When it rains after bushfires, the consequences stretch way beyond temporary relief for those on the front line — and scientists are warning of potentially dire impacts on our waterways.
They say the aftermath of the current fires could bring devastation to freshwater animals and plants, as well as drinking water catchments.
This is because, while it may or may not help firefighters themselves, rain inevitably washes the ash and eroded soil from burnt forests into rivers and streams, shifting the bushfire impact to our crucial freshwater ecosystems.
Waterways can also suffer immediately in a fire just from the temperature increase, according to Ross Thompson from the University of Canberra.
Even if water doesn’t boil (which effectively sterilises it), many animals can’t withstand the sudden increase in temperature, and die.
But it’s the longer term effects that have scientists really worried.
Rain washes ash into waterways
Rain events after bushfires can transfer huge amounts of ash, burnt material, soil and dead animals into our rivers and lakes.
“There will be an increase in nutrients because there’s lots of phosphorous and nitrogen in ash,” said Professor Thompson, a freshwater ecologist who has studied water catchments after the 2003 fires in the ACT and the 2009 Black Saturday fires in Victoria.
“You also see more sediment because trees are falling down and the river banks are getting knocked around.”
As the water fills up with fine sediment and foreign, nutrient-rich material, the water quality can drop very quickly — and stay that way for a long time.
“We’re still seeing higher sediment loads coming out of the Cotter River catchment and those fires were more than a decade ago,” Professor Thompson said.
Freshwater animals lose oxygen
As soon as a fire has passed and the ash settles on rivers and lakes, bacteria in the water will start consuming the carbon in that ash.
In the process of breaking down the carbon, the bacteria will also consume the dissolved oxygen in the water. The more carbon, the more oxygen will be taken out of the system.
And most animals and plants can’t survive in such a low-oxygen environment, as was seen in the Murray Darling Basin last summer.
“Even if the fires didn’t burn immediately adjacent to the river.”
Changes in the turbidity, or amount of sediment in the water, is another factor that can threaten aquatic species.
“We see a loss of a lot of invertebrates that rely on really stony streams, because the sediment and ash smothers the rocks and it changes the habitat.”
Rescuing fish from ‘water like licorice’
Brown trout and rainbow trout also don’t like fine sediment, Professor Thompson said — and as exotic species, they are already right on the edge of the temperature they can cope with in Australia.
“Our experience … is that we lose trout populations, which is obviously a concern to recreational fishers and there’s economic consequences,” he said.
“The native fish tend to be a bit more robust. But we do have some quite rare species that won’t cope with fire.
“River blackfish are in a reasonable amount of conservation trouble and they really need vegetated streams, so the loss of habitat is significant.”
After the initial horrors of the Black Saturday bushfires in Victoria in 2009, scientists rushed to capture the surviving threatened fish from affected streams, and got a glimpse of the potential impact on freshwater ecosystems.
“We saw cooked yabbies in some sites in the days after,” said Fern Hames from the Arthur Rylah Institute for Environmental Research (ARI).
But again it was the threat of heavy post-fire rain that was the biggest concern, said Ms Hames, who is a freshwater fish ecologist.
“The rains did come shortly after the fires, and the water looked like molten chocolate, or licorice,” she said.
The team from ARI removed the threatened barred galaxias and Macquarie perch from streams in headwaters of the Goulburn River and kept them in aquaria while their habitats could be restored.
“This project had great resonance for local communities because many people had also lost their homes, relocated to another place while their homes were rebuilt, and then returned,” Ms Hames said.
Drinking water quality affected
Changes to the insects and other invertebrates in an ecosystem, along with the influx of nutrients from the ash, can result in the growth of cyanobacteria — commonly known as blue-green algae (but it’s not actually algae).
Cyanobacteria produce chemicals which may cause a range of water quality problems, starting with poor taste and odour, according to Stuart Khan, an urban water management expert at the University of New South Wales.
“And they’re the things that will require a lot more focus and attention to ensure that we continuing to provide safe drinking water.”
In the short-term, Professor Khan said, damage to water infrastructure poses the biggest risk, with some fire-affected towns already being instructed to boil water, or drink bottled water only.
But in the long-term, he is worried about the stability of large water catchments, particularly the Warragamba catchment.
“Really, if we had an east coast low now, a mudslide would be exactly what I’d be concerned about,” he said.
“You could imagine tons and tons of mud and sediment eroding and being washed into waterways.
“This is the entire Warragamba catchment, and the impacts are inevitable. It’s just a matter of when they’re going to occur.”