Exploring the multi-decadal temporal dynamics of catchment to coast fine sediment delivery using floodplain sedimentation rates: a case study employing fallout radionuclides on the River Avon, Devon

By comparison of floodplain sedimentation rates, derived using the fallout radionuclides 210Pb and 137Cs, and downcore geochemistry, obtained by XRF, with historic flow data; this study explores the importance of flow magnitude-frequency and land-use change on the sedimentation record of floodplains over medium-term timescales (~40 y). Salt marsh geochemistry is also examined alongside the floodplain’s to determine if floodplains may be used as records of catchment to coast sediment delivery. This is done in the context of attempting to further the understanding of the temporal dynamics of catchment sediment delivery. Using a case study of the Devonshire Avon, this study found that over the last 40 years there was a statistically significant relationship (Pearson’s 0.92, p-value 0.02) between floodplain sedimentation rates and the maximum discharge during the period of sedimentation. It is proposed that this may represent the extension of the current trend within literature, that the largest events are most significant in annual sediment transport, to multi-annual and decadal timescales. Observations of substantial changes in the geochemistry of sediments delivered to the floodplain through time, in the 1800’s potentially driven by mining and more recently (last ~40 y) by intensification of agriculture, are used to demonstrate that these results should not be considered outside of their historic catchment context due to the potential influence of changing catchment processes on the sedimentation rate record of floodplains. Poor correspondence between the floodplain and salt marsh geochemistry found here, potentially due to the influence of complex sedimentation dynamics on salt marshes owing to tidal influence, provides little support for the use of floodplains as a record of catchment to coast sediment delivery. This study finds that the comparison of floodplain sedimentation rates derived from radionuclides with flow records shows potential as a novel method for exploring the medium-term temporal dynamics of catchment to coast sediment delivery, provided consideration is given to catchment land-use and management history. However, the representativeness of floodplains of catchment to coast delivery requires substantially more justification