Changes in the fine sediment dynamics of the Ouse and Tweed basins in the UK over the last 100–150 years

Abstract

AbstractSediment cores (collected from nine headwater lakes and reservoirs in their catchments and adjacent areas and from ten floodplain sites in the middle and lower reaches of the two river systems) have been used to reconstruct changes in suspended sediment yields, rates of sedimentation and suspended sediment sources for the Ouse and Tweed catchments in the UK over the last ca 100–150 years in response to recent environmental change. For the small lake/reservoir catchments in or adjacent to the Ouse basin, average sediment yields for the recent period (1963–95) were generally found to exceed those for the period prior to 1963, by up to 94%, suggesting that sediment yields have increased over time. For the equivalent lake/reservoir catchments in and adjacent to the Tweed basin, sediment yields were generally lower in the period since 1963. In both basins, sediment yields over the past 100–150 years were characterized by considerable temporal variability, which was seen to reflect, at least in part, the significant changes in land use and land management that occurred within their catchments. No clear links between the variability of sediment yields and changes in climate were apparent. Changes in sediment source reconstructed for one of the lakes appeared primarily to reflect catchment disturbance during afforestation in the 1940s. Average rates of overbank sedimentation for the ten floodplain sites varied considerably between sites. For those cores collected from the Ouse basin there was no consistent temporal trend, and average sedimentation rates for the period extending from ca 1895 to 1963 were broadly similar to those for the period since 1963. For the Tweed basin, sedimentation rates for all three cores were higher prior to 1963, reflecting the increased soil erosion and sediment yields associated with the conversion of pasture to arable land in the immediate post‐World War II period and the ditching and planting operations associated with afforestation in the 1940s. Substantial downcore variability in the source of the overbank sediment, considered in terms of both source type and spatial location, was found in both river basins. This was seen to reflect land‐use change, including land drainage, the conversion of pasture to arable land, afforestation, and increased cereal production. The evidence provided by the floodplain cores again showed little indication of any influence linked to climate change.Although the detailed patterns of temporal variation of sediment yield, overbank sedimentation rate and sediment source reconstructed from the sediment cores collected from the lake/reservoirs and floodplain locations are site specific, there is a general consistency between the changes in sediment dynamics reconstructed for the headwater catchments (based on the cores collected from the lakes and reservoirs) and the equivalent evidence for the middle and lowland reaches of the catchments (provided by overbank floodplain deposits). Despite significant changes in climate, and, more particularly, land use over this time period, the general lack of evidence of substantial changes in downstream sediment yield and rates of overbank sedimentation over the last ca 100–150 years in the Ouse and Tweed basins suggests that their sediment dynamics have been generally insensitive to recent environmental change. This finding has important implications when considering the likely response of these, and similar, UK river basins to future environmental change. Copyright © 2003 John Wiley & Sons, Ltd.