The terminal lakes of the Murray River, Australia’s longest river, are some of the most ecologically important wetlands on the Australian continent. These freshwater lakes are protected under the Ramsar Convention which obliges the Australian Federal Government to prevent ecological decline. Lakes Alexandrina and Albert are, however, threatened by rising salinity which primarily results from upstream water extraction for irrigated agriculture. In response to this threat, the Murray Darling Basin Plan allocates water to the lakes to maintain their freshwater condition. The allocation of water for this purpose is politically contested. As part of this debate, questions have been raised about whether the lower lakes were naturally fresh.
Recent research using hydrological modelling has suggested that 8,500-5,000 years ago the lakes were saline due to higher sea levels. By contrast our work using both sedimentary diatoms and hydrodynamic modelling demonstrates that for most of this time the lakes were fresh. In particular, discharge from the River Murray was higher between 7,200-6,600 years ago and this elevated discharge appears to have prevented sea water incursion despite higher sea levels. After this time, the lakes remained fresh.
As a result, the current allocation of water to maintain the freshwater status of Lakes Alexandrina and Albert is consistent with their history. Hence, history has an important role to play in informing contemporary environmental management.
The presentation summarises work published in Tibby et al. (2022) complemented by new radiocarbon dates and new diatom data from Cooke Plains wetlands - a higher elevation site once connected to Lake Alexandrina.