Vertical sorting of bed material in rivers has received increasing attention, but very little experimental data are available so far. A recently developed nondestructive sampling technique allows the determination of bed material composition over vertical surfaces of the bed. This is particularly interesting for determining particle arrangement on the surface of and inside bed forms. Experiments were carried out using bimodal and trimodal sediment mixtures in two recirculating flumes. The development of surface coarsening was detected and measured at various stages of the process. Variations in the initial bed material composition brought very different responses in the bed geometry. The mobile armoring process was also strongly affected by bed material composition. The experimental procedures described in this work indicate a possible path to be followed for further experimental research on armoring processes.¿ 1997 American Geophysical Union