The assumption of the equilibrium state of gravel surfaces in flume experiments under feeding or recirculating conditions is generally justified by three equilibrium criteria based on sediment transport, slopes, and bed features. When these parameters become stable, an experiment is expected to reach equilibrium. This equilibrium state, however, is based on a one-dimensional model, the Exner equation, which may not truly reflect the equilibrium state of the system considering the complex flow and sediment processes. In this paper, the evolutionary process of a gravel surface is investigated based on a large-scale recirculating flume experiment. The performances of the three equilibrium criteria are evaluated first, and then the evolution of the bed morphology is studied. The key findings include the following: (1) the sediment transport rate, slopes of water and bed surfaces, and one-dimensional morphological features reach equilibrium roughly simultaneously; (2) two-dimensional morphology continually evolves after these characteristics reach equilibrium, which is confirmed by the characteristics of the sediment transport process; and (3) the results from a numerical simulation suggest that a much longer time is required to reach an equilibrium state. Our results suggest that sufficient experimental time is required to investigate the equilibrium morphological characteristics of gravel surfaces, which is much longer than the equilibrium time reflected by the one-dimensional equilibrium criteria. Copyright © 2017 John Wiley & Sons, Ltd.