Captain Sturt's diary tells us three important things. Even before irrigation there was no navigable passage between the lower lakes and the ocean;.the lower lakes were not fresh; and to a British explorer the lower lakes could not be considered part of the Murray River.
Over the last 100 or so years, for mostly politically expedient reasons for South Australia, the geography of the area has been redescribed and history rewritten. Indeed by claiming the lower lakes as part of the Murray River, South Australian politicians have been able to argue for additional water allocation and by complaining that there would be a navigable passage from the lower lakes to the Southern Ocean if only there was more water coming down the river, South Australian have over the years successfully played the victim card and been allocated more and more water.
But this recent prolonged drought will perhaps be their undoing. I am not suggesting the drought will be a disaster for the lower lakes, but simply that it is becoming increasingly difficult for South Australians to insist that the only solution to the environmental issues confronting the lower lakes is more water from upstream. At issue is the role of the barrages at the bottom of the lakes.
Prior to 1940 Lake Alexandrina was a mix of seawater and freshwater, under tidal influence and fully connected to a much healthier Coorong. The barrages were completed in 1941 and separated the Coorong from Lake Alexandrina and Lake Albert and shut off 90 percent of the tidal flow. They were designed to raise water levels in the lower lakes by up to 75cm and stop fresh water flowing out to sea. But now, because of the drought, sea level is about 45cm above the level of the lake and some salt water is leaking through the barrages into the lakes which are rapidly drying out.
As the water recedes and more of the lake floor is exposed to oxygen, an acid sulphate soil problem is developing.
Potential acid sulphate soils (ASS) are common along much of the Australian coastline. These soils formed after the last major sea level rise, which began about 10,000 years ago. The soils are harmless as long as they remain waterlogged. But, if the water table is lowered the sulphide in the soils will react with oxygen forming sulphuric acid.
In the case of the lower lakes, the barrages are stopping inundation from seawater; in the same way the dykes in Holland are used to reclaim land. Indeed the Dutch have been managing associated acid sulphate soil problems for more than four centuries.
Acid Sulfate Soils have been associated with fish kills in coastal Queensland and New South Wales when land was inappropriately drained.
For example, about 700 hectares of land near Cairns was drained in 1976, and since then it has been estimated that 72,000 tonnes of acid have flowed into Trinity Inlet. Approximately 50 percent of NSW sugarcane land is underlain with potential ASS and inappropriate drainage of these soils caused a major fish kill in the Tweed River in 1987.
NSW farmers have since largely solved the problem through the implementation of liming and less intrusive drainage. But the South Australians, instead of considering practical solutions, initially just provided money to CSIRO Land and Water to undertake a study, which included establishing the severity and spatial extent of the problem, while issuing media releases claiming that because of "imbalances in the way water has been shared between Murray–Darling Basin states" the lower lake would turn acidic.
The most recent statements from Minister Wong suggest there is a realisation that there is just not enough water upstream to save the lakes, and that the only real solution to this emerging environmental catastrophe is to open the barrages and let seawater flood the area.
If the barrages are opened, irrigators dependent on freshwater from the lower lakes would need to be compensated because they have a freshwater allocation creating a property right and the damage to their businesses from the salt water would be difficult to repair. The alternative, during this protracted drought continuing to send large quantities of water from the upper catchment to the lower lakes is no longer viable.
Some argue that if a permanent weir was constructed just upstream of the lakes at Wellington and the barrages used under "an adaptive management regime", there could be water savings in the order of 750 gigalitres (billion litres) a year. Opening the barrages would immediately take some pressure off the entire Murray-Darling system, because less water would need to be allocated to South Australia. That doesn’t mean the Murray River won’t run dry if this drought continues.
Indeed, it doesn’t matter how many activists and South Australian politicians call for more water for environmental flows, if there's ongoing drought and the upstream dams eventually run dry, there will simply be no water for the river. Thankfully the lower lakes can be saved by the Southern Ocean even if the drought continues.
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