There is a widely held belief that Lake Alexandrina and Lake Albert have always been freshwater lakes and that 'saline intrusions' were rare.
However, there is ample evidence from newspapers, journals of early explorers, and oral histories from locals that indicate that Lake Alexandrina and Lake Albert had varying degrees of salinity depending on the season and weather patterns. Varying degrees of salinity across the water body, where a river meets the sea, is usually categorized as a type of 'estuary'.
To understand the relationship between the river, Lower Lakes and the sea, take a look at a map of the Lower Lakes. Notice the location of the barrages, the location of the Murray Mouth, the town of Wellington, and the proximity of the Coorong to Lake Albert and Lake Alexandrina. It is also important to note the location of Blanchetown, 274 km north and the first Lock on the River aside from the barrages. The Barrages, finished in 1940 were built to prevent seawater from entering the Lower Lakes and ultimately up the River Murray, especially during times of drought.
History: Pre-barrage 1800's to 1940
"Thus far, the waters of the lake had continued sweet; but on filling a can when we were abreast of this point, it was found that they were quite unpalatable, to say the least of them. The transition from fresh to salt water was almost immediate, and it was fortunate we made the discovery in sufficient time to prevent our losing ground. But, as it was, we filled our casks, and stood on, without for a moment altering our course."
In 1844 the South Australia Company needed to know where fresh water and grazing would have been available for the thousands of sheep and cattle being brought into the state. Annotations on types of vegetation and the quality of water around the Lower Lakes and Lower River Murray have been made on this map clearly showing the location of brackish and fresh water around Lake Alexandrina
"Certainly no residents upon the lakes or rivers could complain if the bitter, salt, and useless fluid of the lakes for nine months of the year were changed to a permanent and plentiful supply of fresh water fit for all purposes to which fresh water can be applied."
People who fished for a living could be opportunistic and fish for fresh, estuarine or marine species depending on the varying conditions of the estuary before the barrages. In 1912-1913:
Inland droughts from 1910 affected the River Murray flows and, as a consequence years of low flow, there was a large incursion of marine water into the Lakes and lower reaches of the River Murray itself. This incursion had a disastrous effect on the stocks of freshwater fish and "many hundreds of fine cod and callop were killed in the lower sections of the river". The incursion did not adversely affect the catches of the marine species mostly fished by Goolwa fishers. (Olsen 1991)
From The Advertiser,(1892, May 4) in an article titled THE FISHERIES QUESTION — No. 11 there are several interviews with Lakes fishers as the need for regulation in the fishing industry is examined.
"For his own part he fished chiefly for butterfish and bream in the wide waters of the lake when the sea went up. When this was the case the cod went up stream, as they do not like salt water."
"...examination of Robert Strachan, a fisherman, who said cod fishing was carried on at present, while mullet will be caught next month. Cod are caught with a hook and line. A small mesh net had to be tried with which to catch tukerie for bait. He fished near Milang for the most part, but when the salt water came in he went to the other end of the lake."
Some locals still remember what life was like before the barrages. Their stories have been told in these interviews.
"There was all salt-water in Goolwa in those days. We used to go down with a little net, down opposite the station, Goolwa railway station, then we’d walk around and get a feed of mullet. You could fish off the wharf…there was a lot of garfish around"
"We did a lot of swimming, actually, out from the wharf. There were a couple of walkways with ladies’ and mens’ bathing sheds situated out along this walkway, and we used to swim there. In the early days, before the barrage was built, it was all clear salt water and it was beautiful swimming."
"As the South Australian Government in the meantime had long foreseen, or been made aware of the need, for some sort of a structure at the Murray Mouth. They came up with a scheme, designed it and submitted a proposal and it was approved. Goes back a long way - the early settlers - the country around Mundoo Island, Hindmarsh Island was settled in about 1840-1841. It was mainly cattle grazing because of the ephemeral nature of the Murray and the very flat terrain, it was a constant battle of forces between the sea and the river. You’ve got fresh water coming down - it pushes the sea water out and everything’s hunky dory, but if you get a dry year and the river doesn’t flow, then the tides push the sea water in, and , as in 1915 it was a bad drought year, the sea water penetrated up to Mannum - they were catching Mullet at Mannum, and there was a sighting of a shark at Tailem Bend, and a dolphin at Murray Bridge!...
History: Post Barrages 1940's to now
This report, the Murray River Barrages Environmental Flows Report was produced for the Murray-Darling Basin Commission in 2000. It's purpose was to provide guidance to the Commission from a panel of scientists to better manage the river for the benefit of the environment. A few excerpts below are from the report and clearly acknowledge that the Lower Lakes were once part of a much larger estuary that included the Coorong. Separating the Coorong from the Lower Lakes with the barrages has caused all kinds of environmental problems.
From page 10, Key Issues:
"The Scientific panel identified four key issues driving the serious degradation of environmental values in the Lower Lakes and Coorong. These are:
• the reduced area of the estuary
• changed water regimes of the lakes and river
• freshening of brackish and saline habitats
• reduced habitat for aquatic plants
The first two issues are the most significant in terms of their impact and their influence in driving the other key issues."
"The reduction in the size of the estuary has reduced the size of the tidal prism by around 90% of its original pre-barrage size. In 1914 the lake area affected by tides was 97.3 km2 (75 000 hectares), with a spring tidal prism of 20 000 ML (Walker 1990). These figures indicate that the original tidal prism produced a twice-daily exchange of similar magnitude to the flows of 20 000 ML/day for a month or more which would now be required to substantially clear the mouth of accumulated deposition (Harvey 1988)."
From: page 44, Opportunities for Improvement:
Options for rehabilitation measures to address the key geomorphological issues - Long Term
"Enlarge the diversity of habitat in the estuary by increasing the size of the tidal prism and the flushing effects of tides at the mouth. An option for management is to relocate the barrage system to Wellington or to Pt Sturt–Pt McLeay."
From page 62, Ecological Needs:
"The changes in water regime and the presence of the barrages have both led to a reduced extent of the estuarine system, now approximately 11% of the former area (Bourman ibid). Along with the changes, there has been an increase in the extent of the freshwater environment behind the barrages and an increase in the spatial and temporal occurrence of hypermarine conditions in the Southern Lagoon. There has been the establishment of unvarying marine salinities for long periods in the remnant estuary when the barrages are closed. Clearly, management of the region needs to enhance the environment for estuarine macro-invertebrates and for estuarine-marine fish."
From page 64, Long-term Changes:
"Consider removal of present barrages and investigate options for new structures at Wellington or Pt Sturt. This would greatly enlarge the estuary and return it to its historical form. More information on hydrology, geomorphology and biology of the system is needed before this major change could be implemented."
An article in The Australian from 2008 calling for 'heroic action for the lakes', from Prof Tim Flannery;
"I think it's time for quite heroic measures that will be somewhat risky and probably unpopular," he said. "One of the things that could be done is a barrage built higher up the system and for the Lower Lakes to be flooded by the sea."
More on the problems confronting the Lower Lakes and the Coorong can be found here.