Changes in sediment sources and floodplain deposition rates in the catchment of the river tweed, scotland, over the last 100 years: the impact of climate and land use change

摘要

Evidence from floodplain cores collected from three sites in the middle reaches of the Tweed basin in Scotland is used to reconstruct changes in sediment sources and overbank floodplain deposition rates over the last c. 100 years. Core chronologies and sedimentation rates are established using ~(137)Cs and unsupported ~(210)Pb measurements. The average sedimentation rates since 1963 range from 1.9 ± 0.2 to 2.2 ± 0.2 kg m~(-2) a~(-1) and are lower than the average rates for the period 1894/95 to 1963, which range from 2.7 ± 0.6 to 5.9 ± 0.9 kg m~(-2) a~(-1). There is also evidence of significant downcore variations in sediment source, defined in terms of both type (i.e. topsoil or channel bank/subsoil material) and spatial location (i.e. main geological/topographic zones). There is no clear link between the changes in overbank sedimentation rates and sediment sources and the trends shown by precipitation, weather pattern and river flow records over the past 100 years, suggesting that changes in climate along cannot explain the downcore trends. Instead, the temporal changes in overbank sedimentation rates and sediment source appear to be linked more closely to changes in land use and land management over the past 100 years and, more particularly, the introduction of land drainage at the end of the 19th century, the rapid increase in afforestation since the 1940s and the post-war conversion of grassland to arable land.