The Holocene geomorphic history of the lower Murray River and Murray estuary

Abstract

Palaeo-environmental studies form an important basis for natural resource management and provide an understanding of pre-anthropogenic conditions as well as an indication of a natural system's likely response to change. A palaeo-estuary's response to the Holocene sea-level highstand can provide a useful analogue to predict potential future change due to sea-level rise associated with anthropogenic climate change. Such knowledge is particularly important in the management of intensively modified systems, such as the Murray-Darling Basin. Australia's largest and most important river system has a long and contentious history of intensive water management. Conflicting scientific accounts of the palaeo-environmental history of the Murray estuary diminish our understanding of this system’s behaviour and reduces the efficacy of natural resource management in the region. This study presents a well-constrained model of the geomorphic evolution of the lower Murray River and Murray estuary with a specific focus on the response of the system to the Holocene sea-level highstand. Hydrodynamic modelling of the lower Murray River and Murray estuary was conducted to evaluate the primary drivers of palaeo-environmental change during the Holocene and constrain the plausible response of the Murray estuary to the +2 m higher-than-present sea level of the Holocene sea-level highstand. Sensitivity testing conducted in 2D demonstrates that variation in sea level significantly altered the regional palaeo-environment and dominated the response of the system, with variation in bathymetry, riverine discharge or barrier morphology resulting in minimal change. The elevated sea level of the Holocene highstand generated an extensive estuarine environment with an elongate central basin extending a minimum of 100 river kilometres upstream from the Murray Mouth and into the confines of the Murray Gorge. The gorge-confined lower Murray River acted as a landward extension of the Murray estuary for much of the Holocene, presenting a unique and unusual geomorphic response that does not conform to conventional estuarine facies models for incised systems. The extremely low gradient of this system facilitated this significant marine incursion and generated an extensive backwater environment with very low current velocities. The utility of applying 2D simulations in lieu of complex and computationally expensive 3D simulations for assessments of palaeo-environmental change has been considered. Two-dimensional simulations are inherently unable to resolve any potential saline stratification within the estuary. Consequently, a comparative analysis of 2D and 3D simulations was conducted to determine whether 2D models are appropriate for assessments of palaeo-environmental change within the lower Murray River and Murray estuary. The 2D-3D comparison demonstrates that evaluations of 2D psu limits can be applied as a proxy for the maximum ingression of the salt wedge at depth resolved in 3D. Overall, results demonstrate a consistency in both salinity and flow velocity magnitude outputs between 2D and 3D simulations such that 2D results provide a meaningful representation of results resolved in 3D. Crucially, a comparison between estuarine zonation and inferred morphology derived from both 2D and 3D simulations generates directly comparable and similar results. Together these results confirm that 2D simulations of the lower Murray River and Murray estuary can be adopted as efficient and meaningful alternatives to 3D simulations, a finding which could be applied to other preliminary studies which may not have access to high-performance computing facilities. Best-estimate Holocene highstand and modern pre-modification 3D hydrodynamic models are validated against sedimentologic data acquired at Monteith, 104 river kilometres upstream from the Murray Mouth. This 3D modelling constrains the upstream limit of the Murray estuary's central basin to 140 river kilometres upstream from the river mouth and confirms that conditions within this palaeo-environment promoted and enabled the deposition and preservation of a laminated silt-clay sequence similar to the mid- to late-Holocene sequence that characterises the central basin deposit of Lake Alexandrina. Analysis of a 30 m sediment core, Monteith-A, reveals an uninterrupted sedimentary succession from lowstand, through transgression, to highstand and demonstrates the response of this large catchment river to fluctuations in sea level during the late-Pleistocene and early-Holocene. The initiation of the Murray estuary presents in core Monteith-A as a shift in deposition to a laminated backwater sequence at 8,518 cal yr BP, that continued to be deposited at least until 5,067 cal yr BP resulting in continuous, conformable deposition of an uninterrupted sequence at a rate of 3.2 m per thousand years. A transect of cone penetrometer soundings demonstrates that the Murray estuary's central basin deposit occupied the entire width of the several kilometre-wide Murray Gorge. The accommodation space provided within the Lower Lakes and Murray Gorge generated an extremely low gradient, backwater environment that captured the river's sediment discharge and essentially prevented the delivery of terrigenous sediment derived from the entire Murray-Darling Basin to the offshore marine environment between 8,518 and 5,067 cal yr BP. The existence of this previously unrecognised natural sediment trap located upstream of the point of discharge to the ocean suggests that mid-Holocene climate reconstructions based on fluctuations of terrigenous sediment in marine cores taken offshore of the Murray's Mouth should be re-evaluated.