The well-publicised images of 1800s paddle steamers lying stranded in the middle of a dry Darling River, supports the notion that there have always been long periods of low to zero flows during drought.
But, in a new study, Restoring the ecological integrity of a dryland river: Why low flows in the Barwon-Darling River must flow, Charles Sturt University and CSIRO researchers Dr Martin Mallen-Cooper and Dr Brenton Zampatti found that under natural conditions, the Darling River flowed 85 per cent of the time, even during the worst droughts on record, such as the Federation drought from 1895 to 1903.
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Therefore, they argue that the persistent periods of zero flow and extremely low flows, witnessed during the last drought, were exacerbated by water storage and diversion.
They said the impacts were further exaggerated by weir-pools, which create artificial lentic (still-water) conditions for around 1000 kilometres (40 per cent) of the river.
The report also acknowledged the effect of climate change, which it said by 2030 may reduce runoff in the Barwon Darling catchment by 10pc, but argued its influence was overshadowed by the dominant impact of water storage and diversion.
The research, published in the journal Ecological Management and Restoration, also found when the river flowed during historical droughts, the typical low flows were much higher compared with the Millennium (2001-09) and recent (2013-2020) droughts.
The report stated that during historical droughts, flows exceed 700 megalitres per day at Walgett and 1300 ML/d at Wilcannia and Menindee 50pc of the time, compared with 175 ML/d at Walgett, 70 ML/d at Wilcannia and 195 ML/d at Menindee in contemporary droughts.
Overall, it said conditions observed during the Millennium and 2013-2019 drought showed a change in the Barwon-Darling River from a "predominantly connected lotic (flowing water) river, to a fragmented, lentic (still water) river."
Four ways to "make low flows flow"
Dr Mallen-Cooper and Dr Zampatti suggested four solutions to "make low flows flow."
The first was to manage flow differently using the hydraulics of flow, including its depth, velocity and turbulence, instead of focusing on volume, in order to improve the environment in which aquatic life exists.
The study showed that flowing-water with good hydraulics for fish are present in the river between the weirpools and these can be maintained in droughts with relatively low flows.
Off-stream storage could be covered to reduce evaporation and reduce blue-green algae. The result would be higher security, higher quality water for towns, and more flow in the river when it is really needed.
- Dr Martin Mallen-Cooper, CSU researcher
The second was to better link the management of tributary flows to the management of the Darling.
Dr Mallen-Cooper said the Darling received 99pc of its water from tributaries, the main ones being the Border, Gwydir, Namoi and Macquarie rivers.
"Presently, all that flow is collected, held, and released under rules of individual upstream valley Water Sharing Plans, mainly for agriculture, and there is no requirement to pass any water to the Darling River," he said.
The study found that while the Darling River recorded 433 days of zero flow at Bourke in 2018-19 - when close to a million fish were killed downstream - 100,000ML of water was captured by the upstream tributary dams.
"This is not surprising, as it is the original purpose of the dams and weirs," Dr Mallen-Cooper said.
"But we need to develop more sophisticated water management that does not disadvantage downstream communities, or destroy the health of the Darling."
In response to this suggestion a spokesperson for NSW Water Minister Melinda Pavey said the Department of Planning, Industry and Environment (DPIE) already considered Darling River flows in the management of tributaries upstream, through its representation on the Environmental water advisory group.
"Recently, in cooperation with the Commonwealth Environmental Water Holder, DPIE managed two environmental watering events using water from headwater storages of the northern tributaries to provide flows into the Barwon Darling," they stated.
"New rules in the Barwon Darling, lower Macquarie, and lower Gwydir will now ensure that water released from headwater storages can be protected as it moves down these rivers."
Off-stream storage instead of weirs
The report's third suggestion was to provide off-stream water storage for towns and individual landholders as opposed to weirs.
Dr Mallen-Cooper said weirs along the Darling store water in large pools, capturing the small flows present during droughts and preventing them from moving downstream.
The report suggested off-stream storages could be filled during periods of elevated flows, so people and the environment were not competing for low flows during the drought.
"Off-stream storage could be covered to reduce evaporation and reduce blue-green algae. The result would be higher security, higher quality water for towns, and more flow in the river when it is really needed," Dr Mallen-Cooper said.
"In many towns the weirs also provide a major social amenity. These could still be retained but would not be used to capture 100pc of water - small flows could pass downstream and keep the river alive."
The final suggestion was the removal of some remote weirs, without social amenity, reducing the backwater and creating more flowing-water habitat.
The report noted the Western Weirs project is currently being undertaken by the NSW government to improve the management of the weirs.
The project, along with the rebuilding of Dungowan Dam and expansion of Wyangala Dam, is being reviewed by a NSW Upper House inquiry.
A spokesperson for Ms Pavey said they were developing a Preliminary Business Case for the holistic management and operation of the Western Weirs.
"This may include the construction of a new integrated system of gated weirs to replace current fixed weirs along the river, which would improve connectivity."
