Blackwater widespread after River Murray floods

The Murray-Darling Basin Authority says river users may notice a darker water colour in parts of the River Murray and its tributaries over the coming weeks.

Flood waters during spring have mobilised large amounts of organic matter, such as leaves and wood, from the forest floor and floodplain. This organic matter is now decaying, resulting in low dissolved oxygen levels which may in turn cause fish to die.
 
The Murray-Darling Basin Authority's (MDBA) head of River Management, David Dreverman, said blackwater events like this occur naturally. "The high level of carbon and low levels of oxygen in the water due to the load of decaying organic matter is not unexpected," said Mr Dreverman.

"River users may also notice dead or distressed fish as a result of the sudden drop in oxygen levels." Mr Dreverman said some higher parts of the floodplain, particularly in the Edward and Wakool systems, may not have been flooded for more than 20 years, adding to the high carbon load.

"With high carbon loads and floods over this extent of flood plain, it is possible that impacts will be widespread, extending downstream, potentially into South Australia.
"In most cases it is not possible to dilute the blackwater, but we are working closely with state agencies and environmental water holders to monitor the situation and will identify opportunities as they arise. That includes looking at whether better quality water can be delivered to affected areas to create local refuges with increased oxygen levels," Mr Dreverman said.

Similar blackwater responses could be occurring in tributaries such as the Goulburn and Murrumbidgee valleys.

More information on blackwater can be found at mdba.gov.au/managing-water/water-quality/blackwater. To report sightings of stressed or dead fish, contact the following state authorities:

Victoria — Environment Protection Agency 1300 372 842
New South Wales — NSW Fisheries Fishers Watch 1800 043 536.