Gold rush-era sand slug choking the Murray, report finds

The Murray Darling Basin Authority (MDBA) yesterday released the results of a study looking at the Barmah Choke, a naturally occurring narrow stretch of the River Murray that begins downstream of Cobram, Victoria, and ends upstream of Echuca.

The section of the river is popular with boaters and fishers from both sides of the Victorian and NSW border.

According to the report, the sand slug, created by gold mining and land clearing activities upstream as far back as the 1800s and weighing millions of tonnes, has now started to flow into the choke, making it metres shallower in parts and severely reducing its capacity to carry water.

MDBA river management executive director Andrew Reynolds said the authority had noticed the capacity of the Barmah Choke was diminishing “for some time now”.

“Back in the 1980s, it was about 11,300ML per day, but now it's about 20 per cent less than that, at just 9200ML per day," Reynolds said.

"We set out to investigate the possible causes of that decrease in capacity, and what we found was quite incredible. 

“A survey of a 28 kilometre stretch of river revealed that, for every kilometre of river length, 84,000 cubic metres, or 112,000 tonnes, of sediment was estimated to be on the bed,” he said.

“That's more than 3.0 million tonnes just in the area we surveyed.

"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, and in this part of the river it's averaging more than 1.0 metre deep—with some areas over 4.0 metres deep.”

Researchers looked at a number of ways the sediment would have found its way into the river, including flooding, historical desnagging and regulation of the river system, but everything pointed to land-use changes that introduced a large volume of sediment over a short period of time.

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

Water capacity through the choke is important in regulating water levels in the lower reaches of the Murray River. 

Rather than dredging the river, which could have a knock-on environmental effect further downstream, the MDBA said it was looking at ways to manage the reduced water capacity.