Four large-scale experiments are reported on the transport of graded sediments in a straight channel in the domain of partial sediment mobility (in which not all size fractions are fully mobile). Particular attention is paid to the influence of sediment introduced near the head of the experimental channel in inbank and overbank flows, respectively. In addition to sediment transport rate, the composition and texture of the bed and the composition of the bed load were measured at intervals. The differences in sediment behaviour between the inbank and overbank flows tested were unexpectedly small; however, the input of sediment near the head of the channel changed the sediment mobility profoundly. Transport rate was suppressed ahead of a wave of material that propagated through the channel. This suggests that the artificial introduction of sediment changed the near-bed flow structure for more than 15m downstream, impairing the transporting capacity of the stream. The observed changes of surface composition and texture imply that a complex grain structure is laid down during the recession of sediment-transporting flows in the field, and this structure is important for prediction of subsequent sediment behaviour. Better understanding of the near-bed processes and, particularly, the role of the bed condition will have to be assembled before significantly more reliable prediction methods can be made available. However, awareness of the factors that limit the accuracy of present methods is valuable in deploying those factors, and this paper contributes to such awareness.