A SUSPENDED SEDIMENT BUDGET FOR THE MODIFIED SUBTROPICAL BRISBANE RIVER ESTUARY, AUSTRALIA

Annual suspended sediment budgets, for an average flow year and a wet year, were constructed for the sub-tropical Brisbane River estuary. Th e input of marine sediment from Moreton Bay is the dominant source of suspended sediment (about 456000 t) to the Brisbane River estuary duri ng an average flow year, contributing more than 1.5 times the sediment delivered from the catchment (about 178000 t) and urban areas (about 112000 t) combined. As the volume of water discharged during floods in creases, the sediment retention efficiency of the Brisbane River estua ry decreases rapidly due to the flushing of sediment through its mouth . As such, although the input of sediment from the catchment increased three-fold (600000 t) during 1996 associated with a 20 year return pe riod flood, marine sediment was still the dominant source of sediment deposited in the estuary because 77% of the fluvial sediment was expor ted to Moreton Bay. Dredging has increased the sediment trapping capac ity of the estuary with more than a two-fold increase in the flood wat er volume needed (about 2000 x 10(6) m(3)) to flush the estuary fresh at the mouth compared to pre-1962. An upstream dam traps a large propo rtion of the catchment sediment load, but the upstream retention of fl ood water has also increased the trapping capacity of the estuary by r educing the freshwater flow. As such, about 22% (33000 t) more sedimen t was deposited in the estuary during a 20 year return period flood in May 1996 than would have been deposited prior to dam construction. Th e sub-tropical Brisbane River estuary has a lower and more variable se diment trapping efficiency than typical temperate northern hemisphere estuaries. (C) 1998 Academic Press.