The period of 2019-20 shall be remembered for its devastating Black Summer bushfires across the upper Murray catchment bordering NSW and Victoria in south east Australia. Bushfires in this catchment ended following wide-spread rainfall 20/21 January 2020. Steep terrain within the burnt areas generated runoff loaded with large amounts of ash, sediment and debris, which ended up in the upper Murray and Mitta Mitta Rivers. The cocktail mix of these pollutants threatened aquatic environments of the upper Murray waterways and Lake Hume, a major reservoir of the Murray River.
This presentation includes a detailed analysis of pre- and post-fire long-term water quality data from Jingellic on the Murray and Tallandoon on the Mitta Mitta. Water quality data at both stations represent draining water into Lake Hume. Very high concentrations of suspended solids, nitrogen, phosphorus and dissolved organic carbon accompanied the initial runoff following heavy storms. Concentrations of these pollutants continued to be high in the upper Murray river throughout 2020 and even one year after the bushfires. Concentration spikes were in line with increased flows, which in combination attributed to the main load of pollutants in Lake Hume. In comparison, fire intensity and its spread upstream of Tallandoon but downstream of Dartmouth Dam for the Mitta Mitta River was small, resulting in significantly lower concentrations of pollutants in the runoff.
The increase in nutrient loading and other contaminants in the receiving waters of Lake Hume will impact ecosystem processes over years to come. It can lead to future intensification of harmful algal blooms due to internal nutrient loading from deposited sediments or to oxygen-depleted water and increased levels of iron and manganese, which can then impact downstream drinking water offtake. These lagged effects warrant an intensification of monitoring of sediments and water quality for early warning of potential negative impacts.