Drought, megafires, and flood – citizen scientists reveal impact on river water quality

Researchers from Southern Cross University led a unique study in collaboration with a dedicated team of citizen scientists to help monitor how these climate extremes impact river water quality.

Professor Scott Johnston, a Landscape Hydrogeochemist from the University’s Faculty of Science and Engineering has overseen this water quality monitoring project in the Macleay River since 2016. The Macleay is a large coastal catchment in Northern New South Wales that stretches across the Great Dividing Range from the tablelands near Armidale to the coast at Kempsey and South West Rocks.

“We collaborated with a trained group of local citizen scientist volunteers who were able to regularly collect river water samples, capturing what took place at a level of detail that is really quite unique,” he said. ‘’Without their hard work on the ground, this study would not have happened and it is a great example of a University and community working closely together to help understand a locally relevant issue.’’

Technical and Laboratory Officer Roz Hagan sampling water in the Macleay River

The study examines the combined impacts of drought, fires and floods on river water quality, measuring the concentrations, discharge and fluxes of suspended sediment, major ions, dissolved organic carbon and nutrients, such as nitrogen and phosphate.

“Some of the biggest impacts associated with the fires were in the forested mid to upper catchment areas where we saw extensive soil erosion due to the large area of vegetation ground cover burnt. There was a lot of ash and unconsolidated soil material which was highly erodible,” Professor Johnston said.

“Fires not only cause soil erosion, but they also transform soil nutrients into very soluble forms, so they can be readily transported into rivers after rainfall. One of the first things that happened in the initial rainfall after the fires were large fish kills in the upper section of the river. They were some of the biggest fish kills that have ever been documented in the Macleay,” he said.

Locals in the Macleay catchment area described the water coming down the river in the first few days of rain following the fires as ‘like a cake mix’.

“The sediment, ash and elevated nutrients in the water was through the roof, which is clearly what our data shows. This led to low oxygen conditions and the resulting fish kill,” Professor Johnston said.

‘’Over the following months the river water quality gradually recovered, with most parameters returning to pre-fire levels within about 3-12 months,’’ said Professor Johnston.

According to Professor Damien Maher, an expert in hydrobiogeochemistry at the University, it is generally uncommon for such large fish kills to happen in the upper catchment area of a river, except in very extreme drought and bushfire.

“Fish kills in the lower estuary are reasonably common after flooding. However, fish kills of this size in the upper catchment area due to the bushfires is something which you don’t see very often,” Professor Maher said.

Study details
Drought, megafires and flood - climate extreme impacts on catchment-scale river water quality on Australia's east coast
By Scott Johnston and Damien Maher
Published in Water Research
DOI: https://doi.org/10.1016/j.watres.2022.118510