Geochemical characteristics of overbank deposits and their potential for determining suspended sediment provenance; an example from the River Severn, UK

The sources of suspended sediment are an important factor controlling sediment yield and sediment budgets. Sediment provenance is an essential prerequisite for elucidating the overall sediment delivery system. Only a proportion of the suspended sediment transported by a river during floods may reach the river mouth, and lowland floodplains frequently represent important sediment sinks. The geochemical properties of floodplain sediments have been used in stratigraphic studies of long-term environmental changes in river basins, bur their potential for investigating recent and contemporary sediment sources has not been fully exploited, This paper reports the results of a study that has used the geochemical properties of overbank deposits, including heavy metal, trace metal and cation exchange elements, to establish the main suspended sediment sources within the 10 000 km(2) basin of the River Severn, UK. The results confirm the importance of the upland catchments of the rivers Teme, Vyrnwy and Upper Severn as sources, providing 70% of sediment in the Basin, and of the River Avon as a source of 27% of the <63 mu m material. The catchments of the rivers Perry, Stour and Tern, which drain the central areas of the Severn Basin, are shown to be insignificant sources of sediment, contributing only 0-4% to sediment deposited at Haw Bridge. An attempt was also made to establish historical sediment source contributions, although changes in sediment geochemistry during storage in floodplain sinks cause this to be largely unsuccessful.