A large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated floods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients, both in the channel during the rising stage of the flood, as well as on the floodplains, during a later stage of the flood. These vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment-induced density gradients, resulting in a positive feedback mechanism that produces high deposition rates. Erosion prevails when the sediment is largely held in suspension due to hindered settling, and is strengthened by the reduced wetted cross-section caused by massive sedimentation on the floodplain. Observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment-induced density effects and hindered settling are included. Copyright © 2009 John Wiley & Sons, Ltd.