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 So uth Wales, represent substantial sediment source zones. Contemporary c hannels within these fills are up to 8 m deep and 50 m wide for catchm ent areas < 10 km(2). Virtually all banks are eroding. Downstream of t his sediment source zone, sediment transfer zones are characterised by sinuous channels that are partially choked by sands released from upl and valley fills. Coarse sands stored in point bars deflect flow to ou ter 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 river mouth, the remainder of the catch ment is a bedrock-confined sediment throughput zone, characterised by fluctuations in channel bed elevation. The downstream 10 km of the cat chment 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 (tra nsfer) reaches of the catchment comprised swamps. Vegetation clearance , along with disturbance and drainage of swamps, transformed discontin uous water courses into continuous channels. Working from a base-point of intact upland valley fills, almost 50% of available material has b een removed from the upper catchment sediment source zone, contributin g 2.9 X 10(6) m(3) of material to the lower catchment. Of this, roughl y 65% has been stored along the channel bed and floodplain. This gives a total sediment contribution to the delta of 1.0 x 10(6) m(3). Suffi cient materials are stored in transient storage units along transfer z ones 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 fo llowing European settlement of the catchment (i.e., around 1830). Tran sition in landscape form and associated sediment delivery were rapid, with a negligible lag interval (i.e., within a few decades of disturba nce). However, given the extensive volume of sediment released from up land valley fills, landscape recovery (i.e., refilling of upland valle ys) will likely take thousands of years. (C) 1997 Elsevier Science B.V .