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. The input of marine sediment from Moreton Bay is the dominant source of suspended sediment (about 456 000 t) to the Brisbane River estuary during an average flow year, contributing more than 1•5 times the sediment delivered from the catchment (about 178 000 t) and urban areas (about 112 000 t) combined. As the volume of water discharged during floods increases, the sediment retention efficiency of the Brisbane River estuary decreases rapidly due to the flushing of sediment through its mouth. As such, although the input of sediment from the catchment increased three-fold (600 000 t) during 1996 associated with a 20 year return period flood, marine sediment was still the dominant source of sediment deposited in the estuary because 77% of the fluvial sediment was exported to Moreton Bay. Dredging has increased the sediment trapping capacity of the estuary with more than a two-fold increase in the flood water volume needed (about 2000×106 m3) to flush the estuary fresh at the mouth compared to pre-1962. An upstream dam traps a large proportion of the catchment sediment load, but the upstream retention of flood water has also increased the trapping capacity of the estuary by reducing the freshwater flow. As such, about 22% (33 000 t) more sediment was deposited in the estuary during a 20 year return period flood in May 1996 than would have been deposited prior to dam construction. The sub-tropical Brisbane River estuary has a lower and more variable sediment trapping efficiency than typical temperate northern hemisphere estuaries.