The characteristics of overbank deposits associated with a major flood event in the catchment of the River Ouse, Yorkshire, UK

Overbank floodplain deposits associated with a major flood which occurred in 1995 were collected shortly after the event from transects at 14 locations within the catchment of the River Ouse, Yorkshire, UK. At most sites, the sediment deposited nearest to the channel consisted primarily of sand-sized material, and the absolute particle size composition of the mineral fraction of the deposits evidenced fining with increasing distance from the channel. Most of the sand-sized material was deposited within 20 to 40 m of the channel, although at some sites, sand was present at distances > 100 m from the channels. There was no obvious downstream trend in the texture of the sediment deposited on the floodplains. In most places, the overbank sediment was considerably coarser than the suspended sediment transported in the study rivers, although at several sites, thed50 of the latter was similar to that of overbank sediment deposited furthest from the channel. Although the C:N ratio of the organic fraction of the overbank sediment was approximately constant across the width of the floodplain, the organic carbon and137Cs content of the deposited sediment increased with distance from the channel, and both properties were positively related to the magnitude of the < 63 μm fraction. The texture of sediment in cores collected from two of the sites in 1994 was generally finer than that of the overbank deposits from the 1995 flood at the same locations, confirming the extreme nature of the 1995 flood event.