Saving the Murray: an alternative view
[A response to Jennifer Marohasy’s “The Murray: a fresh perspective”:
Jennifer Marohasy replied with “The Murray: salt water solution”]
Dr. Jennifer Marohasy presented a case for the removal of the barrages at the lower end of the River Murray to allow seawater into Lakes Alexandrina and Albert, and to build a weir near Wellington, where the river enters the lakes, to safeguard Adelaide’s fresh water supply. This, she asserted, would be an environmentally responsible response to ensure adequate water for irrigation purposes upstream during drought years.
She argued that the freshwater Lakes Alexandrina and Albert at the terminus of the River Murray in South Australia were originally estuarine, observing that Captain Sturt described saline water in the southwestern corner of Lake Alexandrina in 1830 and noted his vain attempts to enter the Southern Ocean from the river, being thwarted by sand shoals. She also commented that sharks and dolphins were observed upstream of the lakes in 1915, well before barrage construction, which was completed in 1940. Marohasy also maintained that the barrages block 90% of the flow between the lakes and the ocean, creating an artificial freshwater environment. Her solution to the “problem” of the lower lakes is to remove the barrages and let the sea in to “restore the lakes to their natural estuarine condition”.
In reality the barrages do not block 90% of the flows between the lakes and the sea. This is the amount of water has been abstracted from places upstream.
There has been considerable debate in the press about the original character of the estuary in pre-European times, with extreme views being expressed. Many people believe that seawater previously occupied the lakes and that it was only after barrage construction that they became fresh. I had accepted that view before researching the area, but now know better.
We now know that the last time the lakes were primarily marine was about 6,000 years ago when global sea level was perhaps 1 metre higher than today. The conditions of the estuary vary in time and space, depending on the flows from the River Murray. During major floods the whole system (apart from Coorong Lagoon) would have been essentially fresh but as floods subsided the mixing zone of fresh and salt water would have migrated into the estuary. In times of severe drought, salt water extended into the channel of the river upstream of Wellington, but these were isolated events, not the norm. Later river flows restored freshwater to the lakes delivering nutrients to a dynamic and variable estuary.
The upper part of the estuary (Lakes Alexandrina and Albert) for the past 5,000 years has been dominantly fresh. Long term proxies for this view include freshwater microfossils recovered from cores extracted from the floor of Lake Alexandrina, small freshwater shells on the northern shore of Lake Alexandrina radiocarbon dated at ~ 2,000 years B.P., freshwater mussels at least several thousand years old around the lake margins, with genetics that reveal a long evolutionary history in the lakes and demonstrate that they were not simply introduced following the completion of the barrages in 1940. The freshwater mussels had been a food source for Aboriginal people for many thousands of years. Other indications of long term freshwater conditions include the dominant occurrence of land derived, siliceous sediments on the lake shores, as opposed to calcareous sediments sourced from the sea. Furthermore, there are threatened species of freshwater fish that currently have refuges in places such as the lower Finniss River, a micro-estuary within a larger one.
So the lakes were not permanently occupied by seawater prior to European arrival. One only has to ask the local Aboriginal people. For most of the time the lakes were fresher than under the current controlled conditions. It was a variable and vibrant highly productive ecosystem, which teemed with birds, fish and other aquatic organisms, and supported Aboriginal people for thousands of years.
Upstream irrigation caused the lakes to became saltier. To counter this induced increase in salinity, the barrages were built. It is true that Captain Charles Sturt was thwarted in 1830 from taking his boat out of the Murray mouth by sand shoals, but this was an isolated observation. Sturt revisited the site in 1836 when the river had created a wide mouth. Moreover, the mouth has not been totally closed in the hundred years since it was first surveyed in 1839. In fact, prior to 1981 it has only closed two or three times in the past 1000 years. The saline water that Sturt tasted in the southwestern corner of the lake would have been in the mixing zone of the estuary.
The example quoted by Marohasy of salt water, sharks and dolphins upstream of the lakes was after much water abstraction upstream and the result of a severe drought. In 1914 after decades of upstream irrigation
… the Murray River disappeared in one of Australia’s most memorable and tragic droughts … the waters fell as if a plug had been pulled out … leaving the bed bare with piles of dead fish and rotting weed. In many places a child could cross without wetting its feet … bullock wagons travelled along in the course for miles … Boats were high and dry … All that was left of the mighty Murray for hundreds of miles were a few brackish pot-holes and the dregs of streams polluted with brine, spelling disaster to the vines. (Hill,1937, page 168).
If the natural balance of the system were to be restored not only should the barrages be removed but also no fresh water should be extracted upstream. Not a likely scenario. Unfortunately, it is a totally managed system and must remain so. Perhaps this regulated system could be managed in a more environmentally sensitive manner, using fish ladders, transparent barrages, controlled flows and getting the balance right between environmental and extractive water in the system.
Acid sulphate soils presented a potential problem in the lower lakes, when water levels upstream of the barrages fell as much as 0.8 m below sea level, exposing sulphide rich sediment which produced sulphuric acid when exposed to oxygen. Temporary regulators were constructed to prevent acidification in the Goolwa Channel and Lake Albert by maintaining a water cover. Problems of acid sulphate soils are best avoided by maintaining a fresh water blanket.
Marohasy believes that salinity decreases at Morgan over the past couple of decades are due to refined irrigation practices upstream, but they are more likely to be the result of salt interception schemes in South Australia, which prevent rising saline groundwater from entering the river. Many of the irrigation schemes upstream of South Australia appear profligate with open channels delivering water to flood irrigate farms for rice, cotton and broad acre pasture.
It might be possible to carry out engineering works to simulate the original character of the estuary, but it would be very complicated and expensive to construct and manage. It is critical to have water available for flushing the system, and the more structures that are built across the system the less mixing by wind and changes in water levels that will occur in the river and lakes.
Originally South Australia received all of the flow of the River Murray, now the flow is reduced by >75% and South Australia’s allocated water use is a mere 7% of the total. The only sound solution to the problem is to provide increased freshwater to the system or the river will die. The health of the lower river is an excellent barometer of the wellbeing of the whole system.
The best way to manage the river system is by maintaining the natural system as closely as possible. To turn the lakes into an arm of the sea would be to destroy the freshwater ecosystem that has evolved over the last 5,000 years. It is imperative that sufficient water flows through the River Murray to maintain the system’s ecological health. Certainly, removing the barrages will not “Save the Murray”.
Bob Bourman is a Visiting Professorial fellow at the University of Wollongong
Bourman, R.P., Murray-Wallace, C.V., Belperio, A.P. and Harvey, N., 2000. Rapid coastal geomorphic change in the River Murray Estuary of Australia. Marine Geology, 170(1&2): 141-168.
Cann, J.H., Bourman, R.P., Barnett, E.J., 2000. Holocene Foraminifera as Indicators of Relative Estuarine-Lagoonal and Oceanic Influences in Estuarine Sediments of the River Murray, South Australia. Quaternary Research, 53: 378-391.
Close, A., 1990. The Impact of Man on the Natural Flow Regime. In: Mackay, N. and Eastburn, D. (Eds), The Murray. Murray Darling Basin Commission, Canberra, pp. 61-76.
Hill, Ernestine, 1937. Water into Gold. Walkabout Pocketbooks.
James, Kristine, 2004. Shifting sands at the Murray Mouth: evidence from historic surveys 1839-1938. South Australian Geographical Journal, 103: 25-42.
Murray-Wallace, C.V., Bourman, R.P., Prescott, J.R., Williams, Frances and Price, D.M. (2010): Aminostratigraphy and thermoluminescence dating of coastal aeolianites and the later Quaternary history of a failed delta: The River Murray Mouth region, South Australia. Quaternary Geochronology, 5:28-49.
Paton, D.C. 2010. At the End of the River: The Coorong and Lower Lakes. ATF Press, Hindmarsh. 247 pp.
Sim, T. and Muller, K. 2004. A Fresh History of the Lakes: Wellington to the Murray Mouth, 1800s to 1935. River Murray Catchment Water Management Board, South Australia.
Wood, A. 2007, Poor Man River – Memoirs from the River Murray estuary.
Wood, A. 2008, Of Billabongs and broken dreams, Tales of the River Murray. 180 pp.