TRaCK: Tropical Rivers and Coastal Knowledge

Science and knowledge that governments, communities, industries for sustainable use of Australia's tropical rivers and estuaries

A catchment sediment and nutrient budget for the Mitchell River, Queensland

TitleA catchment sediment and nutrient budget for the Mitchell River, Queensland
Publication TypeReport
Year of Publication2010
AuthorsRustomji, P, Shellberg, JG, Brooks, AP, Spencer, J, Caitcheon, G
Date Published12/2010
InstitutionCSIRO Water for a Healthy Country National Research Flagship
Keywordserosion, mitchell river, nutrients, sediments, water quality

This report presents a sediment and nutrient budget for the Mitchell River catchment, located on the western flanks of Cape York and draining to the Gulf of Carpentaria. A catchment sediment (or nutrient) budget accounts for the major sources, transport pathways and sinks of sediments and nutrients within a catchment. The model predicts budgets for the fine suspended (silt, clay) washload and coarser sand-sized bed material load, as well as nutrients. The model has been calibrated to empirical data which is critical to obtaining sensible model results.

The model predicts 2.9 Mt per year of fine suspended sediment (silt and clay) washload export from the Mitchell River outlet. Alluvial gully erosion is predicted to be the dominant fine sediment source and is spatially restricted to the floodplains and megafan areas adjacent to the main channels once the river leaves the bedrock uplands. Previous SedNet applications in tropical regions have not explicitly represented alluvial gullies as a sediment source, yet based on this study these features appear an important sediment source. Contemporary fine sediment yields are estimated to be approximately twice those of pre-European settlement conditions, although again dating of historical rates of sediment accumulation on floodplains could provide empirical insights into this prediction.

Modelling of bed material load indicated that response times in the lower catchment to variations in upstream coarse sediment input exceed 100 years. This implies that in terms of bed material (sand sized particles and larger), the full effects of the introduction of European land management practices in the upper catchment may not yet be completely manifest.
This modelling exercise has highlighted some major knowledge and data gaps that limit our ability to better predict contemporary sediment dynamics within the Mitchell River catchment (which is comparatively rich in data) and other similar tropical savannah catchments in northern Australia. These are listed in the report.

We regard these model runs as a starting point around which a future program of field data collection should be framed to test hypotheses developed here. We also strongly advise that other future sediment budget studies in tropical Australia should use a combination of geochemical tracer data and station-based load estimates to constrain the modelling.