TY - JOUR
T1 - Holocene freshwater history of the Lower River Murray and its terminal lakes, Alexandrina and Albert, South Australia, and its relevance to contemporary environmental management
AU - Bourman, R.P.
AU - Murray-Wallace, C.V.
AU - Wilson, C.
AU - Mosley, Luke
AU - Tibby, John
AU - Ryan, D.D.
AU - De Carli, E.D.
AU - Tulley, A.
AU - Belperio, A.P.
AU - Haynes, D.
AU - Roberts, A.
AU - Westell, C.
AU - Barnett, E.J.
AU - Dillenburg, S.
AU - Beheregaray, L.B.
AU - Hesp, P.A.
PY - 2022
Y1 - 2022
N2 - 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.KEY POINTS Paleoclimatic, geomorphological and modelling reconstructions, together with sedimentary records based on freshwater diatoms, molluscs, fish, turtles and lacustrine systems and evidence of genetically divergent and locally adapted obligate freshwater fishes, demonstrate that predominantly freshwater conditions were present in the Lower River Murray and its terminal lakes throughout Holocene time (11.7 ka). The empirical observations presented in this paper reveal that a large marine-dominated mid-Holocene estuary was not present in the Lower River Murray, Australia. Proposals to remove the barrages near the Murray Mouth would destroy the long-term freshwater environment of this Internationally Significant Wetland Site with negative ecological impacts.
AB - 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.KEY POINTS Paleoclimatic, geomorphological and modelling reconstructions, together with sedimentary records based on freshwater diatoms, molluscs, fish, turtles and lacustrine systems and evidence of genetically divergent and locally adapted obligate freshwater fishes, demonstrate that predominantly freshwater conditions were present in the Lower River Murray and its terminal lakes throughout Holocene time (11.7 ka). The empirical observations presented in this paper reveal that a large marine-dominated mid-Holocene estuary was not present in the Lower River Murray, Australia. Proposals to remove the barrages near the Murray Mouth would destroy the long-term freshwater environment of this Internationally Significant Wetland Site with negative ecological impacts.
KW - Lower River Murray
KW - Lower Lakes
KW - freshwater environments
KW - mid-Holocene sea-level
KW - archaeology
KW - Aboriginal middens
KW - diatoms
KW - lacustrine sediments
KW - freshwater Aboriginal middens
KW - saprolite (Coorongite)
KW - river management
UR - http://www.scopus.com/inward/record.url?scp=85124194272&partnerID=8YFLogxK
U2 - 10.1080/08120099.2022.2019115
DO - 10.1080/08120099.2022.2019115
M3 - Review article
AN - SCOPUS:85124194272
SN - 0812-0099
VL - 69
SP - 605
EP - 629
JO - Australian Journal of Earth Sciences
JF - Australian Journal of Earth Sciences
IS - 5
ER -