Holocene freshwater history of the Lower River Murray and its terminal lakes, Alexandrina and Albert, South Australia, and its relevance to contemporary environmental management

Recent claims based on hydrodynamic modelling within a sequence stratigraphical perspective of incised valley fill sedimentation have argued that the Lower River Murray and its terminal lakes Alexandrina and Albert represented a marine-estuarine lake system, with marine salinities for some 200 km upstream from the Murray Mouth. These claims have encouraged proposals for the removal of barrages near the Murray Mouth to restore the 'original natural condition' of the lakes. It has also been suggested that fine-grained terrestrial sediments were trapped in this mega-lake, necessitating a re-assessment of the Holocene climatic history of southeastern Australia determined from the study of continental slope cores. We show that throughout Holocene time (the past 11.7 ka), the Lower River Murray remained a freshwater-dominated system, based on a range of mutually complementary sedimentary evidence. Radiocarbon dating of Aboriginal middens adjacent to the river and lakes comprising freshwater mussels (dominantly Velesunio ambiguous), crayfish (Euastacus armatus), turtles (Emydura macquarii) and otoliths of freshwater fish species, such as Murray cod (Maccullochella peelii), confirm freshwater riverine and lacustrine conditions throughout the Holocene. Lake Alexandrina also contains endemic obligate freshwater fishes, including a genetically divergent and locally adapted lineage of southern pygmy perch (Nannorpeca australis), revealing an evolutionary history linked to freshwater habitat in the lakes since the late Pleistocene. Freshwater diatoms from fine-grained fluvial clay successions at Riverglen Marina, and diatoms and lacustrine sediments, including sapropels in the lower lakes and their former embayments of Cooke Plains and Waltowa Swamp, also chronicle a history of freshwater deposition. Lakeshore ridges of terrestrially derived quartz sand formed during elevated freshwater lake levels 8.0 +/- 1.2 ka ago, while consolidated masses of the freshwater clam Corbicula australis, radiocarbon dated at 2650 +/- 90 year BP, also attest to long-term freshwater conditions. An open Murray Mouth is prima facie evidence for sustained river discharge, and the mouth remained open throughout the Holocene based on geomorphological evidence. The barrages that were built to retain freshwater within the lower lakes, in response to upstream water abstractions, which had reduced river flows, provide the closest analogue of the 'original' conditions of this environment. With increased automation, nuanced barrage operation could even better simulate the original environment, whereas removing the barrages and building a weir at Wellington would destroy the character of this internationally significant Ramsar Wetland, with detrimental impacts farther upstream.

Automated summary

Based on various sedimentary evidence, we have demonstrated that the Lower River Murray has been a freshwater-dominated system for the past 11.7 thousand years, contradicting recent hydrodynamic modeling conclusions. These conclusions have influenced proposals for removing the barrages near the Murray Mouth, while we argue that maintaining these barrages aligns with the "original natural condition" of the lakes.