Field and laboratory experiments suggest that gravel in transport as bedload can be trapped on the upstream sides of antidunes by flow disruptions caused by breaking stationary waves. Trapped gravel patches are quickly buried by the migrating antidunes, and if not destroyed by subsequent scour, remain as gravel lenses herein called "dropout armor." If clasts in the lenses include the largest size transportable by the flow. maximum bed shear stress can be determined from Shield's criterion. If channel slope, width, and bed material are known, flow depth and mean velocity, antidune geometry (height and wavelength), and maximum discharge can be calculated. If channel slope and width cannot be determined independently, knowledge of maximum bed shear-stress still allows estimation of mean flow velocity and antidune geometry, and flow depth and slope can be estimated from the velocity profile. However, lack of accurate knowledge of the influence of suspended sediment on the velocity profile compromises the accuracy of these estimates unless channel slope can be determined by independent means, or antidune geometry and water depth estimated from antidune cross-laminae.