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 is up to 4m deep and has accumulated through a narrow section of the river known as the Barmah Choke, or the Barmah-Millewa Reach.
Its tendrils have reduced the water flow through that section of the river by more than 2,000 megalitres a day compared with flow rates in the 1980s. The problem dates back to an excess of sand in the riverbed caused by gold mining and land clearing in the 1800s.
But there are concerns that 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 their property, stretching 300m from the river bend.
Brad Davidson says 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,” he says.
Fallen trees and debris pushed into the river by the floods have allowed even more sand to accumulate.
“It’s like someone’s put a wall and captured all the sand coming around the corner and then it just sort of got stuck in here,” he says.
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.
In the main office of the caravan park hangs a 90-year-old photo of a woman standing on a diving board on this section at the river. Brad Davidson says it used to be so deep swimmers could not touch the bottom.
But the diving board is gone and if you were to jump in, the water would only reach your knees.
Brad’s brother Andrew Davidson, who works at the caravan park, says the slug narrows in sections and tapers out to the sides as it moves down the river.
“And this is what’s so baffling because the sand slug is not just one body – it’s got arms everywhere,” he says.
“You could probably almost walk a good third of the way out there [into the river] and not be underwater.”
‘Not a natural feature’
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, all of which will flow down the river to join the sand slug at the Barmah Choke.
The sand was the result of land clearing, gold mining, de-snagging and river regulation, the report says, meaning the sand slug is “not a natural feature”.
“The presence of this very large quantity of sand on the bed of the river was unexpected,” says MDBA’s senior director of river modernisation, Joseph Davis.
The average depth of sand in more than 100km of the river, between the Yarrawonga Weir and the Barmah township, was 1.2m, with some areas up to 4m deep.
Early indications are that the 2022 floods “accelerated that sand buildup,” he says.
A feasibility study conducted in 2022 recommended the MDBA undertake “sediment management work” in the Barmah Choke.
That may include physically removing the sand with a large vacuum. Early consultation, particularly with traditional owners, has opposed the use of dredging.
The feasibility 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.
“This work is ongoing and results will inform our ongoing management of the river and work undertaken through the Barmah-Millewa Program,” Davis says.