The magnitude of sediment delivery within a rivers drainage basin on the tropical coast of north east Queensland

The 530 km2 South Johnstone River catchment was the focus of a detailed investigation into sediment erosion and depositional processes. The area is subject to some of the highest rainfall totals in Australia with very intense storms and tropical cyclones. The catchment is divided almost equally into three topographical regions, the eastern coastal plain, the lower erosion surface and the middle erosion surface. The latter comprise granite, metamorphic and basalt bedrock forming a deeply incised, mountainous region abutting the Atherton tableland. The coastal plain is up to 14 km wide and comprises rich cropping soils derived from alluvium and basalt. This study aimed to determine the sediment delivery from the catchment and erosional and depositional processes occurring within the catchment. Methods used in the is study included sediment sampling and grainsize analysis, sediment textural analysis, water sampling for suspended sediment, catchment scale computer modelling and GIS applications. Results obtained from this study include the determination of a catchment sediment delivery ratio of 52 % comprising greater than 80 % silt and clay sized sediment. Long term average catchment sediment yield is 165900 t a-1• Mean catchment erosion rate is 3 t ha-1 a-1• These findings are significantly higher than previous estimates of catchment sediment yield. The high content of fine grained sediment b.eing delivered from caneland catchments poses a significant offshore risk in terms of water quality and is a potential hazard to the Great Barrier Reef