Mass fish-kill events in Australian rivers are often linked to sudden declines in dissolved oxygen during low-flow periods when rivers contract into isolated or weakly connected waterholes (Sheldon et al. 2022). Persistent thermal stratification can isolate deeper water from atmospheric oxygen exchange, promoting hypoxia, while subsequent mixing can redistribute oxygen-depleted water through the water column (Boys et al. 2022). However, stratification and mixing dynamics in river waterholes remain poorly understood (Zhai et al. 2023).
A floating monitoring platform was deployed in a waterhole in Deep Creek, Victoria, Australia, to collect minute-scale measurements of water-column temperature, dissolved oxygen, light penetration, and meteorological conditions. Water density and buoyancy frequency were calculated from temperature profiles to quantify stratification strength and identify stratification and mixing events. Detected events were examined in relation to flow and atmospheric forcing, while stratification duration was compared with dissolved oxygen deficit.
Results showed a seasonal transition from weak and intermittent spring stratification to stronger and more persistent summer stratification as flow approached stagnant conditions. Under stagnant flow conditions, intermittent mixing was associated mainly with surface cooling and, to a lesser extent, wind forcing, and was commonly followed by rapid re-stratification. Persistent stratification was associated with deep-water oxygen depletion, with dissolved oxygen deficit increasing by approximately 7% saturation per day of stratification and frequently reaching hypoxic conditions.
High-frequency monitoring and event-based analysis can help managers identify periods of elevated hypoxia, target monitoring around likely mixing events, and detect developing hypoxia early enough to support timely water-quality management.