The River Murray’s geomorphic legacy – shedding light on environmental change from Holocene floods to Millennium Drought extremes

During the peak of the Millennium Drought in South Australia (1997-2010) historically low river levels triggered widespread riverbank collapse along the lower River Murray. The Millennium Drought failures represented rapid geomorphic change in a reach that is normally considered very stable and presented an unusual phenomenon, as bank failures are usually associated with floods and not prolonged drought. Geotechnical investigations undertaken during the drought identified a thick soft clay unit underlying riverbanks that failed and deemed it instrumental to failure. Its regional extent was not well known. This thesis is centrally concerned with this unit; how and when it formed, its regional extent, what it tells us about climate change, and what its geotechnical and geomorphic consequences are. Subsurface investigation of this unit revealed its regional extent along the River Murray between Blanchetown and Wellington (approx. 200km), characterised by fine-grained clays and silts and preserving a laminated sequence at its downstream extent. This finding was unexpected for this fluvial setting. Investigation revealed that the unit was deposited within a central basin environment that formed in response to post-glacial sea level rise during the Holocene, resulting in a drowned valley setting. The Murray central basin sequestered sediments from the Murray-Darling Basin (MDB) for much of the Holocene (at least 3.3 to 7.6 ka), disrupting the sediment delivery offshore with implications for palaeoclimate interpretations from the offshore Murray Canyon. The Holocene legacy of the River Murray was instrumental in contributing to the Millennium Drought failures, predisposing riverbanks to fail through slow drawn-down during prolonged drought. Geomorphic mapping further revealed the role of antecedent controls in dictating the spatial distribution of failure along the River Murray, allowing for identification of failure prone zones for management of the riverbank collapse hazard. The unit also preserves the first multi-millennial palaeoflood record for the Murray- Darling Basin, a proxy record of eastern Australia’s hydroclimatic variability, crucial for establishing long-term records of hydroclimate variability for water security management in the MDB. Overall, this study provides a detailed understanding of riverbank collapse for the lower Murray, which will improve management strategies for this region with social-economic implications. It also represents a paradigm shift in our comprehension of the lower Murray with evidence of a central basin environment during the Holocene. Finally, it fills the gap of the offshore repositories and the palaeoflood record identified in this study presents opportunities for documenting the hydroclimatic history of the MDB at an unprecedented resolution. Future work on this palaeoflood archive will further our understanding of water availability in the MDB and water security, with implications for management of Australia’s food bowl

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

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

Comparison of landscape approaches to define spatial patterns of hillslope-scale sediment delivery ratio

A sediment delivery ratio (SDR) is that fraction of gross erosion that is transported from a given catchment in a given time interval. In essence, a SDR is a scaling factor that relates sediment availability and deposition at different spatial scales. In this paper, we focus on hillslope-scale SDR, i.e. the ratio of sediment produced from hillslopes to that delivered to the stream network. Factors that affect hillslope water movement, and thus entrainment or deposition of sediments, ultimately affecting the SDR, include upslope area, climate, topography, and soil cover. In erosion models, SDR is usually treated as a constant parameter. However, the use of spatially variable SDRs could improve the spatial prediction of the critical sources of sediment, i.e. identification of those areas directly affecting stream water quality. Such information would improve prioritisation of natural resource management effort and investment. Recent literature has described several landscape approaches to represent SDR variability in space, some of which account only for topography, whilst others consider topography and soil cover characteristics. The aim of this study was to evaluate four landscape approaches for their ability to depict spatial patterns of SDR in the Avon-Richardson catchment in the semi-arid Wimmera region (Victoria, South-east Australia). Erosion was assessed using a semi-distributed model (CatchMODS) with disaggregation based in subcatchments of around 40 km2 area. Hillslope gross erosion was assessed with a RUSLE approach. By applying the four landscape approaches using DEM and estimates of land use cover, four landscape index subcatchment distributions were calculated. These were normalised into standard distributions. Then, a sigmoid function was used to transform the standardised indices into SDR-index distributions ranging from zero to one. Finally, subcatchment SDRs were estimated as the product of the SDR-index by a whole-of-catchment SDR value that was estimated by calibration against sediment loads measured at five gauging stations of the study area. The major sources of hillslope erosion were modelled to be located in the southern hilly areas of the catchment. However, a topographic convergence approach predicted as well important contribution of hillslope-erosion sediment loads coming from the eastern flatter cropping land. The introduction of landscape-variable SDRs improved the overall goodness-of-fit of modelled versus observed sediment loads at five gauging stations located in the catchment for only the topographic convergence approach. However, the limited number of observations (11), the location of some gauging stations downstream of active gully erosion, and the lack of gauging stations monitoring the north-eastern part of the catchment hindered the assessment of which spatial distribution of hillslope erosion best represented the real catchment conditions. Further research is needed to define the relationship between landscape indices and SDR; and to evaluate the spatial distribution of erosion against more complete field evidence

Adaptive management of Ramsar wetlands

Abstract The Macquarie Marshes are one of Australia’s iconic wetlands, recognised for their international importance, providing habitat for some of the continent’s more important waterbird breeding sites as well as complex and extensive flood-dependent vegetation communities. Part of the area is recognised as a wetland of international importance, under the Ramsar Convention. River regulation has affected their resilience, which may increase with climate change. Counteracting these impacts, the increased amount of environmental flow provided to the wetland through the buy-back and increased wildlife allocation have redressed some of the impacts of river regulation. This project assists in the development of an adaptive management framework for this Ramsar-listed wetland. It brings together current management and available science to provide an informed hierarchy of objectives that incorporates climate change adaptation and assists transparent management. The project adopts a generic approach allowing the framework to be transferred to other wetlands, including Ramsar-listed wetlands, supplied by rivers ranging from highly regulated to free flowing. The integration of management with science allows key indicators to be monitored that will inform management and promote increasingly informed decisions. The project involved a multi-disciplinary team of scientists and managers working on one of the more difficult challenges for Australia, exacerbated by increasing impacts of climate change on flows and inundation patterns

Joining the dots: hydrology, freshwater ecosystem values and adaptation options

AbstractThe objective of this research was to investigate and test the necessary steps in developing an adaptation planning framework for freshwater biodiversity. We used Tasmania as a test case to demonstrate how downscaled climate model outputs could be integrated with spatially resolved hydrological models and freshwater biodiversity data. This enabled us to scope adaptation actions at local, regional and state scales for Tasmania, and to explore how priorities might be set.To achieve this integration we quantified how different climate change scenarios could affect the risks to biodiversity and ecosystem values (‘biodiversity assets’) in freshwaters, the scope and types of adaptation actions, and assessed the strengths and weaknesses of the policy and planning instruments in responding to climate change.We concluded that downscaled climate modelling, linked with modelling of catchment and hydrological processes, refines projections for climate-driven risks to aquatic environments. Spatial and temporal hazards and risks can now be compared at a variety of scales, as well as comparisons between biodiversity assets (e.g. relative risk to riparian vegetation v. in-stream biota). Uncertainties can be identified and built into adaptation processes. Notwithstanding this progress, we identified a number of issues that need to be addressed in order to increase confidence in this process.The main issues for improved and timely modelling are: frameworks for using and downscaling outputs from improved global climate models as they become available; better data on thermal tolerances of freshwater biota; and, improved methods for predicting key water temperature variables from air temperature and other biophysical predictors. Improvements are also needed in updating and maintaining high quality biodiversity data sets, and better spatially explicit information on the contributions of groundwater to surface waters and rates of recharge.The list of adaptation options available is extensive, but the key challenge is to organise these options so that stakeholders are not overwhelmed. Scenario modelling that incorporates explicit tools for comparing costs, benefits, feasibility and social acceptability should help with setting priorities but require further development.A review of current Australian policies revealed a variety of responses driven by both water reform and climate change agendas. Many agencies are actively revising their policies to accommodate adaptation. However, we note that much of the reform of the water sector in the last 10–15 years has aimed to improve certainty for non-environmental water uses. Under the National Water Initiative, governments have agreed that entitlement holders should bear the risks of reduced volumes or reliability of their water allocations as a result of changes in climate. The key opportunity for adaptive uptake of climate adaptations is by developing and periodically reviewing water management planning tools. Pathways need to be developed for integrating the traditional evolution of planning and policy with the needs for climate change adaptation for aquatic ecosystems. Formal mechanisms for the uptake of knowledge about identified risks into policy and legislative instruments remain under-developed. An even bigger challenge is to integrate multiple adaptation strategies (sometimes at different scales) to achieve specific adaptation objectives within regions or catchments—especially where a mix of water management and non-water management is required

Value of Returns to Land and Water and Costs of Degradation Vol 1 of 2

Volume 1 (Main Report), Consultancy report for the National Land and Water Resources Audit, Theme 6.1. This report presents new datasets developed through the National Land and Water Resources Audit that relate to economic aspects of natural resource management in Australia. There is a focus on resources used to support agriculture and resources impacted by agriculture. Consistent with protocols used by the Australian Bureau of Statistics and the Australian Bureau of Agriculture and Resource Economics, the database provides a new capacity to integrate natural resource information in Australia. The datasets are primarily built for the 1996/97 financial year, the year of an agricultural census.audit;Australia;natural resource management

Avoiding the “fat” of the land: case studies of agricultural nutrient balance

Let’s start with a simple analogy: if a person eats more than they need, they gain weight. That is: if our feed inputs (kilojoules in) are greater than our outputs (exercise — kilojoules out) then we will gain weight (kilojoules in storage).That’s our fat. If, on the other hand, our feed inputs are less than our outputs, then we will lose weight.And if our inputs are the same as our outputs, our weight will remain constant. In general, the further away you are from an ‘ideal’weight, the greater the health risks. And yes, other aspects of your body management — smoking, drinking, too many late nights and B grade movies will also impact on your health — but the excess weight is important. It’s all about balance. An agricultural enterprise is very similar: if inputs of feed and fertiliser (nutrient in) exceed the sum of the products sold or exported from the property (nutrients out), then there will be nutrients for storage in the soil or loss. The immediate nutrient losses can cause eutrophication of waterways, and the stored nutrients represent a potential for loss in the future when stored in the soil. So this is the environmental risk — too much “fat” in the agricultural system

The role of water markets in climate change adaptation

Abstract Water markets were first introduced in Australia in the 1980s, and water entitlement and allocation trade have been increasingly adopted by both private individuals and government.Irrigators turned to water markets (particularly for allocation water) to manage water scarcity and Governments to acquire water for the environment (particularly water entitlements. It is expected that further adoption of water markets will be essential for coping with future climate change impacts. This report reviews the available literature related to the relationship between southern Murray-Darling Basin (sMDB) water markets and anticipated climate change effects; the economic, social and environmental impacts of water reallocation through markets; and future development requirements to enhance positive outcomes in these areas. The use of water markets by irrigators can involve both transformational (selling all water entitlements and relocating or switching to dryland) and incremental (e.g. buying water allocations/entitlements, using carry-over, changing water management techniques) adaptation to climate change. Barriers to both adaptations include: current and future climate uncertainty; poor (or non-existent) market signals; financial constraints; information barriers; mental processing limits; inherent attitudes toward or beliefs about climate change; institutional barriers and disincentives to adapt. A better understanding of trade behaviour, especially strategic trade issues that can lead to market failures, will improve the economic advantages of water trade. There remains community concerns about the impacts of transfers away from regional areas such as reduced community spending and reinvestment; population losses; loss of jobs; declining taxation base, loss of local services and businesses, regional production changes; and legacy issues for remaining farmers. However, it is hard to disentangle these impacts from those caused by ongoing structural change in agriculture. Rural communities that are most vulnerable to water scarcity under climate change and water trade adjustment include smaller irrigation-dependent towns. Communities less dependent on irrigation are better able to adapt. Further, where environmental managers use water markets to deal with water variability and to ensure ecological benefits, irrigators are concerned about its impact on their traditional use of markets to manage scarcity. Climate change and water scarcity management are intertwined, suggesting that policy, institutional and governance arrangements to deal with such issues should be similarly structured. Water users will adapt, either out of necessity or opportunity. The cost of that adaptation at individual, regional and national levels—particularly to future water supply variability—can be mitigated by the consideration of the existing advantages from future opportunities for water marketing in Australia