It led to a warning of a potential hypoxic blackwater event in both systems.
By the last bulletin on December 23, dissolved oxygen at both Toonalook and Deniliquin had improved above four milligrams per litre, reducing the immediate risks to fish health.
‘‘High flows, increasing water temperatures and high loads of leaf litter on the floor of the Barmah-Millewa forests caused dissolved oxygen in the Kolety/Edward River to decline,’’ the bulletin said.
‘‘As flow in the Kolety/Edward River is decreasing, there is less water flowing out into the Barmah-Millewa forests and bringing less organic material back into the main channel.’’
‘Blackwater’ is a term used when high levels of organic material and tannins in a river discolour the water, making it appear black.
The water can then become hypoxic when the material decomposes, reducing the oxygen in the water.
Blackwater events occur during flooding when organic material is washed off the river bank and floodplain and into the river system.
In an effort to improve dissolved oxygen levels, the Commonwealth Environmental Water Office has been delivering environmental water through Murray Irrigation escapes.
At least 200 megalitres of water per day is being released from the Wakool Escape, 500ML/day from the Edward Escape (upstream of Deniliquin) and 200ML/day Niemur Escape (downstream of Werai forest).
Edward Wakool Angling Association scientific officer Dr John Conallin said based on the most recent results, he is confident a hypoxic blackwater event can be avoided in the local area.
But he said careful monitoring is still needed.
‘‘With the water receding at Deniliquin and releases from the escapes, the oxygen levels are starting to increase,’’ Dr Conallin said this week.
‘‘Water levels are coming back to normal and the dissolved oxygen is improving on our end.
‘‘The river levels may start to fall again and with the hot temperature we have to keep monitoring, but at this stage I do not anticipate a blackwater event.’’
Dr Conallin said a dissolved oxygen level of less than 2mg/L can be dangerous for fish health.
At the time of going print yesterday, dissolved oxygen levels in the local area were as follows:
● Edward River at Deniliquin – 6.5mg/L.
● Edward River at Toonalook – 7.1mg/L.
● Edward River at Moulamein – 5.1mg/L.
