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.
Peter Martin
Finley, NSW
If that decision is made we can sustain these communities with fresh water 24/7 is totally sustainable way.
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Given that the normal average depth of the lake today is about 3 metres and this is at least 0.5 metres above mean sea level, then it is a physical impossibility for sea (salt) water to have not penetrated a very long way into the estuary for at least part of the year.
A full moon tide is usually about 1 metre and this volume represents 40% of the original 2.5 metre average depth. Ergo, in the absence of any river flow the tidal flush would introduce sea water to at least 40% of the estuarine area.
Furthermore, as the total area is in the order of 68,000 hectares, this tidal surge would involve an influx of 680,000 megalitres each day over the week of full moon tide.
And it follows that the only days on which no sea water would penetrate at all would be those days when the flow of the Murray was GREATER than 680,000 megalitres. Just to put that in perspective, this volume, if averaged over a whole year would amount to 248 million megalitres, or 10 times the current average gross annual inflows and 25 times the current net flows after extractions.
The numbers behind the claims that the estuary spent any significant time as a completely fresh water body prior to the barrages simply do not add up.
And I challenge any of the proponents of this absurd notion to prepare a simple flow model that is capable of incorporating historical flow cycles and known tide cycles. But they will not do so because this model will clearly demonstrate that the entire annual volume of extractions from the basin amounts to only 22 days worth of tidal volume.
Ergo, neither the natural flow nor the reduced extracted flow had any significant effect on the extent of tidal penetration in the estuary.
And will someone drown that moron they have elected as mayor down there. The guy is dumber than duck $hit.
Charles Sturt found salt water in the lower lakes. I would think that Sturt's observations are evidence that directly challenges rog's assertion that "Flinders Uni say that the last time that saltwater was present was 1,700 years ago".
See for yourself at:
https://climatelab.com.au/wiki/Historical_Records_of_the_Murray_River#Charles_Sturt.27s_Diary_of_his_Expedition_down_the_Murray_River
the above site has links (in the references section) to the full text of Sturt's diary
"coastal lagoons that were formerly
saline have been converted into freshwater lakes by construction of barrages to exclude sea
water. In the estuarine lagoons at the mouth of the Murray, in South Australia, barrages were
completed in 1940 to seal off gaps in the enclosing barriers and exclude sea water.
Previously, the sea entered Lake Alexandrina and Lake Albert freely, particularly during dry
summer periods when fresh water discharge from the Murray was low. The building of
barrages were stimulated by the evidence that during droughts brackish water was spreading
into the Lower Murray and invading irrigation works. Late in summer of 1938 – 39, surface
salinity in Lake Alexandrina reached 28.5º/oo, the salinity of the open sea being a little above
35º/oo. There were extensive tracts of salt marsh bordering the lakes, liable to inundation by
the brackish water. In 1940, after the completion of the barrages, salinity dropped sharply to
1.42º/oo, and it has continued to decline, falling to a lowest annual maximum of 0.08º/oo in
1955 – 56. The lakes are now sufficiently fresh to be used as a source of water for irrigation
of adjacent pastureland, and the freshening of the lakes has been followed by the
development of reedswamp"
http://www.dpi.vic.gov.au/dpi/vro/egregn.nsf/0d08cd6930912d1e4a2567d2002579cb/0da8459bb6f7a4ecca25730600149cc7/$FILE/geomorphology%20gipps%20lakes%20shoreline%20erosion.pdf
During the 30's and 40's extraction was well below half that of the 90's(3000 GL vs 10000 GL approx).
david,
An Adelaide Uni study: http://www.dwlbc.sa.gov.au/assets/files/USE_CoorongWQChangeDraftNov05.pdf
It talks about a 6000 year history of the lakes, and the affect of sea water and how far upstream it went during drought.
I think the barrages very existance are proof that sea water incursions were a problem well before widespread irrigation.
One correction, Richard, the lakes evaporate 2/3rds of SA's allocation, not half.
Funny how few of the so-called 'concerned ecologists' demanding more fresh water when the system can least spare it seem to give a rats arse about the extensive salt water ecosystem that was destroyed by the barrages. A system this size would have had a huge prawn stock that formed a large part of the food chain for the entire coastal fishery in the region.
The destruction of this would have left the Coorong to serve the entire food chain from only 25% of the original resource, and without the regular tidal flush on which so much depends.
And the only possible conclusion to be drawn from it is that this impact has been far more significant than overfishing could ever have been.
The irony is that a floating house on this degraded estuary would prevent 1.6 tonnes of evaporation for every metre of roof or deck area while the total annual runoff from each metre of that roof into a rainwater tank would only be a third, or 0.5 tonnes a year.
All cities in low rainfall zones should be maximising the amount of housing that floats on their water storages. This competitive advantage remains the same regardless of drought or flood while runoff dependent systems exhibit major variations in yield.
Richard - I was only passing on what my friend said on the matter as she lives there and is aware of the problem locally - that's all.
Can we aford to waste 1,000,000 megalitres of fresh water which evaporates from this lake every year. South Australians like to claim how efficent they are, they should include this evaporation in their water use figures.
Jennifer,
By accepting Sturts observations (those of a British Officer too!) you must also accept all other 'observations' (Flinders Uni say that the last time that saltwater was present was 1,700 years ago)
Sturt 1838
"During my late visit I never observed the sea running in, but a strong current always setting out of the channel. From what I observed, I am led to think, that the level of the lake is above high water mark. The narrowness of the channel preventing the body of water thrown into it, by the Murray from being thrown out in the same proportion. The immense body of back water in the chain of lagoons, would be an argument in favour of this supposition."
You dont think there is an argument to open the barrages Janama? How About that it is an artificially created freshwater lake that evaporates/wastes half of SA's water allocation each year. The barrages have resulted in the mouth silting up thus needing constant dredging due to the lack of tidal flow and have destroyed the ecology of the coorong. The coorong and lower lakes are just as much a part of the ecological system of the ocean as the river. It is only during recent times of severe drought and resulting lack of flow from the river that the true disaster of these barrages has become evident. I can see no good reason for leaving the barrages closed. The government needs to spend some money on a weir at wellington and some pipes around the lakes to provide freshwater for people who rely on it and return them to their natural state of a tidal estuary. This would benefit the ecology of the lakes, the coorong stop the mouth silting up and save loads of water. I do not beleive that Adelaide derives any of its water from these lakes as you suggest.