A fluvial sediment budget for upper Wolumla Creek, south coast, New South Wales, Australia

Extensive valley fills have formed at the base of the escarpment in granitic catchments along the south coast of NSW. On the 1865 portion plan, the valley fill surface in the upper part of Wolumla Creek, in the Bega River catchment, was intact, but within a few decades of European settlement of the area the valley fill had been incised. Today the incised channel is up to 10 m deep and 100 m wide. The catchment drains an area of just 18.2 km². Based on detailed field mapping, with extensive drilling and augering, the volume of the intact valley fill in upper Wolumla Creek in 1865 was approximately 5000 × 10³ m³. Between 1865 and the present day, approximately 3500 × 10³ m³ of this material has been removed, leaving roughly 1500 × 10³ m³ of material stored on the valley margins. During an initial period of discontinuous gullying, approximately 230 × 10³ m³ of sand accumulated as a floodout. Subsequently, the incised channel became continuous, cutting through the floodout; over 50 per cent of floodout deposits were removed. Flushing of the materials released from upland valley fills has been very efficient in the Wolumla Creek catchment, with a sediment delivery ratio of around 70 per cent. The efficient downstream transfer of deposits reflects bedrock confinement in downstream reaches. Extensive volumes of material have accumulated along the lower reaches of the catchment, exacerbating the transformation to the geomorphic character of the lower Bega River.