Weir stratification and hypoxic water management (2019)

Funding

CEWO $38,728

Researchers/Investigators

Associate Professor Skye Wassens, Dr Damian Michael and  Dr Daryl Baldwin

Research Theme

Environmental Water

Description

On the evening of January 26 to 27, 2019, thousands of dead fish including Murray cod, golden perch, silver perch, bony herring (bream) and carp died in Redbank Weir on the Murrumbidgee River. The fish kill coincided with the last of three major fish kills upstream on the Darling River at Menindee.

Subsequently, ILWS researchers were engaged to conduct a Short Term Monitoring and Evaluation Activity from February 7 to March 18 to monitor dissolved oxygen (DO) levels and water temperature in the Murrumbidgee River. Data analysis and a summary of observations were provided to the Murrumbidgee Technical Advisory Group on a daily basis at the completion of each field trip.
This “real-time” data contributed to daily water management decisions made during the 2019 Murrumbidgee refuge environmental watering action.

The project’s final report determined the proximate cause or causes of the fish kill that occurred at Redbank Weir in late January 2019; examined management responses to the fish kill, specifically the use of environmental water to improve fish habitat and mitigate poor water quality conditions in Redbank Weir and prevent other fish kills in Maude and Balranald Weir pools; reported on monitoring efforts before, during and immediately after the fish kill in order to inform future monitoring programs; and made recommendations on potential ways the river can be operated into the future to minimise fish kills.

Key findings

• The fish deaths in the Murrumbidgee River at Redbank Weir were caused by destratification
• The management decision to use environmental water to allow for the drawdown of Maude Weir pool prevented fish deaths in the weir pool
• The decision to use environmental flows to export hypoxic (low oxygen) water in Redbank Weir out onto the floodplain using existing infrastructure and replace it with water that had a much higher oxygen concentration meant the rainfall rejection flow (water that had been allocated to irrigators but not used because it had rained) that reached Redbank didn’t lead to hypoxia

Recommendations

Following a review of the data collection undertaken as well as the management decisions taken, the report contained 13 recommendations around the issues of monitoring and reporting, river management and knowledge gaps. The three key management recommendations that relate to monitoring and evaluation and river management are:

• The agency responsible for monitoring and assessment of water quality during events like the one that occurred in the lower Murrumbidgee River needs to be identified.  That agency should undertake weekly depth profiles of dissolved oxygen and temperature in the lower Murrumbidgee River weir pools from the beginning of November to the end of March, and transmit results to relevant staff in a timely manner so that the information can be promptly interrogated to inform management - especially if low dissolved oxygen levels are encountered.

• Telemetered temperature probes to be deployed through the water column in each of the four lower Murrumbidgee River weir pools, and the results routinely inspected to determine the extent and strength of stratification.

• A workshop should be held with key stakeholders to determine the best way to manage rain rejection flows in the Murrumbidgee River so as to maximise economic benefit, but at the same time ensure there are no environmental impacts. For example, if in-river water storages cannot be temporarily drawn down during periods where there is an increased risk of destratification leading to hypoxia, then, if present, hypoxic bottom waters need to be managed as a normal part of the weir management.  This may mean the release of hypoxic bottom water to downstream reaches; and if so the impact on downstream aquatic communities and water quality needs to be assessed.

Outputs

Baldwin, D.S. (2019) Weir stratification and hypoxic water management - Murrumbidgee River 2019. A report prepared for the Commonwealth Environmental Water Office. 45 pp

Outcomes

The findings and recommendations from the report will assist water managers in making future management decisions that may mitigate the risk of similar events in the future.

Contact

A/Prof Skye Wassens email

Dr Damian Michael email

Albury-Wodonga

July 2019