Changing headwater suspended sediment yields in the LOIS catchments over the last century: a paleolimnological approach

Abstract

This paper describes the methods used to reconstruct suspended sediment yields over the last century, based on 210Pb and 137Cs dated bottom sediments in nine lake and/or reservoir catchments. Magnetic susceptibility and dry bulk density are used to correlate between multiple cores taken from each lake/reservoir basin in order to estimate sediment influx through time and reconstruct sediment yield histories. The catchments represent a range of morphological and land use systems, including upland moorland, upland forestry and upland and lowland agricultural systems. The reconstruction provides some background information on pre-20th century sediment yields and permits an evaluation of the effect of 20th century land management systems, especially upland afforestation and agriculture, on changing suspended sediment yields. Reconstructed sediment yields over the last century range from c. 7 to 86 t km−2 yr−1. Average post-1953 yields from pasture, arable, moorland and forested catchments are 13, 31, 29 and 13 t km−2 yr−1, respectively. The history of human disturbance in individual catchments is clearly recorded in the reconstructed sediment yields. Short-lived disturbance, such as afforestation, produces high yields (over 40 t km−2 yr−1) for up to a decade after planting, but yields subsequently decline to pre-planting levels (c. 12 t km−2 yr−1). Sustained increases in sediment yield are associated with arable, mixed agricultural and, to a lesser extent, grazing systems. While the strong controls exerted on sediment yield by human disturbance mask the potential effect of regional contrasts in runoff and topography between catchments, there is some evidence to suggest that a long-term climate change signal is recorded in the temporal trends in sediment yield from an analysis of Lamb weather types. Copyright © 1999 John Wiley & Sons, Ltd.