The Murray-Darling Basin continues to experience widespread flooding in some areas, prompting an increased risk of water quality issues like low-oxygen blackwater as temperatures increase.
Governments and water authorities are working together to monitor the unfolding conditions which may see low-oxygen blackwater and blue-green algae emerge that can lead to fish deaths and increased water treatment.
Given the scale of natural flooding, prevention is not possible and mitigation options are very limited. Only a handful of localised options to improve water quality can be enacted now.
Murray-Darling Basin Authority Senior Director, Environmental Management, Dr Janet Pritchard said the significant and widespread flooding in the Murray-Darling Basin would see water extending to some floodplains that have not had water on them for several years.
"Hypoxic blackwater events occur as leaf litter and other carbon-based debris is swept into waterways. As this material is consumed and broken down by bacteria, oxygen can be sucked out of the water, making it difficult for fish and other aquatic animals to survive," Dr Pritchard said.
"There are already isolated reports of crayfish exiting floodwaters to escape poor water quality.
"With the onset of warmer weather in the coming weeks, water temperatures will start to climb and the possibility of hypoxic blackwater will increase and unfortunately, we could see fish deaths like we did during the 2011 and 2016 floods.
"In the past 20 years, large-scale hypoxic blackwater events have been associated with late spring or summer floods, particularly in the Murray, Edward-Wakool, Lower Darling and Murrumbidgee rivers. This season the risk will apply to several other systems across the Basin in NSW, Victoria and South Australia as well.
"The high flows and widespread flooding will also add nutrients to waterways, which will favour the growth of blue-green algae with warmer conditions. As floodwaters recede back to normal levels, we will also be on the look-out for elevated salinity levels in some locations."
The NSW Department of Planning and Environment's Director of Water Planning Implementation, Allan Raine, said agencies at both state and federal level were closely monitoring the situation.
"The scale and magnitude of this flooding on public and private property means mitigation methods to get oxygen back into the water are extremely limited. Methods used in drought situations, such as artificial aerators in specific river sections, won't be viable for combating large volume and widespread blackwater events," Mr Raine said.
The current watchpoints for potential blackwater events in NSW include:
- Barwon River between Walgett and Brewarrina
- Murrumbidgee River at Balranald
- Wakool River
- Murray River at various locations
- Little Merran Creek
- Thule Creek
- Barber Creek
Mr Raine said dissolved oxygen levels in these areas had been steadily declining over recent weeks.
"We haven't had any reports of large-scale fish deaths so far, which is good news, but as large amounts of organic material enter rivers from the floodplain, we know that this situation could change, which is why we are watching it closely."
The Commonwealth Environmental Water Holder, Dr Simon Banks said state and federal environmental water managers were working with industry and local communities to divert very small volumes of good quality water from the main river channel, via irrigation channels, and then diverting it back into creeks further downstream where there is poor water quality.
"These small flows can create localised pockets of better water quality for fish and crayfish to move into. This worked well last year and is being used again in the Edward Kolety Wakool region, where natural flooding has already resulted in low-oxygen blackwater," Dr Banks said.
"We are working with local landholders and will avoid delivering water where it will exacerbate the impact of floods."
Who to notify of potential blackwater events:
To report areas in NSW where fish may be struggling or a fish death event has occurred, 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).
