Risk of hypoxic blackwater event in inland rivers

Dissolved oxygen levels are declining in some inland NSW rivers as the weather starts to warm up, increasing the risk of fish deaths.

Hypoxic, or low oxygen, blackwater is a natural feature of Australian lowland river systems and occurs when high volumes of organic material, such as sticks, leaves, bark and grass are broken down in the floodwater or washed off the floodplain into the river.

Allan Raine, Director of Water Planning Implementation with the NSW Department of Planning and Environment, said as the material is broken down, it uses oxygen from the water.

“Too much organic material, coupled with increasing water temperatures, can significantly reduce oxygen levels,

which can result in fish deaths,” Mr Raine said.

“We are seeing declining dissolved oxygen levels in the Barwon Darling near Walgett, Lowbidgee (at Balranald), as

well as the Edward-Kolety, Wakool and Niemur systems in the Murray.”

NSW and Commonwealth water agencies are working closely together to monitor the situation.

Environmental water holders have commenced some releases to provide potential refuge for native aquatic fauna in the Edward-Wakool area.

These deliveries will provide some fish refuge, but mitigation of hypoxic blackwater in the mid-Murray system is not possible.

“With large volumes of water currently moving across the floodplain, there is very little that can be done to prevent hypoxic blackwater developing,” Mr Raine said.

“The impacts of blackwater events are an unavoidable by-product of a much welcomed return to abundant water

supply and ongoing heavy rainfall.”

To notify the Department of potential blackwater events please email: waterqualitydata@dpie.nsw.gov.au or waterqualitydata@industry.nsw.gov.au

To report a fish kill, contact the ‘Fishers Watch’ phone line on 1800 043 536.

Further information on this blackwater event is available on our website at: Hypoxic blackwater