Sediment Yield in the Barrier Range Before and After European Settlement.

Abstract

Sediment yield has been estimated for the catchment of Umberumberka Creek, just west of Broken Hill in far western New South Wales, for two periods before European settlement and for three periods after settlement. The approach is stratigraphic whereby pre-settlement yields have been estimated from a radiocarbon-dated alluvial fan while post-settlement yields have been calculated from surveys of sedimentation in Umberumberka Reservoir. The average post-settlement yield of 1.9 m3 ha-' yr-' is about 50 times greater than the average yield for the 3000 years preceding settlement. The highest recorded post-settlement yield of 3.1 m3 ha-' yr-', over the interval 1915-1941, is about 90 times greater. Sediment yield in the interval 3000 to 6000 years BP was higher than in the interval 3000 years BP to European settlement but lower than the post-settlement yield. Changes of sedimentation rate have been detected in the reservoir since it was built in 1915, reflecting variations in the delivery of sediment to the streams and storage of sediment within the catchment. The nature of these mechanisms has not been accurately determined but comparison of sedimentation rates in reservoirs in south-eastern Australia suggests that a regional increase in rainfall during this century has played a role. If this is correct then the reduced sediment yields since the 1940s are no more than partially related to management and the reduction of rabbits.