THERE are a lot of comments in the thread following my blog post ‘Healthy Country Means Less Water for South Australia’. In that thread Peter R. Smith OAM has claimed that if it weren’t for the barrages Lake Alexandrina, a terminal coastal lake at the bottom of the Murray Darling catchment, would become hyper-saline. In the same thread Sean Murphy has replied, but Lake Alexandrina was once tidal, so how could it become hyper-saline?
It could become hyper-saline if the Murray’s sea mouth closed over completely, something that engineers warned in 1903 could happen if the barrages were built stopping inflows from the Southern Ocean – stopping the tide.
Soon after Europeans started farming on the shores of Lake Alexandrina they began devising plans to preventing it from becoming salty. The first such plan was presented to the South Australian parliament in 1890. Prepared by the Engineer in Chief Alex B. Moncrieff it proposed the building of a lock on the Goolwa channel and barrages across the other channels to prevent seawater from entering the lake.
Federation, and the 1895-1902 drought, focused the attention of the communities along the River Murray on the need for cooperation if they were to develop the waters of the River Murray. In 1902 the Corowa Water Conservation Conference led to an Interstate Royal Commission with the purposes of “To inquire and report on the conservation and distribution of the Murray and its tributaries for the purpose of irrigation, navigation and water supply.” It was another twelve years before the River Murray Waters Agreement 1915 was ratified which created water sharing principles for New South Wales, Victoria and South Australia including an annual water entitlement for South Australia to be met in equal share by Victoria and New South Wales, and the development of a program of major works including the construction of dams and weirs which the three states and the Commonwealth where to jointly fund.
In the meantime a more substantial plan was developed to prevent Lake Alexandrina becoming salty, as it had during the federation drought. The plan was presented to government in 1903 as a joint report by T.W. Keele the Principal Engineer of Harbors and Rivers of New South Wales, W. Davidson the Inspector-General of Public Works of Victoria and Mr Moncrieff who was still the Engineer in Chief in South Australia.
The report, dubbed, the “Report by Experts” begins with reference to why the best option for securing “the impounding of the fresh water” should involve the blocking of several channels from the lake that converge on the Murray’s sea mouth rather than placing a barrage across the actual sea mouth of the river. The report also explains why the barrages should be placed such that they exclude the Coorong from the Alexandrina lake system because the Coorong represented “an evaporating area of 90 square miles additional to that of the lakes”.
The report details and quantifies the tidal influence through each of the channels relative to a tidal gauge at Milang. The opening between Mundoo and Hindmarsh Islands is referred to as the most direct outlet from the lakes to the sea and with a tide that rises considerably higher than the tide through the Goolwa channel. Different barrage structure were proposed for each of the channels with a permanent earthen wall pithed with stone across Boundary Creek, while for the Goolwa it was proposed a sheet-pile structure be built with a lock large enough for river steamers.
The Report by Experts includes two important warning: that after construction of the barrages the Murray’s mouth would be expected to close over completely; and before erecting the barrages a more regular supply of fresh water from the river would first need to be secured or the lakes would dry-up during periods of drought. These important caveats have been subsequently ignored by state and commonwealth governments and are never referenced in the very many reports published with increasing regularity by the Murray Darling Basin Authority.
Report by experts. The Murray Barrages. August 20, 1903. The Advertiser p. 8 http://nla.gov.au/nla.news-article4987833