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Add salt at the mouth to save the Murray

By Jennifer Marohasy - posted Wednesday, 6 October 2010


Sturt described the waters of Lake Alexandrina as initially sweet but by the morning of the second day, as they headed to the southwestern corner, he noted the waters suddenly became salty and unpalatable and that “the transition from fresh to salt water was almost immediate”.

On the third day, the expedition attempted to manoeuvre the whale boat from the lakes to the Southern Ocean but was blocked by sandbars. Now a sand dredge works at this site to keep the so-called Murray's mouth open and its image has become a symbol of reduced flows because of upstream irrigation. But before irrigation, in 1830, Sturt wrote:

Shoals again closed in upon us on every side. We dragged the boat over several, and at last got amongst quicksands.

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It was not until the fourth day at Lake Alexandrina that Sturt conceded it would be impossible for his men to drag the whale boat any farther over the sandbars and sand flats. He eventually changed plans, heading back across Lake Alexandrina and up the Murray River because it was futile to try to break through to the ocean. When his diary was later published, Sturt commented:

Australian rivers fall rapidly from the mountains in which they originate into a level and extremely depressed country; having weak and inconsiderable sources, and being almost wholly unaided by tributaries of any kind; they naturally fail before they reach the coast, and exhaust themselves in marshes or lakes; or reach it so weakened as to be unable to preserve clear or navigable mouths, or to remove the sand banks that the tide throws up before them.

With the barrages and flows from the upstream Hume and Dartmouth dams and Lake Menindee, a near constant water level of 0.75m above sea level was maintained until 2006-07. Then, given the severity of the drought, there was not enough water in the upstream dams to supply the Lower Lakes and the barrages functioned as dykes and the lakes started to dry up.

As water levels declined a new problem emerged. Potential acid sulphate soils were exposed. These soils are harmless as long as they remain waterlogged, but if the water table is lowered, as happened in 2007, the sulphides in the soils react with oxygen, forming sulphuric acid.

There are two solutions to this problem. For centuries the Dutch, who know all about dykes, have limed regions that were once exposed to tidal flow to avoid the creation of acid sulphate soils, providing one solution. The other is for the SA government to open the barrages and let seawater into the lakes, as had happened before European settlement.

The history and development of this region, however, now means the only solution is seen to be more environmental flows from the Murray-Darling.

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During the drought, the SA government argued that because of “imbalances in the way water has been shared between Murray-Darling Basin states”, the Lower Lakes would turn acidic. This ignores the possibility of removing the barrages and returning the lakes to a more natural state, or liming the areas to prevent acidification as the Dutch have done.

The Murray-Darling Basin Authority will release a new plan for water sharing in the basin. Various organisations have lobbied hard during the development of this plan to get more water for the Lower Lakes, some claiming up to $8.5 billion of taxpayer money needs to be spent buying more water as environmental flow.

It is seven years since the previous basin-wide plan was developed. At that time salinity was considered the biggest risk to the Murray-Darling Basin and salinity levels at Morgan, just upstream from the offshoots for Adelaide's water supply, were considered the best measure of the health of the entire system.

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A version of this article appeared on Quadrant Online and in The Australian on October 2, 2010.



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About the Author

Jennifer Marohasy is a senior fellow with the Institute for Public Affairs.

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