European impacts on downstream sediment transfer and bank erosion in Cobargo catchment, New South Wales, Australia

Upland valley fills in Cobargo catchment, on the south coast of New South Wales, represent substantial sediment source zones. Contemporary channels within these fills are up to 8 m deep and 50 m wide for catchment areas < 10 km². Virtually all banks are eroding. Downstream of this sediment source zone, sediment transfer zones are characterised by sinuous channels that are partially choked by sands released from upland valley fills. Coarse sands stored in point bars deflect flow to outer banks where colluvial footslopes are eroded. Approximately 50% of banks are eroding in this section of the catchment. Other than the 10 km river reach upstream of the fiver mouth, the remainder of the catchment is a bedrock-confined sediment throughput zone, characterised by fluctuations in channel bed elevation. The downstream 10 km of the catchment is a sediment accumulation zone, in which erosion is restricted to occasional concave banks. Prior to European settlement of Cobargo catchment, upland valley fills were largely unincised, and middle (transfer) reaches of the catchment comprised swamps. Vegetation clearance, along with disturbance and drainage of swamps, transformed discontinuous water courses into continuous channels. Working from a base-point of intact upland valley fills, almost 50% of available material has been removed from the upper catchment sediment source zone, contributing 2.9 x 10⁶ m³ of material to the lower catchment. Of this, roughly 65% has been stored along the channel bed and floodplain. This gives a total sediment contribution to the delta of 1.0 x 10⁶ m³. Sufficient materials are stored in transient storage units along transfer zones to maintain current rates of sediment throughput to the delta for several decades. Cobargo catchment experienced dramatic acceleration of channel incision and bank erosion processes within a few decades following European settlement of the catchment (i.e., around 1830). Transition in landscape form and associated sediment delivery were rapid, with a negligible lag interval (i.e., within a few decades of disturbance). However, given the extensive volume of sediment released from upland valley fills, landscape recovery (i.e., refilling of upland valleys) will likely take thousands of years.