NSW’s Menindee Lakes are facing a flood of deadly blackwater. 

Reports say deadly, deoxygenated ‘blackwater’ is making its way across the plains of Far West NSW, heading south down the Darling Baaka River towards Menindee Lakes.

Blackwater is a natural phenomenon caused by post-drought rains soaking plains across the Barwon-Darling catchment. The flows are carrying a decade’s worth of leaf litter and paddock stubble.

When bacteria breaks down the organic matter, it releases significant amounts of dissolved carbon into the water. The carbon removes dissolved oxygen in the water, making it virtually impossible for aquatic creatures to breathe.
Authorities are fighting to minimise losses when the blackwater hits the Menindee Lakes system.

“When they smell the rubbish water coming, fish will shift into a big lake if it’s available, or head up a tributary that might have better water quality,” says NSW Fisheries aquatic habitat rehabilitation manager Iain Ellis. 

“The thing concerning me is once you get past about Bourke you don’t have tributaries contributing to the Darling.

“Water moves slowly here, so fish will have days dealing with the blackwater, even if they try to swim through it.”

They are desperate to avoid a repeat of the 2019 fish kills in the same waterway, when a million native fish perished.

That event was linked to decisions from around 2016, when the Menindee Lake were drained to water downstream reaches of the Murray Darling system. This turned the river below Menindee Lakes had dried into a series of disconnected pools in which algae proliferated, leading to a lower layer of deoxygenated water in the pools.

When rain came through in 2019, it mixed the deoxygenated lower layers with the surface water, effectively deoxygenating the whole pool and killing fish in droves.

Mr Ellis says fish in Menindee Lakes have been showing signs of a promising recovery.

“Another three or four of good years and we’re well on the way to recovery for Murray cod, as nasty as it was,” he says. 

“And those fish can help reseed the Murray River downstream. There’s massive amounts of juveniles in the lakes and the Lower Darling and the Anabranch, and they’ve been moving back upstream in the past 12 months.”

Water agencies are trying to protect this progress by herding the black tide so that it can mix with the sweetwater sitting in the lakes.

Blackwater is already pooling in Lake Wetherell, which backs up behind the gated entry to the Menindee Lakes proper.

River operators are preparing to strategically open that gate and direct water around the other lakes in separate bursts by opening in turn the gates to Menindee, Pamamaroo and Cawndilla lakes.

This may be enough to create refuges for the fish to survive.