The spatial variability of overbank sedimentation on river floodplains

River floodplains are important landforms, and information on rates and patterns of overbank sediment deposition is essential for understanding their development. Such information is also needed for validating existing overbank floodplain flow and sediment transport and deposition models. The use of bomb-derived 137Cs for documenting rates of overbank sediment deposition on river floodplains offers the potential for assembling data for a large number of points on a floodplain and therefore for investigating the spatial patterns involved. This approach also overcomes some of the operational and sampling problems encountered in traditional monitoring techniques. The study reported uses 137Cs measurements on sediment cores to investigate the spatial variability of sediment deposition on selected floodplain reaches of five British lowland rivers. Attention is given both to the overall reach and to more detailed investigations of small areas. The observed spatial patterns of sediment deposition are interpreted in terms of the major controls involved, which include the floodplain microtopography and morphology. The relationship between the grain size composition of deposited sediment and sedimentation rate is also considered.