The Murray-Darling Basin experienced the wettest November on record, prompting an increased risk of water quality issues as temperatures rise over summer.
MDBA Senior Director of Environmental Management Dr Janet Pritchard said it was the first time for many years that water had filled the Basin's rivers and dams and extended to some of the far floodplains.
"While it's great to see full storages and the floodplains and wetlands getting a much-needed drink, it also means the water moving through the landscape will transport a significant amount of organic matter into the rivers. This is a natural process that helps to grow the plants and animals in our rivers, but the consequences can be far-reaching," Dr Pritchard said.
"There is a greater risk of hypoxic blackwater events as leaf litter and other carbon-based debris is swept into the waterways. This can suck the oxygen out of the water, making it difficult for fish and other aquatic animals to survive.
"In the past 20 years, large-scale hypoxic blackwater events have been associated with late spring or summer floods following drought in the Murray, Lower Darling and Murrumbidgee rivers, and this season the risk will affect northern Basin Rivers such as the Barwon-Darling as well.
"As temperatures start to climb in the weeks ahead, the possibility of hypoxic blackwater will increase if conditions stay wet under the La Niña weather pattern. Warmer temperatures will favour the growth of blue green algae, and with higher river flows we will be on the look-out for elevated salinity levels as well," Dr Pritchard said.
Governments and water authorities are working together to manage the unfolding conditions as part of normal business. River managers, scientists and environmental water holders are monitoring rivers for water quality risks and will work to reduce the impacts of hypoxic blackwater where they can.
Director of Water Planning Implementation for the Department of Planning, Industry and Environment, Allan Raine said NSW agencies were keeping a close eye on conditions and advised people who witness blackwater to notify the department.
"So far, dissolved oxygen levels remain safe for fish and other aquatic animals, but this could quickly change as we see temperatures rise," Mr Raine said.
"In many cases mitigation measures to resolve blackwater events may not be possible. Water agencies, including environmental water holders, will work together to identify any options that may be able to improve water quality."
Current watchpoints being monitored for potential blackwater events in NSW include:
- Barwon-Darling downstream of tributaries in flood such as the Namoi
- Lower Darling 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
- Edward Wakool system in the region of Deniliquin and Moulamein
Mr Raine said dissolved oxygen levels in the Edward River have been slowly declining over recent weeks, possibly due to return flows from flooded areas of the Barmah Forest.
Another area we are keeping a watch on is the lower Darling, with releases from Menindee Lakes expected to flood areas that haven't see flows for 10 years or more. As the releases subside, they will bring some of that leaf litter that has accumulated over many years, back into the river," Mr Raine said.
In Victoria, Executive Director of Catchments, Waterways, Cities and Towns with the Victorian Department of Land, Water and Planning, Deb Brown said winter-spring watering events had helped to move carbon and other nutrients off the floodplain when the weather was cooler.
"Water managers are carefully managing return flows into a high River Murray where possible, so there is little risk of impacts on the Murray under current conditions.
"Rainfall and catchment conditions are being watched closely as we head into the warmer summer months, with significant rainfall events resulting in an increased risk of a large scale low dissolved oxygen event."
To notify the NSW Department of Planning Implementation for the Department of Planning, Industry and Environment of potential blackwater events email [email protected] or to report a fish death event call the NSW Fisheries Hotline on 1800 043 536.
Community members in Victoria can report fish deaths to the EPA's Pollution Hotline on 1300 372 842 (1300 EPA VIC).
