26th March 2020
Managing resumption of flows in the Lower Darling River
How environmental benefits and water quality risks are considered when
resuming flows in the Lower Darling River
Key points
Releasing water to the Lower Darling when the Menindee Lakes receives inflows
maximises benefits to native fish populations. These flows dilute water that is often saline,
contains algal blooms and may be dangerously low in dissolved oxygen. The flows also
provide nutrients critical for native fish survival and breeding.
Using small, slow moving releases to resume flows can cause poor water quality leading to
fish kills.
Higher volume resumption flows are more likely to quickly break-up harmful algal blooms,
replenish low oxygen pools and dilute highly saline water in isolated pools than smaller,
slow moving flows.
Reducing the time fish experience additional stress caused by poor water quality reduces
the risk of fish kills during a resumption flow event. Deeper, fast moving flows can also
provide an opportunity for fish to move to locations with more favourable conditions.
Even with relatively high pulse flows, there are still some temporary risks to fish as the
reconnection of pools may result in the mixing or turn-over of lower quality water through
the water column.
Why are resumption flows important?
Some rivers in the Murray-Darling Basin occasionally stop flowing during the warmer and drier
seasons. Water extraction and diversion into storages has extended the time between flow events,
especially during drought. When surface flows stop, rivers retreat to a series of isolated pools that
play a critical role as wildlife refuges and water supply for local communities.
The first flow of water after a dry period has important social, cultural and environmental benefits.
After a drought the first flow provides critical relief, but after a particularly long and dry period may
pose environmental risks. Water quality in isolated refuge pools can deteriorate, with thermal
stratification, oxygen depletion, algal blooms and concentration of salts.
The size and timing of flows is particularly important in the Lower Darling River where resumption
flows rely on managed releases from the Menindee Lakes. The key environmental issues for the
Lower Darling River are minimising water quality risks and supporting native fish populations.
Managing water quality risks from resumption flows
The release of water from Menindee Lakes into the Lower Darling River could result in water
quality impacts downstream as the river is ‘restarted’. Resuming flow may flush high salt loads
down the river and into the Lock 10 weir pool at Wentworth. It can also mobilise large amounts of
organic material as the head of the flow progresses downstream. This can result in short term
hypoxia (low dissolved oxygen). There is also a risk restarting the river will de-stratify isolated
refuge pools, increasing the risk of low dissolved oxygen water from the bottom of pools mixing
through the water column causing fish kills. Many of these risks can be mitigated.
In February 2004, water was released after an extended drought to restart and reconnect pools in
the Lower Darling River. Small volumes were initially released and gradually increased to
approximately 300 megalitres (ML)/d. The flows were large enough to de-stratify pools, creating
hypoxic conditions, but were not large enough to dilute and refresh the pools. As a result,
significant fish kills occurred. A subsequent much higher flow of 6,500 ML/d was then released to
flush poor quality water from the pools and reduce the impact.
Between 2004 and 2016 the river dried on several occasions and flows of between 1,750 and
4,500 ML/d were successful in managing harmful algal blooms, replenishing low oxygen pools and
reducing pool salinity. The flow volume water mangers used for each event depended on the
season. Only minor fish kills were linked to these flows.
Supporting native fish recruitment with resumption flows
The Lower Darling River extends over 500 km and is home to over a dozen native fish species
including golden perch, silver perch and Murray cod. These large iconic species will spawn and
migrate in response to large flow events, but research suggests that spawning is most successful
when flowing waters are rich in nutrients. When flow events in the Barwon-Darling are large
enough to recharge the Menindee Lakes, the water is extremely rich in nutrients and aquatic life.
In 2014, a flow of 150 and 200 ML/d over several weeks was released to top up two temporary
block banks in the Lower Darling. Monitoring detected small numbers of Murray cod larvae during
this period.
In spring 2016, after significant flows into the Menindee Lakes, a flow pulse reaching 6,000 ML/d
was released into the Lower Darling River. This was followed by a base flow to maintain
connectivity to stimulate fish spawning and movement. Following this event, very high numbers of
Murray cod larvae were detected indicating strong breeding in the Lower Darling. The flow also
allowed large numbers of golden perch juveniles to disperse into the Lower Darling and Murray
rivers. These juveniles had colonised the Menindee Lakes following flood spawning in the Northern
Basin.
Native fish were also found to move from the River Murray into the Darling and from the Darling
into the Murray. This shows a critical breeding pathway for native species and demonstrates the
importance of connectivity within and between different rivers for native fish populations.
Timing and size of current releases
Releases from Lake Wetherell to the Lower Darling will increase from today – Thursday 26 March –
commencing at 500 ML/day and ramping up to a peak of 3,000 ML/day. Releases will be held at
3,000 ML/day for 7 days and then reduced by 250 ML/day back to a flow of 300 ML/day. This will
create the pulse flow necessary to manage water quality and associated fish risks.
Click here to read the whole announcement
19th March 2020
Lower Darling River releases
On 10 March 2020 water began flowing into Lake Wetherell – the first storage in the Menindee
Lakes system. This was as a result of natural inflows from rainfall in the Northern Basin in late
January and February. Temporary water restrictions were used to protect the first flows in these
NSW waterways, some of which had been dry for many months.
As at 19 March Lake Wetherell had joined with Lake Tandure and was holding 85 gigalitres (GL)
and inflows of around 10 GL per day are expected for the next few weeks. WaterNSW is
forecasting some 275 to 340 GL in total to enter the Menindee Lakes from current flows in the
system upstream. More inflows will arrive if further rainfall occurs upstream.
Managing releases
Volumes of well over 200 GL will now ensure the needs of the Lower Darling River can be met for
the next 12 to 18 months with careful management. This will meet all high priority needs along the
river, plus general security carryover water already in accounts. At this stage water security in the
Lower Darling remains the focus, meaning more inflow beyond that already forecast will be
required before a new general security allocation can be announced. The department intends to lift the section 324 restriction on water access along the Lower Darling
River which has been in place since December 2018. This will allow access to remaining water in
high security and general security accounts, but will not impact on the ability to supply water for
critical needs. This is consistent with the approach taken in the northern valleys, where
suspensions of carryover water were lifted in the Border Rivers and Upper and Lower Namoi
Rivers once 12 months of water for critical needs was available in storage.
To provide the best water quality and ecosystem health outcomes, the water will be carefully
released based on specialist scientific advice as an initial pulse flow of 30 to 40 GL. The aim is to
provide a dilution and flushing action to move contaminated water through the system. This ‘first
flush’, required to restore water quality in the Lower Darling, will be followed by lower flows to
maintain connectivity with the Murray River for as long as possible.
With the current expected inflows, Menindee releases are intended to provide river health and
delivery of supplies to the Lower Darling only. They will not contribute to Murray River
requirements or allow trading to the Murray River.
The pulse release requires the removal of all four emergency block banks that were placed along
the Lower Darling River to provide pools of water for the critical needs of landholders during 2018
and 2019. Work on the removal of the block banks is underway.
Timing and pattern of releases
It is expected that releases to the Lower Darling will begin the week of 23 March, once sufficient
water is stored in Lake Wetherell to allow the higher flow rates of the pulse to be achieved and the
block banks have been removed. The final design of the release hydrograph is being prepared in
consultation with fisheries and ecosystem experts, however; it is likely to comprise initial release
rates rising to around 4,000 megalitres (ML) per day for a period of about six days, before stepping
down to lower rates of around 200 – 400 ML/day.
The first flows to enter Lake Wetherell were of poor quality – de-oxygenated, turbid and highly
saline – and have had some impacts on fish with the death of a number of Bony Bream around 12
March, as the inflows reached the stranded pools at Lake Wetherell. However, subsequent flows
behind the front are of better quality and will facilitate system recovery.
A comprehensive monitoring program has been prepared to track releases from Menindee Lakes
along the dry Lower Darling to the Murray and provide feedback on the progress and mixing of the
flow front as it reaches remnant pools. The data gathered will be analysed and used to build
knowledge and skill around the safe re-starting of regulated rivers after prolonged dry periods.
Click here to read the whole fact sheet
