River ¯oodplains have been widely recognized as important sinks for storing suspended sediment and associated contaminants transported by river systems. The grain size composition of ¯oodplain deposits exerts an important in¯uence on contaminant concentrations, and commonly exhibits signi®cant spatial variability in response to the dynamic nature of overbank ¯ow and sediment transport. Information on the spatial variability of the grain size composition of overbank deposits is therefore essential for developing an improved understanding of the processes controlling sediment transport on ¯oodplains, and for investigating the fate of sediment-associated contaminants. Such information is also important for validating existing ¯oodplain sedimentation models. This paper reports the results of a study aimed at investigating the spatial variability of the grain size composition of ¯oodplain sediments at dierent spatial scales, through analysis of surface sediment samples representative of contemporary ¯oodplain deposits collected from frequently inundated ¯oodplain sites on ®ve British lowland rivers. Signi®cant lateral and downstream variations in the grain size composition of the sediment deposits have been identi®ed in the study reaches. An attempt has been made to relate the observed spatial distribution of the grain size composition of the overbank deposits to the local ¯oodplain geometry and topography. The importance of the particle size characteristics of the suspended sediment transported by the rivers in in¯uencing the spatial variability of the grain size composition of the overbank sediments deposited on these ¯oodplains is also considered.