Double trouble: how floods after bushfire affect the health of our rivers

Source: Jason Alexandra & C. Max Finlayson (2020). Floods after bushfires: rapid responses for reducing impacts of sediment, ash, and nutrient slugs, Australasian Journal of Water Resources, DOI: 10.1080/13241583.2020.1717694


Between Christmas 2019 and the  2020 New Year, forested mountain ranges across drought-stricken areas in Eastern Australia came alight, with fires ravaging 11 million hectares of bush (Eucalyptus woodlands and rainforests) – a size comparable to England’s land area. These megafires threw the states of New South Wales and Victoria into a state of emergency. The bushfire crisis took a sudden turn when heavy rainfall flooded the scorched land in the span of just two weeks. Unfortunately, while rainfall might appear to be a blessing in light of the megafires, the resulting floods were ultimately not sweet relief for rivers. 

Satellite view of bushfires burning across eastern Australia in December 2019. Source: NASA Earth Observing System Data and Information System (EOSDIS).

Authors Alexandra and Finlayson, researchers from CSIRO, recently published a perspective review on the damaging impact floods following bush fires are likely to have on Australia’s precious river systems. This piece was written in response to the catastrophic bushfire events that tore through Australia’s eastern coastline during the 2019-2020 holiday season. Although the bushfire crisis is now stable, a series of flood events across burnt catchments, natural areas of land which collect water, may lead to a water crisis.


Slugs of nutrients, sediments and ash

The major concern for creeks and rivers connected to burnt catchments is the development of sediment ‘slugs’ that form following heavy rainfall. Sediment ‘slugs’ are large amounts of soil, debris, and ash that flow through waterways, turning rivers inky black. This happens because soils exposed by bushfires are highly susceptible to erosion, meaning that when it does rain huge volumes of ash-enriched topsoil are whisked away into flowing waters. Additionally, most of Australia’s bushfires were located in mountain ranges and these steep slopes increase the severity of sediment ‘slugs’ flooding into rivers. The paper by Alexandra and Finlayson summarized three major concerns associated with these phenomena.


Firstly, burnt forest catchments greatly increase the risk of flooding. When rain falls heavily, higher water flows are generated in comparison to unburnt catchments due to the lack of ground cover that usually slows water movement. These higher flows collect even greater amounts of soil and debris from the burnt landscape which causes even more water issues. 

Heavy rains in burnt forests cause large amounts of ash and topsoil to wash into rivers. Source: US Department of the Interior
Water pollution 

Rivers flowing through burnt catchments will experience the direct effects of degraded water quality as a result of nutrient enriched ash. Because rivers are the hydrologic ‘arteries’ of Australia’s landscape, the water pollution problem becomes far reaching. 

Post-fire runoff can increase river nutrient concentrations by 100-fold, triggering toxic algal blooms. These waters can feed into wetlands, lakes, and estuaries which are important habitats for native fauna and flora. The increased sediment load turns water into a dense brown-black cloud that can smother aquatic vegetation, which is a key ecological element that helps to filter out impurities and reduce nutrient concentrations. These adverse effects on water quality end up contaminating urban water supplies which then requires costly and intensive treatment to make water suitable for drinking again. Australia’s largest metropolitan, Sydney, had 80% of its water supply catchment burnt during the latest bushfire event and could potentially be facing drought level water shortages if heavy rainfall occurs. 

Flooding of sediment polluted water in a recovering burnt forest (Martin’s Creek Flora and Fauna Reserve, Victoria). Source: US Department of the Interior
Fauna at risk 

Rivers charged with sediment and nutrient ‘slugs’ threaten populations of fish, invertebrates, and iconic wildlife like the platypus. Lack of oxygen in the water due to algal blooms fuelled by excess nutrients are the main culprit of fish kills, but increased levels of heavy metals such as mercury in sediment ‘slugs’ also pose a threat. Some catchments burnt from the recent bushfires host rivers that provide critical habitat for endangered fish species such as the Clarence River Cod. Sadly, in these rivers translocation of fish before a runoff event is likely the only way to ensure survival of the species.

Long term ecological impacts

Unfortunately, catastrophic flooding coupled with fire events can have a long lasting impact on the health of affected waterways. The review highlighted some past studies that have documented the recovery of fish populations post fire disturbance and found signs of population recovery after two years. An overall decrease in normal water flows is also anticipated in burnt catchments due to increased water loss through evaporation and these flows may take decades to recover. However, the intensity and close timing of these two disaster events is something new to Australia’s ecosystems and predictions remain unclear. Researchers are currently monitoring the recovery of impacted river systems and these observations will prove critical to informing preventative and response plans in the coming years.

Urgent policy action is needed

We need to plan ahead to avoid a bushfire crisis becoming a water crisis, and quickly. The authors highlighted urgent actions such as fish rescue programs that are needed to ensure the survival of endangered species. Due to the scale of the bushfire water crisis, rescue missions will be limited to priority areas where species risk is highest. An increase in water quality surveillance is also urged to detect early warning signs of toxic algal blooms in affected water supply areas.

Fire and flood events will continue to occur, so long term planning is necessary for proactive action. A focus on catchment-scale management is key to reduce impacts due to the interactive nature of rivers across landscapes. Policies need to be developed that ensure that longer term preventative measures are enacted. Programs should focus on erosion control in areas near drainage culverts by revegetating riparian areas along river banks and reassessment of infrastructure and treatment requirements to ensure urban water supply is minimally affected by bushfire-flood events.


Cover image: Burnt hills from Australia’s 2019-2020 megafires in Victoria. Source: U.S. Department of the Interior.

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Jackie Webb

I’m an environmental scientist specializing in issues relating to water quality of aquatic systems in agricultural landscapes. My interests resides in ecosystem biogeochemistry, with a focus on hydrological monitoring, carbon and greenhouse gas accounting, and development of quantitative models to solve environmental issues. I gained my PhD from Southern Cross University in Australia, where I studied terrestrial and aquatic carbon cycling in agricultural floodplains. I am particularly interested in the broader ecological importance of artificial waters that play a critical role in water resources for agricultural and urban areas. My postdoctoral research involved working on greenhouse gas and carbon accounting in agricultural dams. I'm currently working as a Research Fellow at Deakin University, in rural NSW (Australia). Developing new collaborations and pursuing underrepresented ecosystems/research topics is something I value the most in my work. When I'm not doing science I can be found enjoying yoga, trail running, swimming, barre, reading, and in the kitchen fermented things! Twitter: @JackieRWebb

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