A CSIRO review of the lower lakes of the Murray-Darling system has found that the lakes were largely fresh before European settlement, and that environmental flows and barrages used to keep out seawater are appropriate, a finding that will please South Australia but disappoint irrigators in New South Wales and Victoria.
Sending water down the Murray to maintain its flow and keep seawater out has been a major point of contention under the Murray-Darling basin plan.
Irrigators upstream in NSW and Victoria have criticised the continued release of water into the lower lakes in South Australia as wasteful during the drought, and NSW has campaigned for the lakes to be allowed to become salty rather than “waste” water.
But the review of scientific studies by a panel of five eminent scientists, led by Dr Francis Chiew, has found that the science of the Coorong, lower lakes and Murray mouth was well understood, and that the lower lakes were being managed appropriately.
It found that, despite some claims that the lakes were naturally saline, the weight of evidence pointed to the main body being largely fresh before European settlement.
“This is informed by palaeoecological records, water balance estimates, hydrological and hydrodynamic modelling, and traditional knowledge of the Ngarrindjeri People and anecdotal evidence of early explorers and colonists,” the panel found.
Pre-development the long-term average annual inflow from the Murray River was more than 13,000 gigalitres and this volume would fill the lakes on average more than eight times a year.
It found that upstream agriculture had reduced the river inflow by about half, resulting in more frequent incursion of seawater.
The barrages were built in 1940 in response to these changes, isolating the Coorong and the sea from the lower lakes.
“Removing the barrages would have significant ecological and socio-economic impact,” the report found. “Without the barrages, the freshwater values in the Lower Lakes cannot be maintained.
“This will significantly change the ecological character of the Ramsar-listed site, which is a wetland of international importance and which we have an international obligation to maintain.”
It would also also impact traditional owner values and other socioeconomic values that rely on a healthy Coorong, lower lakes and Murray mouth system.
Removing the barrages would not result in any water savings if targets informed by science and modelling and envisaged under the basin plan are to be met.
The report also warned that climate change would make managing the Coorong, lower lakes and Murray mouth more challenging.
The Wentworth group of Concerned Scientists reported this week that current flows would not be sufficient to keep the Murray mouth open without dredging. This was one several goals of the Murray-Darling basin plan.
The report found that sea-level rise would alter the hydrodynamics of the Coorong and Murray mouth, and cause more seawater to flow into the lower lakes. It also found that evaporation from the lakes would be higher and that more environmental flows would be needed to achieve the outcomes for the Murray mouth.
The report found there were gaps in the scientific knowledge about the impact of climate change on the lake system.
The Murray-Darling Basin Authority said it would continue to support evidence-based decision making around water management. “We will use the results of the review to further support our current work,” it said.