Centuries-old sand slug slowing choking the Barmah Choke

Research by the Murray-Darling Basin Authority found the sediment bank, thought to be created by gold-mining and land clearing upstream in the 19^th and early 20^th centuries, was accumulating at the natural chokepoint.

It's believed the 200-year-old legacy issue is the contributing to the Barmah Choke's diminishing capacity, which has dropped almost 20 per cent since the 1980s, from 11,300 megalitres a day to just 9200ML.

MDBA executive director Andrew Reynolds said the independent report prepared by Streamology found three million tonnes of sediment in the 28km stretch of river surveyed, or 112,000 tonnes for every kilometre of river.

For years, locals have blamed excessive flows under the Basin Plan for eroding the banks and clogging up the choke.

However, Mr Reynolds said given the size and make up of the sand slag, it was not from local erosion.

"This coarse sandy sediment has a very different profile and composition to the surrounding river banks, so it's pretty clear that it's not caused by local erosion," Mr Reynolds said.

"In this part of the river it's averaging more than one metre deep - with some areas over 4 metres deep."

"Our analysis found that the most likely explanation is that land-use changes - in particular gold-mining and land clearing - were responsible for introducing large volumes of sediment to the River Murray over a short period of time."

While no gold mining took place near the Barmah Choke, the primary hypothesis is such activity in the Upper Murray, Kiewa and Ovens rivers, dating back to the late 1800s and early 1900s, created a large amount of sediment.

"During the gold rush, river channels were used to process huge volumes of sediment, with the gold dropping out into cradles and sluice boxes," Mr Reynolds said.

"This would have introduced a large pulse of sediment, known as a sediment slug, to the river system.

"This slug has been gradually moving through the river-and is now appears to be contributing to problems in the Barmah Choke."

Managing the Barmah Choke has been a difficult task for water authorities, as the naturally-narrow section of river makes it difficult to get water to where it's needed, which increases the risk of shortfalls.

Only 20 gigalitres of water can be traded from upstream to downstream of the Barmah Choke this water year, because of its restricted capacity.

Mr Reynolds said the findings did not change the current trade restrictions for the Barmah Choke and more research was needed before the MDBA settled on a solution.

"Obviously you can't just rashly start dredging the river," he said.

"As an independent science-based organisation, what we do must be based on evidence."

"We also need to properly understand the longer-term rate of sand build-up and associated loss of channel capacity."

Federal Water Minister Keith Pitt said the research was commissioned in response to concerns from the community.

"These include erosion of the banks and potential for bank collapses caused by well-known issues, and a reduction in the amount of water that can move through the Choke before flooding over the river bank," Mr Pitt said.

At the end of last year, Mr Pitt and state Water Ministers directed the MDBA to investigate ways to take pressure of the Barmah Choke in a separate study.

"The study will consider a range of potential solutions to deliver more efficient operations in the best interests of all parties, and communities and other stakeholders will be consulted through that process," he said.

"I expect the results of the work will be available around mid-2022."

Mr Reynolds said the challenge around the Barmah Choke was dealing the issue, while keeping in mind the "ecologically and culturally important forest" that surrounds the river.

"Ensuring a sustainable future for the river is a mammoth undertaking, and it is not something that can happen overnight," Mr Reynolds said.