The Provenance of Sediment in Three Rural Catchments in South East Queensland, Australia

Abstract

Moreton Bay, in South East Queensland, Australia, is a Ramsar wetland of international significance. A decline of the bay’s ecosystem health has been primarily attributed to sediments and nutrients from catchment sources. The Healthy Country (HC) project, a Queensland Government funded ‘proof of concept’ initiative, is dedicated to reducing rural diffuse sediments and nutrients entering waterways and ultimately Moreton Bay. Three focal catchments, Blackfellow Creek, Knapp Creek and the Upper Bremer River, were selected to trial rehabilitation techniques for the region. Sediment budgets for these focal catchments, developed using catchment scale modelling (SedNet), indicated gully erosion dominates the supply of sediment in Knapp Creek (90%) and the Upper Bremer River (80%) whereas erosion from cultivated soils is the primary sediment source in Blackfellow Creek (40%). These sediment budgets, that include predictions of both the spatial sources of sediment and the primary erosion processes, form a series of hypotheses in each focal catchment that are tested in this thesis. Fallout-radionuclide activity concentrations (137Cs and 210Pbex) of in-stream sediment sampled in each focal catchment were used in conjunction with regional source data to test hypotheses pertaining to dominant erosion processes, specifically the relative sediment contributions from surface and subsoil (gully and channel bank) sources. Distribution modelling results indicated subsoil erosion processes dominate the supply of sediment in both Knapp Creek (100%) and the Upper Bremer River (100%). In Blackfellow Creek, the similarity of cultivated (surface) and subsoil source fallout-radionuclide activity concentrations resulted in no discrimination between these sources that were modelled in combination to contribute >90% of sediments. Importantly there was no discrimination between gully and channel bank erosion sources in Knapp Creek and the Upper Bremer River.