Floods deliver, as Lake Menindee opened up: Battle over Lower Darling River

As NSW prepares to fill Lake Menindee for the first time since 2018, how much will be left to flow down the Lower Darling?

The NSW Government is letting water flow out onto the dry Menindee Lake bed, losing at least 100GL to dead storage.

Analysis: The NSW Government is preparing to wet up the biggest of the four main lakes in the Menindee system to capture 650 to 850 gigalitres of floodwater heading down the Upper Barwon-Darling River basin.

Satellite images show the first flow of water out onto Lake Menindee’s dry bed, as a result of the NSW Government breaking the seals on the regulator connecting it to neighbouring storages.

Up until now only two of the four main lakes at Menindee — Wetherell and Pamamaroo — have been used to store Darling River flows, holding 317GL as of this week.

But as more of the Upper Darling River’s floodwater reaches Menindee the NSW Government has opted to divert the flow into Lake Menindee, rather than see it all flow down the Lower Darling and eventually into South Australia’s Lower Lakes and the sea.

While Lake Victoria, near the junction of the Darling and Murray Rivers, could be used to capture almost 500GL of the Darling River flood flow, NSW and Murray Darling Basin. Authority operations teams estimate much of its capacity will be needed to store Murray water this autumn and winter.

Plans are already in place to store spills from the Murrumbidgee’s Burrinjuck Dam in Lake Victoria, plus Goulburn River environmental pulses and transfers from Lake Hume tied to other environmental flows and the need to meet SA storage rights.

But choosing to store the bulk of the Darling floodwater in the Menindee Lakes comes at a cost.

Pouring water out onto the dry Menindee Lake bed means losing at least 100GL as dead storage, with past work by the NSW Office of Water showing a further 300-400GL would be lost to seepage and evaporation – depending on how much is eventually diverted to the storage.

However once the lakes hold 640GL it will trigger the handover of their management from the NSW Government to the MDBA.

Once the 640GL trigger is reached, NSW Murray irrigators share of the lakes’ water will increase to 167GL, Victorian irrigators will gain 67GL and those in SA 117GL.

It will mean earlier general security allocations to NSW Murray Irrigation customers, as it the Menindee water can be used to help pay off the 314GL they have borrowed from the Barmah-Millewa Environmental Water Account over the past four years.

The Weekly Times understands the NSW Government is also considering how much of the floodwater it allows to flow down the Lower Darling River, given its obligation is simply to maintain daily winter flows at just 300 megalitres.

The NSW Government wants state and Commonwealth environmental water holders to first agree on releasing 48GL of water allocated against their general security Darling system entitlements.

But it’s understood the CEWH and other state EWHs want the NSW Government to release operational water to flush the Lower Darling.

Whatever happens the NSW Government first priority is to divert and dilute the first slug of low-oxygen water at the head of the floodwater flowing down the Darling River into Lake Pamamaroo.

As that slug is diluted, fresher water can then be released back into the Lower Darling River to stimulate fish breeding.

The big question is just how much will be released down the Lower Darling River.

Once the lakes are under the MDBA’s control and water sharing arrangements they do not return to the NSW Government’s control again until they fall to 480GL.

However just 280GL of this water could be used to maintain the Lower Darling River’s flow, as the remaining 200GL would be dead storage that could not be released.

In February 2019 the Academy of Sciences released a report, commissioned by the Labor Party, into the December 2018 and January 2019 fish kills in the Lower Darling that recommended 400GL be retained in the lakes to maintain environmental flows.