Long-term impacts of the ‘Black Summer’ bushfires on water quality and constituent loads in the catchment of a large water-storage reservoir

Context Fires in 2019–2020 burned over 500,000 ha in the upper catchment of Lake Hume, a major reservoir at the confluence of the Murray and Mitta Mitta Rivers. Aims This project assessed changes in key water quality constituents entering the lake following the fires. Methods Weekly and annual loads were estimated for 5 years post-fire at Jingellic (Murray River) and Tallandoon (Mitta Mitta River) and compared with historical loads. Results No fire-related water quality effects were observed at Tallandoon; this was likely to be due to Lake Dartmouth (an upstream storage) buffering poor-quality runoff. At Jingellic, sediment loads remained elevated for at least 5 years, peaking at ~600,000 Mg year−1, which is 15 times the long-term average. Nutrient loads (total phoshorus, total Kjeldahl nitrogen, nitrogen oxides) were up to 3 times higher than normal for 3 years. By contrast, soluble reactive phosphorus and salt impacts lasted only weeks to months. Conclusions These findings highlight that fire impacts on water quality vary depending on constituent type and location in the landscape. Implications Fire impacts on water quality can last for a long time, so studies looking at the impacts should be appropriately designed and resourced, with careful consideration of sampling locations.