USING RESERVOIR DEPOSITS TO RECONSTRUCT CHANGING SEDIMENT YIELDS AND SOURCES IN THE CATCHMENT OF THE OLD MILL RESERVOIR, SOUTH DEVON, UK, OVER THE PAST 50 YEARS

In the absence of long-term records of sediment transport by UK rivers , sediment deposits in lakes and reservoirs offer considerable potenti al for estimating sediment yields and reconstructing changing patterns of sediment yield and sediment sources during the past century in res ponse to environmental change. An investigation of the sediment deposi ts within the Old Mill Reservoir, located in the East Hams region of S outh Devon, UK, has shown that suspended sediment yields from the 1.58 km2 catchment have averaged ca. 54 t km-2 year-1 over the period 1942 to 1991. Bedload transport from the catchment during the same period averaged 15 t km-2 year-1. The sediment record also provided evidence of increasing productivity in the reservoir during recent years and of the impact of a single extreme event. The availability of several dat eable horizons within the sediment cores made it possible to subdivide further the reconstructed record of sediment yield into four periods. This subdivision indicated that suspended sediment yields have increa sed more than fourfold since the Second World War from about 20 t km-2 year-1 to ca. 90 t km-2 year-1. A variety of sediment properties, inc luding mineral magnetic characteristics and caesium-137 activity, was used to fingerprint the dominant source of the deposited sediment. Thi s fingerprint analysis indicated that, with the exception of the extre me event, the sediment sources have remained essentially constant thro ughout the period of record and that surface material from pasture are as represented the dominant source. The increase of suspended sediment yields during the postwar period was attributed to increased livestoc k numbers and grazing intensity in the catchment of the reservoir.