Sediment sources and channel dynamics, Daly River, Northern Australia

The Daly River occupies a mainly undisturbed large catchment in the Australian wet–dry tropics. Concerns about possible increased sediment input to the River from clearing and cropping have motivated this study of fine sediment sources. Using geochemical tracers for both modern sediments and alluvial bench deposits, it is shown that, for the last ~ 30 years, 89–97% of the fine sediment originates from erosion by gullying and channel change. There is no discernible input of top soil from the cleared land adjacent to the Daly River in the study area. The analysis and OSL dating of the alluvial benches have also provided data on the age of (and inferences about the causes of) bench formation, flood frequency change, sedimentation rate change, and episodes of sand transport. The benches are being destroyed as the channel widens (contributing sediment to the river) and the bed of the Daly appears to be shallowing, both responses to increased overbank flows. The sediment source created by channel widening is almost all the result of hydrologic change, with no discernible role for land use.