Fluid flow interacts with sedimentary beds forming waves of different kinds, which are of considerable practical importance since they influence significantly the near-bed flow, both over and below the bed, sediment transport, and wave height attenuation. We focus here on steep bed forms capable of producing flow separation. In this case, the large-scale vorticity generated in the phenomenon of separation rules the process of friction, which appears to be practically unaffected by sediment motion. Under the crests of the bed forms, the mean shear force due to friction is balanced by the force that bed forms exert on the flow via pressure, which can be calculated from the work of Gim¿nez-Curto and Corniero <2002>. Bed forms grow until they have a height such that friction at their troughs is negligible, thus ceasing the motion of fluid and sediment. This condition leads to a very simple expression for the limiting steepness, which compares favorably with existing observations on bed form geometry under steady open channel flow as well as under oscillatory flow.