Bimodality in river bed state suggests critical transitions at high flow

River bedforms affect bed roughness and, with that, fluvial hydraulics. With increasing flow velocity, subaqueous bedforms grow from flat beds to ripples to dunes, before diminishing again to an upper stage plane bed. Previous studies report an increase in the standard deviation of bedform height with increasing transport stage (a measure of flow strength), and rapid switches in time between contrasting bed configurations. Not much attention has been given to this phenomenon despite its importance in, for example, flood prediction. This study reanalyzes experimental data from earlier experiments. We show that there are statistically strong indications that the increase in standard deviation is due to the emergence of bimodal distributions in river dune height for transport stages larger than 18. This is consistent with our understanding of the physics, as time series of observed dune heights exhibit flickering between low and high dunes, suggesting critical transitions between two alternative morphological states. We hypothesize that local sediment outbursts drive temporary shifts from suspended- to bed load conditions, causing dunes to form transiently before returning to an upper stage plane bed. Flickering behavior of dunes at high transport stages implies that one single snapshot is not enough to capture the state of a system, with far-reaching implications for field measurements and experimental designs. The possibility of alternative dune states also calls for a reconsideration of classical equilibrium relations. This study implies a presence of tipping points in geomorphology and calls for further research to understand and quantify flickering behavior in sediment beds at high transport stages.