Tracing Sediment and Phosphorus Source Hysteresis During an Extreme Flood in a Subtropical Australian River

Abstract. Extreme flood events are increasingly recognised as important drivers of suspended sediment (SS) and particulate phosphorous (PP) transport. However, their dynamics remain poorly understood due to the stochastic distribution and limited high-frequency sampling of these events. This study is the first to combine high-resolution rising-stage sampling, hysteresis analysis, and a novel sediment fingerprinting approach based on elemental fractions to quantify the changes in SS and PP sources during the 2025 Cyclone Alfred flood in a subtropical Australian catchment. Results show a pronounced figure-eight hysteresis in both the SS and PP concentration-discharge relationship, characterised by an early first flush and a secondary peak at highest discharge. Sediment tracing shows a shift in source contribution, with initial dominance of alluvial bank material (60–80 %) transitioning to hillslope sources (~70 %) near peak discharge. Despite its short dominance, hillslope material contributed ~60 % of the total storm event sediment load, compared to ~30 % from alluvium sources. In parallel, PP shifted from predominantly oxide-bound forms during the rising limb (~44–75 %) to exchangeable (bioavailable) at peak flow (~70 %), wherein the latter contributed ~53 % of total event PP export. The Cyclone Alfred event alone accounted for ~57 % of annual sediment yield and ~65 % of annual particulate P export. Extreme events thus non-linearly activate distinct SS and reactive PP sources and dominate yearly PP and SS loads. These findings highlight the need to extend monitoring and management strategies to hillslope sediment mobilisation and identifying the sources of nutrient pulses under intensifying urban development and climate extremes.