Accurate forecasts of bed forms and their roughness during a flood wave are essential for flood management. Bed forms remain dynamic even under steady discharge and are subject to a continuous process of creations and destructions of individual bed forms. Dune evolution during the rising limb of a flood wave is quite well understood and can be modeled. However, dune evolution during the falling limb remains poorly understood. The objective of this paper is to explain the bed form evolution and roughness during the receding limb of fast flood waves. Therefore, bed profiles of two flume experiments were analyzed in detail and individual dune creations and destructions were classified. The results showed that for fast flood waves in subcritical water flow: (1) dune length grows during both rising and falling limb due to amalgamation of bed forms, (2) dune length has a longer adaptation time than dune height, resulting in short, high dunes during the peak discharge, and (3) this hysteresis difference between dune height and length results in a larger roughness than predicted by equilibrium bed form dimension equations, which may result in a larger roughness of the main channel during floods than expected.