The role of overbank floodplain sedimentation in catchment contaminant budgets

Abstract

Overbank sedimentation on river floodplains can result in significant reduction of the suspended sediment load transported by a river and can thus represent an important component of the catchment sediment budget. Such conveyance losses will also exert an important influence on sediment-associated contaminant fluxes and budgets. This contribution reports the results of a study of sediment-associated contaminants (i.e. total-P, Cr, Cu, Pb and Zn) fluxes in the River Swale (1346 km²) and River Aire (1002 km²) in Yorkshire, U.K., aimed at quantifying the role of overbank floodplain sedimentation in the sediment-associated contaminant budgets. The catchment of the River Aire is dominated by urban and industrial land use in its middle and lower reaches, whereas the River Swale drains a largely rural catchment, although there is a legacy of metal mining in its headwaters which impacts on heavy metal transport by the river. The results for the River Swale indicate that the conveyance losses associated with the deposition of sediment-associated contaminants on the floodplains bordering the main river can be as high as 47% of the total flux through the main channel system. Equivalent values for the River Aire range up to 26%. Contrasts between the two rivers reflect both the location of the contaminant sources within the catchments and the relative magnitude of the fine sediment deposition fluxes associated with their floodplains.