Sedimentation in a semi-arid zone reservoir in Australia determined by 137 Cs

Abstract

Sediment has accumulated behind a masonry dam built in the 1880s in the Barrier Range, north - west of Broken Hill, New South Wales, Australia. The contributing catchment has an area of 12.5 ha, and is used for rangeland grazing for sheep and goats. It is estimated from surveys and measurements of heavy metals and caesium-137 that over 1000 tonnes of sediment have accumulated in the reservoir since its inception. Approximately one-third of the sediment has been delivered since the mid-1950s. Sediment accumulation is equivalent to a catchment erosion rate of 0.78 t ha-1 y-1 prior to the mid-1950s, whereas from the mid-1950s to the present it is 0.59 t ha-1 y-1. This indicates that the trap efficiency of the reservoir has decreased and/or the sediment supply has become relative ly exhausted and/or there has been a progressive increase in vegetation cover related to reduced grazing pressures. The estimated rates of erosion are considerably lower than for other sites in semi-arid New South Wales probably because sediment has been transported in overflows of the dam wall.