MBDA system review director Ben Dyer said sediment at the bottom of the choke was increasing by nearly two centimetres a year.
“It’s amazing and quite terrifying to think about the volume of dirt moved by gold mining,” Mr Dyer said.
“We’ve found there is roughly 77,000 cubic metres of sand in the choke. That is 7700 truckloads.”
The build-up of sediment in the narrowest part of the Murray River, coined the ‘sand slug’, was discovered this year by the MBDA during its most recent 10-year survey of the choke.
The authority has been surveying the Barmah Choke every decade since the 1980s in an effort to figure out why flow capacity through the stretch was mysteriously decreasing.
In the most recent survey, the decision was made to not just inspect erosion on the bank but to use new technology to build a virtual map of the river bed.
“When we saw the results, we were very surprised. We were not expecting to see so much sand,” Mr Dyer said.
In certain places underwater sandbanks rise as high as a metre and scours in the bends have been filled in with four metres worth of sand.
Cores taken from the river bed by independent scientists showed the sand itself was strange.
“It is very unusual, the fine sand is gone and we’re left with a coarse sand,” Mr Dyer said.
The missing fine sand has been located downstream in South Australia.
Mr Dyer said the MDBA was ‘pretty confident’ the slug was from historic gold mining activity, de-snagging, dredging and land clearing.
“It all comes together to leave sediment in the river,” he said.
“I don’t think any of us could foresee such a large amount of sediment basically filling the river from the bottom.
“It is finally all making sense, and now the challenge is what are we going to do about it and how are we going to manage it?”
In the mid-1800s gold mining activity was at full-throttle upstream of the Murray in tributaries such as the Ovens and Mitta Mitta rivers.
Many local mines at the time used the hydraulic sluicing method to discover gold, which involved diverting water to mine sites and using it to hose away dirt and turn areas into slurry.
It’s estimated the sand has been travelling downstream ever since, but lost a lot of speed after entering the Barmah Choke.
MDBA river management executive director Andrew Reynolds said only a small amount of the human-disturbed sand seems to be successfully getting through the choke.
“Ten to 20 per cent of the coarse sand is getting down to the Torrumbarry Weir pool, but most of it is getting stored in this narrow stretch,” Mr Reynolds said.
“We know it is moving a little bit, just as people report the beaches moving, but it’s slow.”
The Murray-Darling Basin ministers have commissioned the MDBA to develop a “pre-feasibility study” on how to deal with the sand slug.
“That work is only just kicking off now,” Mr Reynolds said.
“We are looking at everything from delivery pattern changes to engineering work … one thing we are not considering is relaxing the choke trade restriction.”
Mr Dyer said a solution could be anything from "do nothing" to "the big dream of a bypass channel".
“In reality it will probably be somewhere in the middle,” he said.
“If you take anything home today, it is that these issues are ongoing, they’ve been developing over decades, it is not something we are going to solve quickly or easily.”
Mr Dyer and Mr Reynolds were speaking in an MDBA webinar, Capacity Issues in the River Murray, on March 9.
The MDBA’s next public webinar will be held on April 20 at 10 am and cover the topic of water quality and fish deaths.
You can watch the capacity webinar at: www.mdba.com.au/webinars
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