Slope-channel decoupling in Wolumla catchment, New South Wales, Australia:the changing nature of sediment sources following European settlement

Within a few decades of European settlement, channel incision transformed d iscontinuous river courses throughout Wolumla catchment, on the south coast of New South Wales, Australia. The development of continuous channels grea tly increased sediment delivery from the catchment. This paper documents th e character, timing and proportion of sediment sourced from upland valley f ills, channel expansion sites, and gully networks. Volumes of material tran sferred from these sources are compared with estimates of sediment eroded f rom hillslopes, and the movement of sediment off the slopes to the valley f loor is assessed. Although disturbance of slopes resulted in significant mo vement of materials, most of this material has been stored on-slope, in tra pped tributary fills and along lower order drainage lines. The slopes are e ffectively decoupled from the channel. Sediment accumulation in farm dams o ver the past few decades has been negligible. Around 75% of the total volum e of material released from creeks in Wolumla catchment since 1865, i.e., 5 500 x 10(3) m(3), has been derived from channel incision into valley fills at the base of the escarpment. Sediment flushing occurred within a few deca des of catchment disturbance. Bedrock confinement in the middle and lower c atchment resulted in very efficient downstream transfer of materials. Altho ugh gully networks and channel expansion sites have released a relatively s mall volume of material, these sources are the greatest contemporary source of sediment in Wolumla catchment. (C) 1999 Elsevier Science B.V. All right s reserved.