The 2022 floods have accelerated the build up of sand in the Murray River at the Barmah Choke, further reducing the delivery of water to downstream communities and water holders.
Deep in the Barmah-Millewa forest on the New South Wales-Victorian border, a giant “sand slug” is slowly choking the Murray River.
The sand has accumulated through a narrow section of the river known as the Barmah Choke, or the Barmah-Millewa Reach, and has reduced the water flow by more than 2,000 megalitres a day compared with flow rates in the 1980s.
But now there are concerns a massive increase in sand in the river, caused by run-off from the 2022 floods, has accelerated the problem and is significantly reducing the amount of water flowing downstream toward Adelaide.
The Picnic Point caravan park is on the upstream side of the Barmah Choke, giving owners Brad and Paula Davidson front-row seats to the growing slug. One of its arms runs half of the length of the property, stretching up to 300m from the river bend.
Brad Davidson said he has never seen the sand slug as wide, or the river as shallow, as it is this year, “It’s definitely grown over the 22 years we’ve been here.”
Fallen trees and debris pushed into the river by the floods have allowed even more sand to accumulate.
The sand slug has doubled in height over the past decade and now sits just 30cm below the surface. Beyond it, the river drops back to 4.5m deep.
Brad’s brother Andrew Davidson who works at the caravan park said, “you could probably walk a good third of the way” across into the river and not be underwater.
An investigation by the Murray Darling Basin Authority in 2019 found there were more than 20 million cubic metres of sand between the Yarrawonga Weir and Picnic Point – enough to fill around 13 Melbourne Cricket Grounds – all of which will flow down the river to join the sand slug at the Barmah Choke.
According to the report, the sand was the result of land clearing, gold mining, de-snagging and river regulation – meaning the sand slug is “not a natural feature”.
The average depth of sand in more than 100km of the river was 1.2m, with some areas up to 4m deep.
“Early indications are that the floods have accelerated the sand buildup,” MDBA’s senior director of river modernisation Joseph Davis said.
“Now that the river bed is settling after the flood we are in the process of getting an updated set of flow measurements in the river and resurveying the river bed to assess the changes.”
A feasibility study conducted in 2022 said the flow capacity through the reach has reduced by 20% over the past 30 years, impacting “the condition of the adjoining forests and the delivery of water to downstream communities and entitlement holders”.
It recommended the MDBA undertake “sediment management work” in the Barmah Choke, which may include physically removing the sand with a large vacuum.
But early consultation, particularly with traditional owners, has opposed the use of dredging.
The study also recommended changing the management of water through upstream storages to reduce unseasonal river flows and limit water shortfalls to ensure downstream water licences and environmental flow levels can be fulfilled.
Those steps were approved and received $2.35m in funding at a meeting of the basin ministers in February, and will now go through a cultural and environmental assessment process.