Murray-Darling Basin Authority’s 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,” Dr Pritchard said.
“This is a natural process that helps to grow the plants and animals in our rivers, but the consequences can be far-reaching.
“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 lookout for elevated salinity levels as well.”
Governments and water authorities are working together to manage the unfolding conditions as part of normal business.
Waterways being monitored for potential blackwater events in NSW include:
- Murray River from Tocumwal to the South Australian border;
- Murrumbidgee River downstream of Narrandera;
- Edward-Wakool system in the region of Deniliquin and Moulamein;
- Barwon-Darling downstream of tributaries in flood such as the Namoi;
- Lower Darling River downstream of the Menindee Lakes; and
- Lachlan River downstream of Forbes.
NSW Department of Planning, Industry and Environment’s Allan Raine said dissolved oxygen levels in the Edward River had been slowly declining over recent weeks, possibly due to return flows from flooded areas of the Barmah Forest.
In Victoria, the Department of Environment, Land, Water and Planning’s 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,” she said.
“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 agencies of potential blackwater events, email waterqualitydata@industry.nsw.gov.au 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).
WET WEATHER WARNING
La Niña conditions in summer will increase the risk of further floods.
With the forecast pointing to continued wet conditions, communities should be alert to the risk of flooding through summer as catchments and dams remain near maximum capacity.
According to the Bureau of Meteorology, the Murray-Darling Basin’s previous November rainfall record was set in 1924.
For Australia as a whole, 2021 was the wettest spring since 2010, breaking the previous record set during a La Niña in 1973.
WHAT IS BLACKWATER?
Dark or tannin-coloured water is natural and indicates nutrients present in the water, which is an important part of river health.
It can be a good food source for waterbugs, fish, frogs, birds, platypus and turtles.
It becomes a problem when carbon levels are so high that the process of decomposition sucks up the oxygen that fish and other creatures rely on.
More information on blackwater can be found at: https://www.mdba.gov.au/issues-murray-darling-basin/blackwater
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