The River Murray Barrages are 7.6 km of weirs separating Lake Alexandrina from the sea and the Coorong. The barrages keep seawater out and create an artificial, exclusively freshwater environment. They were constructed (along with other locks and weirs on the River Murray) between 1935 and 1940 to create a pool of freshwater to stabilise the river level for both irrigation and transportation. Without the barrages seawater periodically entered the Lower Lakes, providing a rich estuarine habitat and a productive mulloway fishery.
From the report "Ecological Assessment of the Lakes and Coorong Fishery", September 2005:
This report "River Murray Barrages Environmental Flows" from the MDBC in 2000 analyzes the condition of the Lower Lakes.
Before the barrages were built in 1940, scientists agree that the Lower Lakes were part of a vast estuarine ecosystem connected with the Coorong.
As Charles Sturt in 1830 approached Lake Alexandrina, at Pomanda Point, he wrote in his journal,
"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."
Barrages in red, separating freshwater from seawater since 1940
A very interesting historical interview in 1999 with Jim Marsh, Barrage Superintendent, Goolwa. Mr. Marsh says,
"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!...
More historical sources from original newspaper stories as well as the history of the region is captured on this page History of the Lower Lakes and Coorong .