The installation of the barrages across the bottom of the Murray River is the greatest single change that has adversely affected the health of the Coorong.
Prior to 1940 Lake Alexandrina, at the bottom of the Murray River, was a mix of seawater and freshwater, and was under tidal influence through the Murray mouth, and fully connected to a much healthier Coorong.
The Murray River barrages were completed by 1941 and separated the Coorong from Lake Alexandrina and Lake Albert.
Before the barrages, fish could move between the lakes and the Coorong. Lake Alexandrina was an important fish nursery for replenishing the Coorong. After the installation of the barrages the Coorong was cut off.
The barrages have shut off 90 percent of the tidal effect, and as a result have made the Murray mouth much more prone to closing over.
The barrages have caused much greater deposition of silt above and below the barrages, and have caused a sand island called Bird Island to form directly in front of the Murray mouth on the landward side.
The barrages were designed to hold Lake Alexandrina up to a maximum of 75cm above previous levels, and as a result shoreline erosion has accelerated. The higher level of Lake Alexandrina has prevented an enormous quantity of fresh water reaching the Murray mouth.
But now, because of the drought, sea level is 45cm above the level of the lake with plenty of sea water leaking into the lake despite the barrages.
The salinity at Goolwa is currently around 20,000 EC units. This is very high. The sea is about 45,000 EC units and the upper limit for drinking water is just 800 EC units.
In short, the barrages were designed to turn a saltwater lake into a freshwater lake, but they weren’t successful.
At best Lake Alexandrina remains brackish, with current salinity at Milang which is about the centre of Lake Alexandrina is about 4,000 EC units.
Furthermore, the barrages that were meant to hold Lake Alexandrina at a higher water level, have resulted in a requirement of up to 1,000,000 megalitres annually just to cover the evaporation loss. This loss has to be supplied from Hume and Dartmouth Dams and at times from Lake Menindee. If the barrages were to be opened, this quantity of water would be immediately saved annually.
Whether our climate scientists are correct or not, the need for this quantity of water to cover evaporation is simply unsustainable.
The value of that water to irrigation at the end of last season was in the order of $400 million.
The South Australian government should now open the barrages, particularly the Goolwa barrage, which is responsible for regulating 70 percent of the Murray River flows, and the Mundoo barrage which is responsible for 10 percent, as these two barrages would have the greatest impact on keeping the Murray mouth open, and improving the health of the Coorong.
In summary, just because the barrages were put in does not mean they have to stay there. It is wrong for South Australians to keep demanding the upper states of Victoria and New South Wales empty their dams to unsuccessfully keep a saltwater lake fresh.