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 gr anitic catchments along the south coast of NSW. On the 1865 portion pl an, the valley fill surface in the upper part of Wolumla Creek, in the Bega River catchment, was intact, but within a few decades of Europea n settlement of the area the valley fill had been incised. Today the i ncised channel is up to 10 m deep and 100 m wide, The catchment drains an area of just 18.2 km(2). Based on detailed field mapping, with ext ensive drilling and augering, the volume of the intact valley fill in upper Wolumla Creek in 1865 was approximately 5000 x 10(3) m(3). Betwe en 1865 and the present day, approximately 3500 x 10(3) m(3) of this m aterial has been removed, leaving roughly 1500 x 10(3) m(3) of materia l stored on the valley margins. During an initial period of discontinu ous gullying, approximately 230 x 10(3) m(3) of sand accumulated as a floodout. Subsequently, the incised channel became continuous, cutting through the floodout; over 50 per cent of floodout deposits were remo ved. Flushing of the materials released from upland valley fills has b een very efficient in the Wolumla Creek catchment, with a sediment del ivery ratio of around 70 per cent. The efficient downstream transfer o f deposits reflects bedrock confinement in downstream reaches. Extensi ve volumes of material have accumulated along the lower reaches of the catchment, exacerbating the transformation to the geomorphic characte r of the lower Bega River.