ABSTRACTBomb-derived fallout137Cs has been used as an environmental tracer to study rates of overbank sedimentation on river floodplains. The depth distributions of fallout137Cs in sediment cores collected from depositional sites on the floodplains of twelve lowland rivers in Britain have been employed to estimate mean annual overbank sediment deposition rates over the past 30–40 years, assuming a constant rate of sediment deposition and a correspondence between the shape of the observed137Cs profiles and the history of radiocaesium inputs. Values of sedimentation rate estimated for the twelve sites range from 0.11 g cm-2yr-1to 0.95 g cm-2yr-1. Variations in the levels of137Cs activity associated with surface sediment from the individual sites can be used to interpret the dominant sediment sources in the individual catchments.