So during the 1967 to 1968 drought when there was no flows from the Murray Darling Basin for 529 days yet the Murray Mouth never closed over? The answer is that South Australia had not yet completely stuffed up the South East unconfined aquifer in 1967 that supplied ground water flows into the Coorong from the south but they had by 1982. The situation was about to get a lot worse when the upper South East drainage works were started in the mid 1990s and only completed in 2011. With hardly any flows now making their way into the Coorong from the south, the southern lagoon of the Coorong started undergoing severe hyper salinisation with also frequent dredging of the Murray Mouth now being required. The following excellent report into the former estuary of Lake Alexandrina is very enlightening reading. From the June 2000 Murray Darling Basin Commission report into the River Murray Barrages Environmental Flows.