Estimation Of Long-Term Sediment Loads In The Fitzroy Catchment, Queensland, Australia.

Abstract

With a catchment area of nearly 140,000 km2, the Fitzroy discharges a significant amount of sediment to the Great Barrier Reef lagoon. Sediment discharge is related to water turbidity which has a direct impact on coastal ecosystems. Previous estimates of the long-term average sediment load of the Fitzroy vary greatly 1.3 - 2.5 Mt (1 Mt = 109 kg). These estimates, however, were mostly based on an extrapolation of sediment loads from other catchments in Queensland. In addition, sediment loads from major tributaries of the Fitzroy have not been assessed for internal consistency in the estimated sediment loads. Measured total suspended solids (TSS) concentrations from six sites in the Fitzroy were used to develop sediment rating curves. Sediment rating curves were corrected using smearing estimates to remove an inherent bias as a result of log-transformation. These rating curves combined with streamflow data were used to estimate long-term mean annual sediment load of the Fitzroy and its main tributaries for a common 30-year period 1974-2003. Sediment rating curves for the six sites show considerable similarity, and variation in sediment discharge at given flow rates is considerably larger than between-site variations in the expected sediment discharge at the flow rate. The mean annual sediment load was estimated to be 3.09 Mt for the Fitzroy at the Gap for the 30-year period (see Table below). Most of the sediment of the Fitzroy comes from the Nogoa and Comet subcatchments. The combined loads of the four major tributaries are consistent with the total load estimated for the Fitzroy at the Gap. Load estimated for the MacKenzie site appeared to be low. At given level of runoff, sediment concentration on average was highest for the Nogoa and Comet subcatchments, and the lowest for the Isaac. An intensified period of water quality monitoring since 1993 coincided with a period of relatively low streamflow. Flow for the period 1994-2003 was about half, on average, of the 30-year period investigated.