by Roger Cooke, 19 September 2011
The evaporation rate from the lower lakes is 2m per sq m per annum. The width of the river at Wellington is 200m and the width of the final 40 km through Lake Alexandrina is 20 km. Therefore the loss due to evaporation from the Lake Alexandrina alone (without Lake Albert) is 100 times per meter run than in the river. In fact the total loss to evaporation from the lower lakes is 600 to 900 GL per annum. This is fresh water!
The total allocation from the Murray Darling Basin for the state is 1850 GL pa of which an average of 800GL pa goes to the Lower Lakes to simply evaporate. That is 43 % of the state’s water allocation goes up in steam. At the going cost of water (ie $1.00 / KL) this waste is worth $0.8 billion pa. Going on the food production in the rest of the Murray Darling Basin, this water could be better used irrigating additional food production of $1 billion pa in S.A.. The answer is to replace the fresh water with salt water so that it is the ocean water that evaporates.
There are two engineering mistakes made on the Murray River. The first was the construction of the barrages at the bottom of the Lower Lakes to provide fresh water to abutting farms and communities. The recent drought exposed this mistake. The State Government originally proposed the correct scientific solution. (ie open the barrages and build the Wellington Weir.). However, the State Government gave into ill informed community pressure and perpetuated the mistake by insisting there was only a “fresh water solution” to the Acid Sulphate Soil that was exposed in the lakes. This was an out and out fabrication to “con” money out of the Federal Government to fund white elephants (ie Clayton, Currency Creek and Finnis River regulators ) for $ 26 million.
The other Engineering mistake was the putting of the pumping station for the Adelaide water supply below Lock 1. This meant that, if the Lower Lakes were returned to an estuary by opening the barrages, the water supply would be vulnerable to salt water intrusion up the river. The original plan to install the Wellington Weir was a move in the right direction ( except for the location which should be at Newport and simply high enough to stop a high tide intruding upstream ). Ironically, the Government did one positive thing . It delivered mains water supply to all farms , towns and communities which had become reliant on the lower lakes being fresh. Therefore there was no reason why the Lower Lakes could not be opened to the ocean.
The addition of the water from the Lower Lakes to that from the Coorong would mean that the volume that would pass through the mouth twice a day at the change in tides would be sufficient to keep the mouth open like had happened before the barrages (ie 200 m wide and 12 m deep). What is more, this would be done naturally and not require $6 million pa in dredging.
The Lower Lakes have been turned into a cess pool by the barrages. For 70 years the water coming down the river has brought with it contaminates which precipitate out when the flow comes to a halt in the Lower Lakes. There was no longer the progressive flushing out to sea of the contaminates twice a day at the changes in tide. The Lower Lakes cannot wait for a flood every 30 years or so for a flush.
The logic is questionable as to how the Lower Lakes were ever included onto RAMSAR. The agreement protects wetlands that are kept unnaturally wet year in year out by the barrages all the way up the river system and in the case of the Lower Lakes keeps them fresh when they are meant to be an estuary.
South Australia’s credibility will always be questioned by upstream states whilst it insists the Lower Lakes have to remain fresh because of the Acid Sulphate Soils. Upstream states have realised the fact that 800 GL pa of valuable fresh water is wasted keeping the lakes fresh when they are meant to be estuarine.