterns and those that deviate due to factors like incoming streams or human impacts like roads. Understanding these variations helps us to better plan for infrastructure projects, river restoration, and managing natural hazards. Our method provides a new way to understand how changes in the environment affect rivers, which can lead to smarter, more resilient design and planning. 

Data compilations of bankfull downstream hydraulic geometry for alluvial rivers, lidar derived high-resolution spatial series of bankfull width for 67 sites, and hydrograph metrics for sites with USGS hydrographs. This compilation is composed of three datasets: (1) a compilation of alluvial river geometry at bankfull for a variety of hydraulic attributes; (2) x, y, and z coordinates of channel bank lines for a selection of sites and their associated river widths derived from high-resolution lidar topography; and (3) statistics describing the hydrographs for a subset of the larger compilation. These data are divided into two primary sets, the larger compilation and a smaller set of sites where the bankfull width was derived from lidar topography. For the high-resolution dataset, the data are available as the coordinates of the bank lines, spatial series of distance downstream and bankfull width, and the spatial series filtered for quality.