River Murray fish kill concerns grow as flood waters peak in South Australia

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.

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.

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.