While flood waters look to have peaked in South Australia, authorities are concerned about the potential for a blackwater event that may lead to fish kills along the River Murray.
Flows at the border into South Australia had not increased for the last seven consecutive days with authorities confident peak flows had reached South Australia and would work their way downriver.
But as the extent of the flooding was known, there were growing concerns a blackwater event may follow leading to fish kills as waters recede.
Blackwater events are caused when heavy rains wash leaf litter that has collected on a flood plain into a major river such as the Barwon-Darling, turning it the colour of dark tea.
As this leaf litter breaks down and bacteria decomposes, the organic compounds dissolve. They consume oxygen and dissolve carbon dioxide into the water, making it difficult for fish to breathe. They can also be made more severe by warm weather.
Craig Copeland, the CEO of OzFish, said in New South Wales there had been a delay between flooding and blackwater events that led to fish kills.
“There is still a risk of a fish kill occurring,” Copeland said. “The government in South Australia are aware that that may be the case, as are many recreational fishers who are aware of what’s been happening in NSW and Victoria.”
“Every part of the river system is different, so I don’t exactly know how it might transpire down there, but so far it hasn’t and we can only keep our fingers crossed that it doesn’t.”
Though there were potential environmental benefits from massive flows into parts of the river that have not flooded in decades, Copeland said there were also risks to people and wildlife as “everything that’s on a flood plain ends up in the water”.
“If there’s pesticides on the flood plain, there’s pesticides in the water. If there’s plastics on the flood plain, it’s in the water,” he said.
In November, when Victoria’s Environmental Protection Authority tested flood waters in the state it found E coli and chemicals such as PFAS and pesticides, residents were warned not to swim in the water.
SA Health’s principal water quality adviser, Dr David Cunliffe, said “there is no evidence of increased E coli” but people were advised not to enter flood waters.
“While there is no evidence of increased E coli in flood waters in the Riverland, we advise people to not drink flood water or tank water which may have been contaminated,” Cunliffe said. “All recreational boating and water activities are banned on the river, including swimming, bathing and diving.”
The latest update published to the SA Water website advised there were no impacts to drinking water in flood-affected communities and no known blackwater events, though the potential remains.
A spokesperson for the Department of Primary Industries and Regions South Australia (Pirsa) said authorities were aware of “the potential movement of blackwater into South Australia” and “have been working to monitor and plan”.
“Pirsa has an emergency response plan in place to respond to large-scale fish kill events. Pirsa will lead coordination of clean-up operations and the disposal of fish carcasses,” they said.
“Fish kill clean-up operations will be assessed on a case-by-case basis and will aim to reduce the risk of adverse socio-economic, human health and environmental impacts.”
Further up river, the Mildura councillor Jason Modica, who chairs Region 4 of the Murray Darling Association, said conditions were beginning to ease but residents had noticed issues.
“The water’s been dark for quite sometime,” he said. “There’s been some people that have seen small spots of dead fish but nothing like the fish kills that have been seen in Menindee three years ago.”
He said any such event would be concerning as native fish populations had declined to “about 10%” of what they were at the time of European colonisation.