Abstract
Three-dimensional numerical modelling of the marine and fluvial dynamics of the lower Murray River demonstrate that the mid-Holocene sea-level highstand generated an extensive central basin environment extending at least 140 kilometres upstream from the river mouth and occupying the entire one to three kilometre width of the Murray Gorge. This unusually extensive, extremely low-gradient backwater environment generated by the two metre sea-level highstand captured most, if not all, of the fine-grained sediment discharged from the 1.06 million square kilometre Murray-Darling catchment. This material was sequestered within a >60 kilometre long, >10 metre thick valley-wide deposit of finely laminated mud. This previously unrecognised sediment trap persisted from 8,518 to 5,067 cal yr BP preventing sediment delivery to the marine environment. Its identification requires that mid-Holocene climate reconstructions for southeastern Australia based on fluctuations in the delivery of fine-grained sediment to the ocean offshore the lower Murray River’s mouth must be re-evaluated.
Highlights
Effective natural resource management benefits from a thorough understanding of how a system functioned prior to anthropogenic modification
A best-estimate 3D Holocene highstand scenario supports and extends the 2D model results of Helfensdorfer et al.[23], which showed that the +2 m sea level of the Holocene highstand generated an estuarine palaeo-environment throughout the Lower Lakes and upstream into the lower reaches of the lower Murray River (LMR) (Fig. 2a)
This high-resolution 3D model suggests that the lower Murray Gorge flooded completely, with the depth-averaged marine-brackish (10 psu) limit penetrating upstream as far as Tailem Bend
Summary
Effective natural resource management benefits from a thorough understanding of how a system functioned prior to anthropogenic modification. Managers will become increasingly reliant on high quality palaeo-climatic data to inform their policies[1,2] This is the case for intensively managed river systems, such as Australia’s Murray-Darling Basin (MDB), that support large-scale agriculture whilst being important ecological refuges (Fig. 1). Landward migration of fluvial, estuarine, and marine environments caused a continuing decrease in the depositional gradient of coastal plain rivers which typically resulted in the deposition of an upward fining sequence within fluvial deposits in www.nature.com/scientificreports coastal incised valleys[17] This commonly presents as a transition from high-energy fluvial sands to low-energy mud-dominated sediments[17]. A previous assessment of the extent of the palaeo-Murray estuary demonstrated that the +2 m higher-than-present sea level of the mid-Holocene highstand generated an estuarine environment throughout the Lower Lakes and well upstream into the LMR23. The immense longitudinal extent of the Murray estuary’s depositional response is atypical and presents an end-member example of an extremely low gradient coastal plain system
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