High floodwaters that are low in dissolved oxygen levels are continuing to move downstream from the Barwon-Darling River to the Menindee Lakes 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 is broken down in the floodwater or washed off the floodplain into the river.
Allan Raine, Director Water Planning Implementation with the NSW Department of Planning and Environment, said as this material is broken down, it uses oxygen from the water.
"Too much organic material can significantly reduce oxygen levels which can result in fish deaths," Mr Raine said.
"With large volumes of water currently moving across the floodplain, there is often very little that can be done to prevent hypoxic blackwater developing.
"This is typical of the Barwon-Darling River, which has one of the highest flow variabilities in Australia."
Mr Raine said there are reports of blackwater events as far back as the 1870s for this 'boom or bust' river system.
"Floodwaters from heavy rain in the Northern Murray-Darling Basin during November recently peaked at Louth and are continuing to make their way down the Darling River," he said.
"The peak flow is expected to reach Menindee Lakes in the third week of February.
"The scale of this hypoxic blackwater event is substantial and there is a very high risk of fish deaths unless cooler temperatures continue.
"The two major risks over the coming weeks are the floodwaters that are low in dissolved oxygen levels moving into the Menindee Lakes and the potential for floodwaters on the lower Darling floodplain to flow into the lower Darling River.
"As the water moves through the system we are actively trying to mitigate the impacts of low dissolved oxygen in both the Lakes and the lower Darling, while ensuring the Lakes are full at the end of the inflows."
Adrian Langdon, Executive Manager Systems Operations for WaterNSW, said releases from Menindee Lakes into the lower Darling will shortly be reduced.
This will allow low dissolved oxygen water, returning from the lower Darling floodplain, to start mixing so it is diluted before the blackwater front flowing downstream from the Barwon-Darling is released from Lake Wetherell.
"Releases from the other lakes will continue into the lower Darling via Weir 32 to help dilute the water being released from Lake Wetherell and to provide smaller, higher dissolved oxygen, refuge areas," said Mr Langdon.
"The smaller lakes will act as a refuge for fish, particularly Lake Tandure, allowing fish to move into these areas to escape low dissolved oxygen water in Lake Wetherell."
Mr Raine said the impacts of blackwater events are an unavoidable by-product of a much welcomed return to abundant water security.
"The department, together with WaterNSW and other agencies, is continuing to monitor the event closely to actively manage the impacts when it is feasible to do so."
