29 August, 2011

Murray Mouth

I today sent the following letter to The Australian. Probably too long for publication-but I felt better!
The article by Jennifer Marohasy (The Weekend Australian- Sea Will Save the Murray-27th August) is a very accurate, well balanced review of the Lower Lakes issue in the context of the Murray Darling Basin Plan.

The key characteristic of the Australian rainfall pattern is massive variability. So much so that average statistics, with the massive spread around the average, are effectively meaningless. This variability results in fresh water flows at the Murray mouth being highly irregular. The Murray mouth was regarded by explorer Sturt as being the entrance to Lake Alexandrina with Lakes Alexandrina and Albert and the northern end of The Coorong forming the river estuary, with the actual entrance to the Southern Ocean being an ever changing channel, difficult to negotiate at the best of times and sometimes closed altogether. At times the lakes were mostly fresh water, sometimes mostly salt, depending on tides, winds and most of all upstream river flows.

In the 1930's the South Australians constructed "The Barrages" between the lakes and the ocean outlet to convert the Lower Lakes (Alexandrina and Albert) into an all times fresh water storage held above sea level. This was beneficial for navigation, irrigators and more recently for recreation/canal style housing development. This action carried with it a quite unreasonable expectation that there would always be sufficient fresh water flows from upstream to maintain the lakes at their elevated fresh water levels, notwithstanding very high evaporation. When this failed to happen, the failure was always blamed on upstream extractions for irrigation when the major cause was simply lack of run-off.

This situation was dramatically demonstrated in the 2002-2008 period when catchment run-off was at record low levels and there simply was not enough water to maintain the lakes in their elevated fresh water state even though upstream irrigation extractions were effectively prohibited. Fortunately upstream storages were sufficient to meet critical human needs and maintain minimal river flow.

The bottom line is that under dry conditions there is simply not enough water to maintain the lakes in their unnatural elevated fresh water state and better management of the Basin strongly suggests that, at least under these conditions, The Barrages should be opened to allow salt water to enter. A weir (or lock) would need to be built on the Murray above Lake Alexandrina to provide fresh water for river and lake irrigators (piped) and to protect Adelaide's fresh water supply from downstream salt water intrusion.
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