Eastern Australia’s subtropical rivers are highly prone to extreme flooding due to climatic conditions, shallow soils, and distinct topography. Two centuries of land clearing have further increased runoff and connectivity between land and rivers, accelerating the transport of sediment and particulate phosphorus (PP). This degrades water quality and increases the risk of harmful algal blooms. Although most sediment export occurs during storm-event flooding, the dominant source processes and hysteresis patterns of sediment and P during extreme events remain poorly understood.
Using grab and rising-stage samplers, we quantified hysteresis loops in suspended sediment and PP concentrations during the flood generated by Tropical Cyclone Alfred. Changes in sediment sources were quantified using soil elemental fractions as a novel geochemical fingerprinting approach. In the Albert River, the Cyclone Alfred flood accounted for ~60% of annual sediment export. Distinct figure-eight hysteresis patterns in sediment and PP concentrations indicate sequential activation of different sediment sources throughout the event. Sediment tracing shows dominance of alluvial bank erosion early in the event, followed by exposed dry sclerophyll forest soils from recent urban development during peak flows. Because peak discharge contributed most of the total load, disturbed soils dominate the overall sediment budget. This contrasts with earlier radionuclide tracer assessments that suggested a predominantly subsurface signal interpreted as bank erosion alone. Elevated exchangeable P during peak flows further indicates inputs from sewerage or latrine overflows.
Overall, these results reveal dynamic shifts in sediment sources during extreme floods, improving process understanding and informing targeted management under increasing climate extremes.