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Hypoxic blackwater in western and southern NSW

The Department of Planning and Environment (DPE) is warning hypoxic blackwater conditions are occurring along the Barwon-Darling River, which have impacted fish, causing some recent deaths.

Low or zero dissolved oxygen levels have been recorded as high flows coming off the northern floodplains are moving down the river. Combined with high water temperatures, this could put fish and other aquatic organisms at risk if they cannot move away to other areas of better-quality water.

“NSW has experienced one of its wettest Novembers on record and the Bureau of Meteorology predicts that above average rainfall across eastern Australia could continue until late summer or early autumn 2022,” DPE Director of Water Planning Implementation Allan Raine said.

“Flood events may cause short-term impacts to water quality, which can impact fish. Despite the current risk, flooding is a natural occurrence and provides long-term benefits to native fish populations by providing opportunities for movement, reproduction and supporting food webs.

“We’ve started seeing some fish deaths in the Barwon-Darling, between Brewarrina and Bourke, and poor water quality conditions look set to continue.”
Mr Raine said that there are limited options to manage hypoxic blackwater events, particularly in the Barwon-Darling where the sheer volume of water is so large. However, some mitigation will occur when river inflows reach the Menindee Lakes and mix with better-quality water.

As water arrives at Menindee Lakes, WaterNSW will work closely with Department of Primary Industries (DPI) Fisheries to monitor the situation and WaterNSW will manage releases to reduce the impacts of the hypoxic water downstream as much as possible.

Low levels of dissolved oxygen have also been recorded in the Kolety/Edward River in the Murray. To provide fish refuge, environmental water holders are delivering water through irrigation escapes, which deliver pockets of high dissolved oxygen.

Other areas being monitored for potential blackwater events in NSW include:

  • Lower Darling/Baaka River downstream of the Menindee Lakes
  • Lachlan River downstream of Forbes
  • Murray River communities from Tocumwal to the South Australian border,
  • Murrumbidgee River downstream of Narrandera, including the Lowbidgee floodplain from Maude to Balranald
  • Yanco and Billabong Creeks

“Agencies will continue to work together to monitor the situation over summer, and to assess available management options,” Mr Raine said.

To report areas where fish may be struggling or a fish death event has occurred, call the NSW Fisheries Hotline on 1800 043 536.

More information on the cause of hypoxic blackwater, the associated risks and information on recent dissolved oxygen levels in the Barwon-Darling and Edward River can be found on the department’s website.