In August 2006, a large flood following saltcedar control efforts through a 12-km long segment of the Rio Puerco arroyo resulted in extensive lateral erosion of the streambanks. Almost all woody vegetation on the floodplain and channel banks had been killed by aerial spraying with herbicide in September 2003. During the flood, dead woody bank stems were either removed by the >4-m-deep flood flow or flattened against the bank, eliminating the source of drag that would have protected the banks from erosion. Owing to downstream variation in the shear stresses on the channel banks and floodplain, lateral erosion of the channel banks was highly variable within the sprayed reach, but channel width increased by an average of 84%. Locations and magnitudes of channel bank erosion were documented from high-resolution imagery and a post-flood (January 2007) high-precision Global Positioning System survey. Topographic data collected during the January 2007 field survey combined with geomorphic mapping from imagery provided a means to infer the progression and relative timing of bank erosion during the flood. Observations and calculations indicate channel widening resulted from a combination of direct fluvial erosion of the lower banks and mass failures of the upper banks. Applications of physically based models of flow and sediment transport demonstrate the relative influence of local floodplain slope, arroyo topography, and orientation of the channel centerline relative to the down-valley axis on bank erosion. Differences in suspended sand concentrations computed using model-calculated “skin friction” shear stress quantify the erosion rate at a site where channel width doubled during the flood.