At river bifurcations water and sediment is divided among the downstreambranches. Prediction of the sediment transport rate and division thereof atbifurcations is of utmost importance for understanding the evolution of thebifurcates for short-term management purposes and for long-term fluvialplain development. However, measured sediment transports in rivers rarelyshow a uniquely determined relation with hydrodynamic parameters. Commonly ahysteresis is observed of transport rate as a function of discharge or shearstress which cannot be explained with the standard sediment transportpredictor approach. The aim of this paper is to investigate the causes ofhysteresis at a bifurcation of the lower Rhine river, a meandering riverwith stable banks, large dunes during flood, and poorly sorted bed sediment.The hydrodynamics and bed sediment transport were measured in detail duringa discharge wave with a recurrence interval larger than 10 years.Surprisingly, the hysteresis in bedload against discharge was in theopposite direction upstream and downstream of the bifurcation. The upstreamclockwise hysteresis is caused by the lagging development of dunes duringthe flood. The counter-clockwise hysteresis downstream of the bifurcation iscaused by a combination of processes in addition to dune lagging, namely 1)formation of a scour zone upstream of the bifurcation, causing a migratingfine sediment wave, and 2) vertical bed sorting of the bed sediment by duneswith avalanching lee-sides, together leading to surface-sediment fining andincreased transport during and after the flood. These findings lead tochallenges for future morphological models, particularly for bifurcations,which will have to deal with varying discharge, sediment sorting in thechannel bed, lagging dunes and related hydraulic roughness.