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REPORT OF THE RIVER MURRAY SCIENTIFIC PANEL ON ENVIRONMENTAL FLOWS
River Murray – Dartmouth to Wellington and the Lower Darling River
June 2000
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RIVER MURRAY SCIENTIFIC PANEL ON ENVIRONMENTAL FLOWS
REPORT OF THE RIVER MURRAY SCIENTIFIC PANEL ON ENVIRONMENTAL FLOWS
River Murray – Dartmouth to Wellington and the Lower Darling River
June 2000
AUTHORS: MARTIN THOMS, PHIL SUTER, JANE ROBERTS, JOHN KOEHN, GARY JONES, TERRY HILLMAN AND ANDY CLOSE
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RIVER MURRAY SCIENTIFIC PANEL ON ENVIRONMENTAL FLOWS
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FOREWORD
The need to provide adequate flows to assist decision-making processes on future water allocation arrangements. These include for the environment to ensure that the development by the Murray-Darling Basin our rivers remain healthy has been Commission of an Interim Flow Management Strategy for the River Murray being undertaken acknowledged as one of the primary by Victoria, New South Wales and South environmental issues in current times. Australia, as well as the water allocation processes of each of the States. It should be This has been recognised by both noted that this study has been undertaken to Federal and State Governments in the provide one input to those decision-making processes. Other inputs include further Council of Australian Governments information on the needs of other water users. (COAG) Water Reform Agenda It should be also noted that each of these water allocation decision-making processes (ARMCANZ 1995), in the National incorporates some form of public consultation Principles for the Provision of Water before final decisions are made. The study was established to provide scientific for Ecosystems (ARMCANZ and advice to the decision-making processes listed ANZECC 1996) and through funding above on possible actions to improve the environmental flow regime of the River Murray provisions of the National Heritage which may be implemented by the MDBMC, Trust. whilst providing for the needs of other users. It was not established to identify flows that will return the river to natural or pristine condition. All States now have mechanisms for water allocation which recognise the need to provide The study was required to deal with the entire water for the environment whilst providing River Murray and the lower Darling River. for other users. For example, Victoria is Given the length of the rivers and the complexity implementing Bulk Entitlements to water, of the regulation systems involved, it was only Queensland is developing a series of Water possible to undertake this type of study at a Allocation Management Plans, South Australia broad scale. Therefore, it concentrates on is preparing Catchment Water Management identifying the major factors affecting river Plans and Water Allocation Plans and New health at the macro-scale and makes South Wales is developing Environmental Flow Objectives for its major rivers. The recommendations at that scale, rather than Murray-Darling Basin Ministerial Council providing recommendations to solve problems (MDBMC) has provided a broader context for at local sites. Within these larger scale each of these water allocation processes by its recommendations, there is considerable scope cap on all diversions within the Murray-Darling for local environmental managers to deal with Basin. A key aspect for the implementation of local issues. all these initiatives is the actual determination of appropriate environmental flow regimes using the ‘best scientific information available’. This study has been established to provide a compilation of the best scientific information available on environmental flow regimes for the River Murray and lower Darling River. DON BLACKMORE This will provide information on the CHIEF EXECUTIVE OFFICER environmental requirements of these rivers MURRAY-DARLING BASIN COMMISSION
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RIVER MURRAY SCIENTIFIC PANEL ON ENVIRONMENTAL FLOWS
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CONTENTS
FOREWORD 3
ACKNOWLEDGMENTS 10
INTRODUCTION 12
1. THE PROJECT 13 1.1 BACKGROUND 13 1.2 SCIENTIFIC PANEL 13 1.3 PROJECT BRIEF 14 1.3.1 Objective 14 1.3.2 Project Requirements 14 1.3.3 Reach Selection 14
2. SCIENTIFIC PANEL APPROACH 17 2.1 CONTEXTUAL FRAMEWORK 17 2.1.1 Principles for Ecosystem Health 17 2.1.2 Water Management Context 18 2.1.3 Adaptive Management 19 2.2 ZONE ASSESSMENT METHODOLOGY 19 2.3 STRUCTURE OF REPORT 21
PART I – OVERVIEW 23
3. SCIENTIFIC OVERVIEW OF THE RIVER MURRAY & THE LOWER DARLING RIVER 23 3.1 PHYSICAL SETTING 23 3.1.1 Landform 23 3.1.2 Geomorphic Changes 24 3.1.3 Hydrology 36 3.1.4 Water Quality 31 3.2 RIVER ECOLOGY 37 3.2.1 Floodplain Ecology 37 3.2.2 Vegetation and Plant Community Ecology 39 3.2.3 Phytoplankton and Benthic Algae 44 3.2.4 Invertebrate Distribution and Ecology 45 3.2.5 Fish Distribution and Ecology 47 3.3 CONCLUSION 51
4. COMMON ISSUES AND MANAGEMENT ACTIONS 55 4.1 CHANGES TO FLOW REGIME 55 4.1.1 Constant Flows 56 4.1.2 Sustained Unseasonal In-channel Flows 58 4.1.3 Reduction in the Occurrence of Floods 59 4.2 LINKAGES 61 4.2.1 Unseasonal High Flows Providing Artificial Linkages 62 4.2.2 Barriers to Fish Passage 62 4.3 HABITAT 64 4.3.1 Conservation of the Floodplain 65 4.3.2 Reduction in Snag Numbers 65 4.4 DISRUPTION OF METABOLIC FUNCTIONING 66 4.4.1 Unseasonally Low Water Temperatures 66 4.4.2 Increased Turbidity During Summer 68 4.5 EFFECTS OF WEIR POOLS 70 4.5.1 Constant Water Levels in Weir Pools 71 4.5.2 Effects of Weir Pools on Connected Wetlands 71 4.5.3 Bank Instability Downstream of Locks 72
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5. REFLECTIONS ON THE PROCESS 75 5.1 MACRO VIEW OF THE RIVER 75 5.2 KNOWLEDGE GAPS AND FURTHER RESEARCH 76 5.2.1 Linking Hydrology and Ecology 76 5.2.2 Management of Menindee Lakes and Lake Victoria 77 5.2.3 The Role of Temperature in the Functioning of the Instream Environment 77 5.2.4 Drying Wetlands Connected to Weir Pools 77 5.2.5 Optimisation of Environmental Water 78 5.2.6 Regeneration Strategies 78 5.2.7 Biodiversity Conservation 78 5.3 POTENTIAL ISSUES FOR FUTURE MANAGEMENT 78 5.3.1 Migration of Abstraction 78 5.3.2 Relative Ecological Significance of Demand Changes 79 5.3.3 Release and Recapture of Environmental Flows 79 5.3.4 Redistribution of Recovered Water 79
PART 2 – RIVER ZONE ASSESSMENT 81
6. ZONE 1 – MITTA MITTA RIVER – DARTMOUTH DAM TO HUME DAM 81 6.1 HYDROLOGIC MANAGEMENT 81 6.2 ENVIRONMENTAL CONDITION 84 6.3 THREATENING PROCESSES 85 6.4 PRIORITY ISSUES 86 6.5 MANAGEMENT RECOMMENDATIONS 86 6.5.1 Low Water Temperatures 86 6.5.2 Constant Flows 86 6.5.3 Reduction in the Inundation of Some Features of the Floodplain 87 6.5.4 Reduction of Instream Habitat 87 6.5.5 Evaluation of Low Flow Levels 87
7. ZONE 2 - HUME DAM TO TOCUMWAL 89 7.1 HYDROLOGIC MANAGEMENT 89 7.2 ENVIRONMENTAL CONDITION 92 7.3 THREATENING PROCESSES 95 7.4 PRIORITY ISSUES 95 7.5 MANAGEMENT RECOMMENDATIONS 95 7.5.1 Unseasonal High Flows 95 7.5.2 Conservation of Anabranches 96 7.5.3 Constant Flows 96 7.5.4 Reduction in the Occurrence of Floods 97 7.5.5 Low Water Temperatures 97 7.5.6 Evaluation of Low Flow Levels 98
8. ZONE 3 - TOCUMWAL TO TORRUMBARRY WEIR, INCLUDING BARMAH CHOKE 99 8.1 HYDROLOGIC MANAGEMENT 99 8.2 ENVIRONMENTAL CONDITION 100 8.3 THREATENING PROCESSES 101 8.4 PRIORITY ISSUES 101 8.5 MANAGEMENT RECOMMENDATIONS 102 8.5.1 Unseasonal Summer-Autumn Flooding 102 8.5.2 Reduced Frequency of Winter-Spring Flooding 103 8.5.3 Constant Flows 103 8.5.4 Conservation of Anabranch Channels 103
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9. ZONE 4 - TORRUMBARRY WEIR TO WENTWORTH 105 9.1 HYDROLOGIC MANAGEMENT 105 9.2 ENVIRONMENTAL CONDITION 106 9.3 THREATENING PROCESSES 109 9.4 PRIORITY ISSUES 109 9.5 MANAGEMENT RECOMMENDATIONS 109 9.5.1 Reduced Frequency of Inundation of Flood Runners and the Floodplain 109 9.5.2 Reduced abundance and distribution of snags 111 9.5.3 Negative Impacts of Weir Pools 111 9.5.4 Barriers to Fish Passage 113
10. ZONE 5 - WENTWORTH TO WELLINGTON 115 10.1 HYDROLOGIC MANAGEMENT 115 10.2 ENVIRONMENTAL CONDITION 116 10.3 THREATENING PROCESSES 119 10.4 PRIORITY ISSUES 119 10.5 MANAGEMENT RECOMMENDATIONS 119 10.5.1 Unseasonal Wetting and Drying of Fringing Riverine Wetlands 119 10.5.2 Reduction in the Frequency of Flooding of Areas other than Fringing Riverine Wetlands of the Floodplain 120 10.5.3 Barriers to Fish Passage 120 10.5.4 Bank Erosion Downstream of Weirs due to Rapid Rate of Fall after Reinstalling Weir 121 10.5.5 Increase in Abundance and Distribution of Snags 121 10.5.6 Risk of Algal Blooms 121 10.5.7 Increased Turbidity Sourced from the Darling River in Summer Months 121
11. ZONE 6 - LOWER DARLING AND THE GREAT ANABRANCH 123 11.1 HYDROLOGIC MANAGEMENT 123 11.2 ENVIRONMENTAL CONDITION 126 11.3 THREATENING PROCESSES 128 11.4 PRIORITY ISSUES 128 11.5 MANAGEMENT RECOMMENDATIONS 128 11.5.1 Barriers to Fish Passage 128 11.5.2 Reduced Frequency of Flooding 129 11.5.3 Constant Flows 129 11.5.4 Unseasonal Flows including Rapid Rates of Rise and Fall 130 11.5.5 Risk of Algal Blooms 130 11.5.6 Permanent Inundation of Anabranch Channel 130
12. GLOSSARY 131
13. REFERENCES 137
APPENDIX 1 – Steering Committee 141
APPENDIX 2 – Project Brief 143
APPENDIX 3 – Operational Modelling 147 Section 1 – Modelling Results for the Expert Panel on Environmental Flows in the River Murray and Lower Darling 147 Section 2 – Modelling Scenarios for the Murray Expert Panel 157 Section 3 – Impact of Reducing the Channel Capacity Downstream of Yarrawonga 165 Section 4 – Feasibility of Flow Pulsing 167
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FIGURES
2.1 The six river zones identified for assessment by the River Murray Scientific Panel
3.1 The four distinct river tracts, and major wetlands and lakes of the River Murray
3.2 Difference between natural (unregulated) and current flows in the River Murray at Albury and Yarrawonga, and in the Edward River at Deniliquin
3.3 Difference between natural (unregulated) and current flows in the River Murray at Euston and Wentworth
3.4 Difference between natural (unregulated) and current flows in the Darling River at Burtundy
3.5 Difference between natural (unregulated) and current flows in the River Murray at the South Australian border and at the Barrages
3.6 Comparison of Mitta Mitta River temperatures at Colemans and temperatures of Dartmouth Dam release
3.7 Temperatures of Dartmouth Dam release into the Mitta Mitta River between January 1975 and June 1996
3.8 Effect of regulating pond on the Mitta Mitta River temperatures at Colemans between May 1987 and April 1990
3.9 Temperature of surface water in Hume Dam and in the River Murray at Heywoods Bridge for the periods (a) January 1991 – January 1996 and (b) July 1981 – July 1985
3.10 Concentration of TKN and nitrate in the River Murray at Heywoods Bridge between January 1980 and January 1997
4.1 Relationship between major ecological issues and river zones
4.2 Aspects of the flood pulse that may be ecologically important
4.3 Impact of flow regulation in the River Murray at (a) Torrumbarry and (b) Euston
4.4 Diagrammatic graph showing variation in flow according to a step function via adjustments to releases from storages on the River Murray
4.5 Impact of flow regulation in the River Murray downstream of Hume Dam at Albury
4.6 Typical Barrier to fish passage
4.7 Ladders assist migration of fish
4.8 River Murray macroinvertebrate numbers and turbidity at Murtho in South Australia
4.9 Abundance of (a) the shrimp Paratya australiensis and (b) the prawn Macrobrachium australiense in the River Murray at Murtho South Australia
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4.10 Impact of Lock 6 weir on the recession of the 1981 flood
4.11 Impact of refilling Lock 6 weir pool, both at and below its desired level, on the recession of the 1981 flood
6.1 Map of Zone 1: Mitta Mitta River between Dartmouth Dam and Hume Dam
6.2 Comparison of natural and current regulated flows in the Mitta Mitta River below Dartmouth Dam
6.3 Change in monthly flow in the Mitta Mitta River downstream of Dartmouth Dam
7.1 Map of Zone 2: River Murray between Hume Dam and Tocumwal
7.2 Change in monthly flow in the River Murray at Albury
7.3 Typical change to the flow regime in the River Murray at Yarrawonga Weir
7.4 Change in monthly flow in the River Murray downstream of Yarrawonga Weir
7.5 Hume Dam pre-release operation in 1989
8.1 Map of Zone 3: River Murray between Tocumwal and Torrumbarry Weir
9.1 Map of Zone 4: River Murray between Torrumbarry Weir and Wentworth
9.2 Typical pre and post regulation flow in the River Murray at Euston
9.3 Change in monthly flow in the River Murray at Euston
9.4 Relationship between cyanobacterial abundance and discharge in the lower River Murray
10.1 Map of Zone 5: River Murray between Wentworth and Wellington
10.2 Change in monthly flow in the River Murray at South Australian border
11.1 Map of Zone 6: Darling River between Menindee Lakes and Wentworth, including the Great Anabranch
11.2 Typical pre and post regulation flows in the Darling River at Menindee
11.3 Change in monthly flow in the Darling River at Menindee.
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TABLES
1.1 List of river zones, reaches proposed by the Steering Committee, and the sample and observation sites visited by the River Murray Scientific Panel
3.1 Sources of streamflow at Albury
3.2 Major tributaries in the Riverine Tract of the River Murray
3.3 Water diversions in the Riverine Tract
3.4 Tributaries in the Mallee Tract
3.5 Water diversions in the Mallee Tract
3.6 Water diversions from the lower Darling River
3.7 River flows and water diversions from the Lower Murray Tract
3.8 Freshwater fish species of the River Murray
3.9 Spawning likelihood at Colemans and Tallandoon in the Mitta Mitta River for Murray Cod, Trout Cod and Macquarie Perch between 1968 and 1993
3.10 Flow management activities that threaten the key components of river-floodplain ecosystem health – habitat diversity, natural linkages and metabolic functioning
3.11 River and land management activities (other than flow management) that threaten the key components of ecosystem health – habitat diversity, natural linkages and metabolic functioning
4.1 The impact of current development on the frequency of occurrence of the 10, 20 and 50 percentile natural conditions flood flow for consecutive stations down the River Murray
4.2 Optimal spawning temperatures and times for major fish species in the Mitta Mitta River
4.3 Ecological characteristics and consequences of weir pools considered under the three key principles of Ecological Variability (EV), Linkages (L) and Habitat (H)
9.1 Percentage of years in which the maximum mean monthly flow for the year exceeded 20,000 ML/day, for natural and current (1994) conditions – Torrumbarry to Euston
9.2 Natural and current (1994) frequency of 40,000 ML/day floods with 2 month duration between August and December – Torrumbarry to Wentworth
9.3 Natural and current (1994) frequency of 60,000 ML/day floods with at least 1 month duration between August and December – Torrumbarry to Wentworth
9.4 Natural and current (1994) frequency of selected floods in the Torrumbarry to Wentworth river zone under 20% reduced demand scenario
9.5 Percentage of years in which the mean modelled flow is less than 4000 ML/day at Torrumbarry, Euston, Mildura and Wentworth weirs for the months from November to April under current (1994) conditions
9.6 Modelled natural flows (ML/month) for 20, 50 and 80 percentile frequencies at Torrumbarry, Euston, Mildura and Wentworth weirs, for the period November to April
10.1 Information on the locks and weirs on the River Murray between Euston and Blanchetown
10.2 Percentage of years in which the maximum mean monthly flow at the South Australian border exceeds 30,000 ML/day, 60,000 ML/day and 100,000 ML/day for at least one month
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ACKNOWLEDGMENTS
he River Murray Scientific Panel would like to thank the project team who provided considerable support to the study. Members include Jane Doolan, Paul Wilson, Shelley Heron, Alieta Donald, TJulia Reed and Tarnia Kruger. Jane Doolan and Alieta Donald in particular took responsibility for collating inputs from individual Panel members and preparing drafts of the resultant reports. The Panel would also like to thank Anne Jensen who provided a large part of the organisation of the South Australian section of the study, and Dick Francis who assisted in writing the hydrological sections of this report. The Murray-Darling Basin Commission, in addition to providing a Panel member, also supplied invaluable hydrologic data and undertook modelling throughout the exercise. A number of people attended various sites on the field trips and provided a valuable source of local information. The Panel would like to thank:
• Ken Harris and Paul Lloyd, Department of Land and Water Conservation, Deniliquin;
• David Leslie, State Forests, Deniliquin;
• John Bartell and Peter Liepkalns, Dartmouth Dam operators;
• Colin Fitzpatrick, Yarrawonga Weir operator;
• Terry Court and Shane McGrath, Torrumbarry Weir operators;
• Alf Richter, Lake Victoria operator;
• Dennis Moy;
• John Harris;
• Joe Murphy, Barmah-Millewa consultative group;
• Neil Eagle, Murray and Lower Darling Management Committee; and
• Don Reid for help on the Great Anabranch.
In addition, individual Panel members sought advice from others in their field. They would like to specifically mention the valuable input of Bryan Pierce on fish information for South Australia. The Panel would also like to thank Prof. Tom McMahon and Dr Fran Sheldon who refereed the report and provided many invaluable suggestions. Finally, the Panel would like to acknowledge the input of the Steering Committee (see Appendix 1) and thank them for what turned out to be a highly challenging project.
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INTRODUCTION
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1–THE PROJECT
1.1 BACKGROUND al. 1996). First derived in Australia following on from the work in South Africa on the Building Blocks procedure (King and Louw 1998), it employs The Murray-Darling Basin Ministerial a sound scientific method to provide a range of Council (MDBMC) implemented a Cap recommendations on environmental flows to water resource managers. The method is recommended in on all water diversions effective June situations that require a relatively rapid appraisal 1997. The Cap was seen as an essential and recommendations for water management (Burgess and Thoms 1997). first step in implementing a more 1.2 SCIENTIFIC PANEL sustainable flow regime. The Steering Committee established to oversee However, it is recognised that further work is the project (see Appendix 1) considered that an required to establish management systems that will: analysis of flow regimes required expertise in the • maintain and, where possible, improve existing following seven areas: geomorphology, flow regimes to protect and enhance the macroinvertebrate ecology, riparian and aquatic riverine environment; and vegetation, fish biology, water quality and algal • achieve sustainable consumptive use by ecology, floodplain ecology, and river operations. developing and managing the water resources of The Steering Committee then established a the River Murray system to meet ecological, Scientific Panel with the following areas of expertise: commercial and social needs. The next step is the development of a flow • Geomorphology: Dr Martin Thoms management plan for the major river systems (Scientific Coordinator), Senior Lecturer, which will maximise environmental benefits whilst University of Canberra meeting the general requirements of existing users. These flow management plans will recognise that • Macroinvertebrate Ecology: Dr Phil Suter, improving river health is not just a matter of Lecturer - Environmental Management & improving flow regimes but may require Ecology, Latrobe University, complementary activities. Albury/Wodonga Campus A key issue to be resolved before a flow management plan could be developed for the River Murray system was the establishment of • Riparian Vegetation & Macrophytes: Dr Jane environmental flow regimes for the River Murray Roberts, Senior Research Scientist, CSIRO and the lower Darling River. Land and Water The MDBMC Water Policy Committee considered that the most effective way to proceed with the • Fish Ecology: Mr John Koehn, Principal establishment of environmental flow regimes would Scientist, Marine & Freshwater Resources be to review the adequacy of current flow regimes by Institute, Victoria the use of a Scientific Panel. The Scientific Panel approach is a rapid appraisal of the environmental • Water Quality & Algae: Dr Gary Jones, condition and requirements of the riverine Principal Scientist, CSIRO Land and Water ecosystem by a multi-disciplinary team with local and scientific expertise. Advice provided by the • Floodplain Ecology: Dr Terry Hillman, Scientific Panel would be considered by the MDBMC Director, Murray-Darling Freshwater and contribute to the development of a Flow Research Centre Management Strategy for the River Murray and to negotiations with the States on the actions necessary • River Operations: Mr Andy Close, Manager to implement the cap. Water Policy, Murray-Darling Basin Commission The Scientific Panel method has been successfully employed in Australia on other large The Department of Natural Resources and river systems, for example along the Darling River in Environment (Victoria) provided the project Queensland and New South Wales (Thoms et al. manager (Dr Jane Doolan) and executive 1996), and the Snowy River in Victoria (Erskine et support as required.
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1.3 PROJECT BRIEF • after discussion with river operators regarding the feasibility of the range of management 1.3.1 Objective actions, set overall priorities for management actions. This would establish a range of short- The major objective of the project as stated in term, medium-term and long-term objectives the project brief is to: for the management of the river which would improve its ecological condition. Identify changes in river operations for the River In addition, the Panel were requested to comment Murray and lower Darling River that should result on the integrated management of the river. in general improvements in the environmental condition of these river reaches whilst considering 1.3.3 Reach Selection the current needs of existing water users. The Steering Committee originally proposed The full project brief is provided in Appendix 2. thirteen reaches within the River Murray from Advice provided by the River Murray Dartmouth Dam to the Barrages (Appendix 2). Scientific Panel is intended to identify both These reaches were based on major control short-term actions and longer term recommendations structures where alterations to operational to improve the environmental flow regime of procedures could influence environmental flows the River Murray which may be implemented within the river. After a brief overview of the by the MDBMC. The Panel was not asked for study area, the Scientific Panel combined a recommendations that would return the river to number of reaches into seven river zones to natural or pristine conditions. Instead the Panel provide a larger-scale overview (Table 1.1). was instructed to identify a range of options that The Panel was able to assess all but the last could be undertaken which will provide some river zone (Wellington to the Barrages) which improvement in environmental values. focused on the operation of the Barrages. This zone was beyond the collective areas of 1.3.2 Project Requirements expertise of the Panel as it required expertise in lake and estuarine ecology and considerable For the purposes of the study, the area – the local knowledge. For this reason, a separate River Murray from Dartmouth Dam to Scientific Panel was established to undertake the Wellington and the lower Darling River from assessment of this zone. This part of the project Menindee to Wentworth – was divided into was managed by Anne Jensen, Department of thirteen river reaches. For each river reach, the Environment and Natural Resources (South project brief required the Panel to: Australia). The Barrage Scientific Panel has • establish the current habitat types and the reported to the MDBMC separately. The current condition of these habitats including documenting document is the report of the River Murray changes from the likely natural state; Scientific Panel and therefore provides an assessment of the River Murray from • identify the major aspects of the flow regime Dartmouth Dam to Wellington and the lower which would maintain or restore ecological Darling River (Menindee to Wentworth). habitats and/or communities and thus set However, the project managers of the two long-term flow objectives for each reach; groups have ensured that the approaches of the two Scientific Panels were complementary. • identify current threats to each habitat type Initially, the Steering Committee included and/or community, including those related to the entire Edward and Wakool River systems as flow and those related to other factors; part of the study area. However, the Steering Committee and the Scientific Panel agreed that • identify management actions which could be as the hydrology and operation of this section taken to alleviate threats and improve was exceedingly complex, it would not be ecological values; possible to make recommendations without a detailed understanding of the system and the • set priorities on management actions from an capacity to visit more sites than was possible. ecological perspective; and Consequently it was agreed to limit the study to
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TABLE 1.1 List of river zones, reaches proposed by the Steering Committee, and the sample and observation sites visited by the River Murray Scientific Panel.
Zone Proposed Location Sample Sites Observation Sites Reach 1 1 Mitta Mitta River Colemans Dartmouth Dam between Dartmouth Tallandoon Dam and Hume Dam 2 2 Hume Dam to Howlong Doctors Point Yarrawonga Corowa 3 Yarrawonga Weir to Bruces Bend Cobram Tocumwal 3 4a Tocumwal to Barmah Picnic Point Tocumwal 4b Tocumwal to Deniliquin Edward River Lawson’s Siphon Steven’s Weir 5 Barmah to Torrumbarry Weir Moama Torrumbarry 4 6 Torrumbarry Weir to Marrabbit Robinvale Euston Tooleybuc Boundary Bend Wakool Junction 7 Euston to Wentworth Tammit Station 5 8 Lock 10 to Lock 7 Lock 7 (d/s of Walpolla Rufus River) Lock 6/Pilby Creek Moorna Station Lake Victoria 9 Lock 7 to Lock 3 Overland Corner Berri Lock 4 Rilli Island Kingston Causeway 10 Lock 3 to Wellington Swanport Swan Reach Marne confluence 6 11 Darling River from Burtundy Flight over the reach Menindee Lakes to Wentworth 12 Darling River Anabranch Bulpinga Flight over the reach 7 13 Wellington to the Barrages
flows in the River Murray and at the Edward the assessment was divided into two sections: River off-take. • Dartmouth Dam to Wentworth; and Assessment of the river zones was based on • lower Darling and Wentworth to Wellington. detailed inspections of a number of sample sites, Initially it was intended that both sections observations at a range of other sites and the would be completed by March 1997. However, knowledge and experience of the zone by Panel due to early flooding of the River Murray in members. Sites for assessment were chosen by September 1996 and releases from Hume Dam the Panel in consultation with members of the in late 1996, work on the second leg of this Steering Committee and from additional advice sought from other experts and local authorities. study and the Barrage study had to be delayed Sample sites were selected to be, as far as until early 1997. The first field assessment of the possible, representative of the entire river zone. River Murray from Dartmouth Dam to The full list of sites visited is given in Table 1.1. Wentworth was undertaken from 16 June to From a practical perspective all sites could not 21 June 1996. The second stage was undertaken be assessed by the Panel on one field trip, therefore from 3 March to 7 March 1997.
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2–SCIENTIFIC PANEL APPROACH
The Scientific Panel approach • Principles for ecosystem health These outline the major aspects of the health consists of a series of observations of rivers and floodplains which should be considered in management decisions. made on the geomorphological, • Water management context hydrological and ecological character This indicates the Panel’s understanding of the water management context for the report and of a river, in a systematic and rigorous outlines the objectives of the Panel in making manner, and these are combined with recommendations to improve river health. current and historical data to provide 2.1.1 Principles For Ecosystem Health
three main outputs: In order to provide a context for its analysis and 1. A series of statements on the condition of the assessment, the Panel developed three major river system. These statements are derived from principles which, in its view, should govern the the observations made by the scientists, based management of rivers and their associated on their experience, training and knowledge of floodplains to maintain ecosystem complexity the functioning of river ecosystems. and health (ecological integrity). These principles 2. A series of questions (hypotheses) about the provide the necessary point of agreement across interactions between the river ecosystem and disciplines which use and respond to different its flow regime. These can be seen as the longer time frames. These principles are: term issues that need to be addressed or tested. 3. Based upon 1 and 2, a list of 1. Natural diversity of habitats and biota within recommendations are provided to assist the the river channel, riparian zone and the water manager in implementing floodplain should be maintained. environmental flow strategies. 2. Natural linkages between the river and the The combination of observation with individual floodplain should be maintained. or collective knowledge is an important first step 3. Natural metabolic functioning of aquatic in the formulation of questions or hypotheses in ecosystems should be maintained. any scientific method (Schumm 1991). The Scientific Panel approach integrates knowledge The Panel recognises that flow management to across a range of disciplines and can provide restore these components of ecosystem health short-term and long-term recommendations from must occur at a catchment-wide scale, rather an ecosystems perspective. Furthermore, it than just locally. Although this is an area of employs a whole of catchment approach (Burgess active research, there is limited information upon and Thoms 1997). The method in itself is which to make quantitative links between adaptive and iterative, based upon the river hydrology and the ecological health of the river under consideration, the composition of the and floodplain (see Chapter 5). At present the scientific team and the requirements of the water Panel, as well as other scientists and resource managers. However, it should be recognised that managers, are working on the assumption that, this method is one of the first steps in deriving environmental flow requirements and is not in the absence of hard information, the best necessarily a reproducible analysis of determining response is to restore as much of the actual discharge rates for particular target species. pre-regulation regime as possible. The Panel is thus adopting a precautionary principle approach 2.1 CONTEXTUAL FRAMEWORK when discussing these issues. Of particular concern were the changes that regulation has In order to undertake this approach for the River made to the variability of flow regimes with Murray and lower Darling River, the Panel major reductions in variability at the daily, developed a contextual framework which built seasonal and inter-annual time scales (see section on and extended the work of other Scientific 3.1.3 and Crabb 1997; Walker and Thoms 1993). Panels (see Erskine et al. 1996; Thoms et al. Variability of flow refers to the fluctuations 1996). This allowed recommendations to be in flow that occur on a daily basis, where the developed in a way that was consistent, easily flow is not constant from day to day rather, it understood, environmentally defensible and varies in response to conditions such as amount scientifically valid. This framework consisted of: of rainfall in catchment. Regulation in the River
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Murray has lead to capture and storage of flows and 2.1.2 Water Management Context the subsequent release of water at a constant flow over sustained periods of time, negatively impacting The Murray and lower Darling are regulated rivers. the environment (see Chapter 3). Variability also They support large areas of agriculture, a number of operates at other scales, seasonal and inter-annual, significant towns and industries and considerable and these have also been impacted by river recreation interests. As a consequence, these rivers regulation in the River Murray, (see section 3.1.3). have undergone considerable change to their The Panel identified the following flow management ‘natural state’. Given this, the Panel as a whole had practices as causing significant alterations to the to determine its objectives and philosophical variability of the natural flow regime and developed approach in making recommendations to improve recommendations, where possible or required, to environmental values as required by the project redress these (see Chapter 4): brief (see Appendix 2). The Panel worked on the basis that the Murray- • constant flows for sustained periods; Darling Basin cap on water diversions was in place • unseasonal flow; and would be adhered to. Given this, it was considered that the role of the Panel was to • increased minimum flow; comment on current operations. Therefore, • decreased frequency of flood periods; operational changes suggested in this report are to improve environmental condition under the current • reduced duration of individual floods; water allocation arrangements and should not be • rapid rates of rise and fall; and used to increase abstractions in the future. Recent • constant weir pool heights. history indicates that attempts to provide management flexibility by increases in capacity or Recognising that the natural diversity of, and flow have invariably been eroded as the ‘extra’ linkages between, rivers and floodplains has becomes a form of further allocation. Thus storage is developed in association with a highly variable Dartmouth Dam, originally intended to compensate natural hydrology, the Panel developed two further for one-in-ten-year droughts has been increasingly principles specifically related to the natural flow incorporated into the normal irrigation supply. regime (see section 4.1 for detailed information). Similarly, the addition of 300 mm of head space in These principles are: Lake Mulwala to buffer rain-rejection flows in now fully allocated to routine storage of water for 1. Elements of the natural flow regime, in diversion. Also, increased diversions of summer particular, seasonality should be retained as far as peaks to the Edward/Gulpa system to avoid flooding possible, in the interests of conserving a niche for at Barmah were followed by increased abstractions native rather than invasive exotic species and in further along that system. In each case, actions to maintaining the natural functions of the river. relieve stress in the system have resulted in 2. Consistent and constant flow and water level increased stress and reduced ‘room to maneuver’. It regimes should be avoided as much as possible, is hoped that the ‘cap’ means an end to these because this is contrary to the naturally variable trends, in spirit and in reality. Development of flow regime of the River Murray. flexible management procedures for environmental purposes, including those suggested in this report, These five principles provided a framework for the will be of little use otherwise. Panel to make its assessment. In taking this Given this, the Panel did not comment on approach in developing the principles, the Panel possible impacts of any new developments. In their recognises that there is a danger of being overly view, any proposals for new developments should simplistic and not considering all possible impacts. comply with government policy in that it should However, these principles provide a useful fully incorporate the needs of the environment. framework for both this one-off assessment and for Furthermore, the Panel strongly recommends that managers to whom the recommendations are any new proposals must improve the environmental directed. Once the recommendations had been condition of related areas of the river. developed based on these principles, the Panel Given the history of flow regulation and reviewed the recommendations in an holistic way to development along the River Murray, the Panel ensure they would provide an integrated approach worked with the premise that it may not be to improving river health along the River Murray possible to return the rivers to their pristine or and lower Darling River. pre-European settlement state. Therefore the
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recommendations (see Chapter 4 and Part II) aim 2.1.3 Adaptive Management to provide the following: In its approach, the Panel has integrated its Maintain and, where possible, improve the natural experience and has utilised the available data to habitats and ecological functions of the Murray define the relationship between hydrological change and lower Darling Rivers as they are today. and ecological outcome, and has made recommendations on this basis. However, it needs to In developing their recommendations, the Panel be recognised that in many cases this knowledge base also considered that: is limited. The Panel considers that implementation of its recommendations needs to be undertaken in an Habitats and ecosystems resulting from adaptive way where the outcomes are monitored and anthropogenic changes do not have precedence hypotheses actually refined. Increasing knowledge in over identifiable ecological benefits (habitat or this way will improve the efficiency with which function) derived from modifying flow towards water is used for environmental purposes. natural patterns. 2.2 ZONE ASSESSMENT METHODOLOGY In essence, the Panel aimed to maintain the current ecological values and functioning of the As a starting point, the Panel prepared a brief rivers. Where there were opportunities to overview of the attributes of the River Murray and the restore aspects of the natural flow pattern, lower Darling River (see Chapter 3). After particularly in relation to variability, and it was consideration of this overview and the identification of considered that there was likely to be ecological the reaches with common environmental problems or or ecosystem benefits as a result, then these issues, the Panel considered that the study area could changes were recommended (see section 4.1). be seen as six river zones (Figure 2.1): In taking this approach, it was recognised • Dartmouth Dam to Hume Dam; that there may be some changes in current environmental values as a result and that it may • Hume Dam to Tocumwal; take time for the predicted ecological benefits to • the Barmah Choke Area; become apparent. However, where these types of recommendations were made, the changes • Torrumbarry Weir to Wentworth; were considered to be warranted because in the • Wentworth to Wellington; and longer term, the health of the river ecosystem would be improved. • lower Darling and Great Anabranch. In developing recommendations, the Panel The Panel provided an assessment of each river zone was also aware that some minor modifications of based on: operating rules may have local beneficial effects which could cause further degradation in other • individual knowledge of conditions within the zone; areas, i.e. simply moving the problem elsewhere. • current ecological theory and concepts of large In these cases, informed environmental river systems; and trade-offs may have to be made. As a general • joint inspection of a range of sites within the zone. principle, a total catchment management These included a detailed examination of one or philosophy prevailed and recommendations that more sites within the zone and observation of a attempted to resolve the problem locally or number of sites (listed in Table 1.1). upstream were favoured over actions that might transfer it downstream. The Panel only made At a number of sites, the Panel met with local and recommendations which, in their view, would regional river operators, environmental officers and result in a net ecological benefit. selected landowners to gain the benefit of their Given the existing level of resource expertise on specific regional issues. This was not development for the River Murray and the undertaken as part of a public consultation process. relatively small amount of water available to the There was no intention to canvass a complete range environment, the Panel considers that should of views at any site or for any perceived problem. any water be saved as a result of implementing The Scientific Panel process depends on Panel its recommendations, this water should be made members forming judgments based on their available for environmental purposes and not knowledge and experience, interactions with each for further consumptive use. other and from assessing the merits of
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FIGURE 2.1 The six river zones identified for assessment by the River Murray Scientific Panel.
Menindee Lakes Menindee
WEIR 32
r
e
v
i R NEW SOUTH WALES
SOUTH g
AUSTRALIA n i
l ZONE 6
Pooncarie r
a D
Lake Burtundy Victoria LOCK 6 LOCK 2 LOCK 10 Wentworth Morgan LOCK 7 LOCK 3 LOCK 11 LOCK 5 LOCK 8 Waikerie Renmark LOCK 9 Blanchetown LOCK 1 LOCK 4 Hay Euston M LOCK Balranald u r r 15 u m b i d g e e R i v e M r Mannum u r r a y R i v e ADELAIDE r W a Wagga Wagga Swan Hill k B ZONE 5 o i l l o a b o n l R g C r e e k Goolwa Wellington i v e r Deniliquin
Barrages ZONE 4 Kerang Tocumwall Yarrawonga Hume Murray Mouth Weir Reservoir Torrumbarry Albury
r
e Lock 26
v
i
VICTORIA R Echuca M n Shepparton o Ove
i d n s t
d r e t o R
e v a L i v i v i e M R r R i t
n r ta r e Dartmouth
e
u v i R p b Reservoir
s l R i
v
a u
e
o p
a r
G
m
w
a
e
i C ZONE 3 ZONE 2 K ZONE 1
environmental issues in the first instance, free of To do this, the Panel used detailed hydrological what may be considered ‘practical’ constraints. analyses provided by the MDBC to establish the Public consultation will be undertaken as part of the changes in the current flow regime from natural development of an interim Flow Management conditions. This information was used during the Strategy for the River Murray by the MDBMC, on-site assessments and included plots of time series, which will include the Panel‘s recommendations as frequency distributions, monthly means and one of its inputs. monthly medians for monthly data, time series plots Assessment of each river zone focused on the for daily data and information presented in this current condition of the river, riparian zone and report (see section 3.1.3 and Appendix 3). Once floodplain, and the impact of current flow and other these factors had been established, the Panel then management practices on the components of river developed: health as defined by the principles outlined in section 2.1.1. It should be noted that the key • broad operating principles that would need to components are interrelated and that management be applied to redress the extent of change in actions which impact primarily on one component flow regime; (e.g. the diversity of habitats) may have secondary • specific management actions required within effects on other aspects of river health. For each that river zone; river zone, the Panel‘s approach was to identify: • specific operational options for meeting that • the different habitat types and the current management action; and ecological condition of these; • priorities for implementing operational options. • the priority ecological impacts to be addressed. Judgments of priority were made collectively by As part of this process, the major features of the the Panel and were based on experience and current flow regime and operational procedures knowledge of the severity of the problem and its were examined and those aspects where some impacts within the zone and across the river as a change could improve environmental condition whole; and were identified. Where the Panel identified operational options which required modelling to test • the factors (flow-related and others) causing their feasibility, this modelling was carried out by these impacts. Andy Close (MDBC), the river operations expert on
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the Panel. Modelling results were provided to the In undertaking the assessment of each of the Panel and were essential in the development of river zones, the Panel noted that a number of zones recommendations. The results of the modelling have similar environmental issues or threats. In exercises are given in Appendix 3. The Panel also these cases, a generic management approach to deal identified those procedures where operators have with these issues was developed and applied to all taken ecological needs into consideration and river zones where that particular issue was evident. which are currently not having any apparent In some zones, the management approach was deleterious impacts. further refined to be included in the operations of Management actions that were not flow-related the river zone. Chapter 4 outlines these common were also considered such as grazing of riverbanks, threats and issues and the generic approach floodplain development and removal of snags from suggested to deal with these. the floodplain. This gives a sense of the significance of flow changes relative to other aspects of river or 2.3 STRUCTURE OF REPORT land management which may be impacting on the ecological values of the river system. Importantly, The remainder of this report has been structured the Panel considers that it is not sensible to look at in a way which follows the Scientific Panel only one aspect of management (i.e. flows) in approach to the task whilst fulfilling the attempting to improve river condition. There is a requirements of the project brief (see Appendix 2). need to develop an integrated package of river Consequently, it consists of two parts. Part I gives restoration management actions that will improve an overview by discipline of the River Murray and river and floodplain health. This is particularly the lower Darling River which lead in part to the relevant in situations where expected benefits from development of the river zones. It then discusses changes to flows are likely to be thwarted by other river health issues that are common to a range of factors. The information provided for each river river zones and suggests generic solutions. It then zone could be the starting point for the development comments on the holistic management of the of such a package. river, current knowledge gaps and research Finally, on the basis of this analysis, requirements. Part II describes in an integrated recommendations for each river zone were way the condition of the various river zones and developed for the priority issues identified. These makes specific recommendations for those zones. include actions that can be undertaken in both the It should be noted that no one chapter can stand short-term and long-term. Programs need to be put alone but needs to be read in conjunction with the in place which ensure that all priority others in order to understand fully the thinking recommendations are taken up, not just those easily and scientific information behind the implemented in the short-term. recommendations.
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PART I – OVERVIEW 3–SCIENTIFIC OVERVIEW OF THE RIVER MURRAY & THE LOWER DARLING RIVER
The first part of this chapter is an The Murray Basin started to develop more than 50 million years ago during the Tertiary period overview of the physical character and when the Eastern Highlands of Australia began to rise and the basin subsided. This created a hydrology of the two rivers. large saucer like depression that gradually infilled with sediment over the Tertiary period. This includes an examination of historical and Sediments that infilled the basin were derived current river operations, and a comparison of from two main sources: the Eastern Highlands the natural and current flow regimes. The and through marine incursions. This long second part of the chapter provides an overview history as a depositional basin is reflected in the of the river ecology considered by discipline. very low gradient of the river. For example, Important features of the flow regime are 89% of the length of the lower River Murray, identified, and the impacts of changes to these downstream of Wentworth, has a channel slope features on biological aspects of the river of less than 0.00017 m/km. Hence, low stream ecosystems are illustrated. These are slopes and low energies relative to its catchment summarised in the conclusions to the chapter. area characterise the Murray and the lower Darling rivers. 3.1 PHYSICAL SETTING The Eastern Highlands is not only the area of maximum relief but also maximum bedrock The structure and functioning of rivers are outcrop in the catchment. It is a complex area of influenced by many parameters. These are sedimentary, metamorphic and igneous rocks, parts outlined below. It is important to understand of which form the Lachlan Geosyncline. The these because river morphology has an near-meridional disposition of rock types in the important influence on chemical and biological highlands has controlled dissection so that, in many processes and hence is a key component of cases, valleys run north-south, joining east-west overall ecosystem functioning. streams. The highlands are characterised by steep slopes and streams, which give rise to limited 3.1.1 Landform mass-movement of the slopes and moderately large-scale sediment movement in channels. This The River Murray drains over 420,000 km2 of area is likely to be a relatively high sediment yield the South Eastern Highlands of Australia. It zone in the River Murray catchment. originates from a wet upland region at an Despite the overall stability of underlying elevation of 1430 m above sea level and flows in basement rocks, there has been some movement a north-west direction through a predominantly of the basement blocks along ancient fault lines. semi-arid region, before turning due south in The Cadell Fault is one of the most significant South Australia to meet the sea. Over 40% of tectonic influences on the River Murray. the water yield of the Murray comes from its Displacement along the fault occurred catchment area above Albury (only 2% of the approximately 25,000 years ago, tilting the total catchment area). Given this drainage Cadell Block to the west and resulting in a rise of characteristic, the River Murray can be described some 8–12 m to the east. Before this as a dryland or semi-arid river. The uniqueness displacement, the Murray flowed across the top of these types of river systems, in terms of their of the faulted block to join the Goulburn River hydrology, geomorphology and ecology, is north of Echuca. The block diverted the river discussed in detail by Davies et al. (1994). The around its northern end whilst the Goulburn present geomorphology of the Murray and lower River formed a large lake (Lake Kanyapella) at Darling rivers is heavily influenced by their its southern end. Between 25,000 and 13,000 underlying geology and past geomorphological years ago this lake emptied and the Goulburn activities extending back over several million cut a new path around the block. Around 8000 years. Downstream of Albury, in its middle, the years ago the River Murray abandoned its course River Murray is joined by several major to the north to rejoin the Goulburn near Echuca, tributaries, the Kiewa, Ovens, Goulburn, establishing the present course of the Murray. Campaspe, Loddon, Murrumbidgee and Darling The former course of the Murray to the north of rivers. the block is now occupied by the Edward River.
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The presence of the Cadell Block has The River Murray has four distinct river produced three low angle alluvial fans. The tracts (see Figure 3.1): Barmah Fan was formed by backwater deposition 1. The Upland or Headwater Tract (river zone 1) to the east of the Cadell Block and has produced is short (359 km) and steep with average an area of occluded drainage. This region has river slopes of approximately 16 cm/km. This become the Barmah-Millewa river red gum tract drains the Pre-Cainozoic fold belt rocks wetland complex. The northern and southern of the relatively wet Eastern Highlands and is ends of the Cadell Block are the fan head dominated by relatively high stream power locations for the Gunbower and Wakool fans. (see Brookes 1988) and a bed-to-mixed load channel system. 3.1.2 Geomorphic Changes 2. Downstream of Albury the river enters the Riverine Tract (includes river zones 2, 3 and Climatic changes over the last 30,000 years have part of 4 to the Edward River confluence). also had a profound influence on the Here the physical nature of the river system morphology of the River Murray. The vast array is heavily influenced by geomorphic activities of relic channels on the River Murray floodplain over the last 2 million years. The river flows of different sizes is a testament to the vastly across a broad floodplain, up to 25 km wide, different hydrological regimes of the past. Three and has many anabranches, billabongs and phases of river activity have been identified wetlands. The distinctive multi-channel river (Bowler, 1978). The oldest channels pattern is a product of relict river systems. (Tallygaroopna channels) were formed between 3. In the Mallee Tract, (river zone 4 downstream approximately 50,000 to 20,000 years ago. They of the Edward River confluence) which have much larger river channel dimensions than extends from near Swan Hill to the Darling the present Murray, and carried a predominantly River Junction, the river generally has a single channel. The river is inset within a sandy bedload. Discharges in these rivers were large trench cut 10–20 m into ancient lake much greater than today and were fed by and marine sediments. The river has an snowmelt from small glaciers and areas of average slope of 5 cm/km. permanent snow in the Eastern Highlands. 4. The channel in the Lower Murray Tract (river Climatic conditions changed dramatically around zone 5) from the Darling Junction is a 15,000 years ago and, as a consequence, the suspended load channel characterised by low rivers began to carry a more muddy sediment bed slopes (< 5 cm/km), sinuousities load with reduced discharges of water. These (1.2–2.1), low stream power (0.44–5.25 W/m, (Kotupna) channels have smaller dimensions see Brookes 1988) and highly cohesive bank with a distinctive meandering channel and materials (silt:clay ratios 12–41%). There are narrower meander belt than the Tallygaroopna two distinct geomorphological sub-sections channels. Goulburn type channels formed within this tract. In the Gorge section below approximately 14,000 to 11,000 years ago and Overland Corner, the river channel is are typically narrow sinuous, suspended load confined to a limestone gorge 2–3 km wide channels. These can be inset within the older and 30–40 m deep. Floodplain development is Tallygroopna and Kotupna channels. limited and the soils are heavy saline grey The modern River Murray channel flows in clays with low infiltration rates and hydraulic and out of these older channel courses. Thus conductivities. Upstream of Overland Corner, there is a variable inherited influence on the in the valley section, the river flows in a channel of the modern Murray. There are valley 5–10 km wide; the channel is flanked clearly defined reaches that are either confined by a broad floodplain containing alluvial and within the floodplain of the older channels, or lacustrine sediment. The soils have a higher are unconfined. Within the confined reaches, sand content with comparatively high the modern Murray has developed a more infiltration rates and hydraulic conductivities. sinuous and deeper channel, and the overall The modern River Murray is a large, low morphology is determined largely by the older gradient, anabranching river system that is River Murray channels. The modern River characterised by low water and sediment yields. Murray in the unconfined reaches has many Low flow energies and highly resistant boundary anabranches and the shape and size of these material combine to make the river very stable. channels are dependent upon the age of the The river channel has not changed its course surrounding floodplain and the nature of the dramatically over the last 200 years (Rutherfurd contemporary discharge and sediment regimes. 1991; Thoms and Walker 1992a, 1992b).
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TABLE 3.1 The four distinct river tracts, and major wetlands and lakes of the River Murray.
NEW SOUTH WALES SOUTH AUSTRALIA
Mallee Tract
Lake Victoria Riverine Tract Hay Euston M Balranald u r r u m b i d g e e R i v e M r Headwater Tract u r r a y R i v e ADELAIDE r Lower Murray Tract Swan Hill Wagga Wagga
Deniliquin
Murray Mouth Albury VICTORIA Echuca
However, the morphology of the in-channel The channel morphology of the River environment has changed considerably. Murray is highly variable or compound in Nevertheless, a complex evolutionary history and nature, particularly in terms of its ‘flashy’ flow and sediment regime has produced a cross-sectional shape. It contains a number of highly variable physical river (floodplain in-channel features or horizontal benches. These environment with an array of physical habitat features were notable on the surveys of the compartments along the river. Studies by river channel undertaken at the turn of the Rutherfurd (1991) and Thoms and Walker century and are similar to those found in other (1992a, 1992b, 1992c) have highlighted some of semi-arid rivers. The geomorphology of these the natural changes in river morphology and the features are not well known, although Woodyer impact of flow regulation on the water and (1968) suggests they are depositional and are a sediment regimes, and river-floodplain response to the highly variable flow regime of morphology in particular. For example, a series of the river (Thoms and Sheldon 1996). 20 cutoffs since the 1860s has shortened its length The complexity of the in-channel environment by 64 km between Hume Dam and Wentworth. of rivers like the River Murray is critical for Moreover, changes in the flow regime subsequent ecosystem health. The benches play an important to river regulation have been associated with an role in the transfer of carbon and nutrients increase in river channel widths along the entire through the river. Organic matter accumulates on length of the river from Hume Dam to these surfaces during low flow and when flow Wellington. River widths have increased on levels rise inundating the benches, the average by 6 m and by 40 m at some locations. accumulated material becomes available to aquatic Detailed investigations by Thoms and Walker organisms and then part of the food chain. The (1992a, 1992b) have highlighted that downstream benches act in effect very much like wetlands. of many weirs in South Australia, cross-sectional Water resource development has changed areas have increased by 285% and some are still the physical character of the River Murray. A enlarging their cross-sectional dimensions change in the hydrology of the river and the 54 years after the completion of the weirs. way water flows are managed have contributed
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to subsequent siltation and loss of bed diversity sections of the report rather than in this general and in-channel habitat. Bank erosion is overview section (see Part II and Appendix 3). In common in this river and many of its addition to using the information presented in tributaries. This has occurred despite the fact this report, the Panel accessed daily and monthly that the channel banks contain highly cohesive hydrological information supplied by the MDBC sediments and stream energies are very low. during on-site inspections. Water resource and catchment development The Murray-Darling Basin extends from have changed the sediment regime of the latitude 24–37˚S and 138–151˚E and regional Murray. The variability of sediment discharges climates vary accordingly. The headwaters of the has decreased and, as a consequence, the River Murray and its major tributaries are located sediment transport regime of the Murray is in the temperate high country whilst much of the considered to be uncharacteristic of its catchment is semi-arid. Average annual environmental setting (Thoms and Walker precipitation increases rapidly with elevation and 1992a, 1992b, 1992c). Presently, the largest ranges from less than 500 mm at lower elevations fraction of the annual suspended sediment load near Mildura to 1470 mm at Mt Hotham, is transported during the regulated irrigation 1600 mm at Mt Buller, 1890 mm at Mt Buffalo, flow periods. The main sediment source is now 2400 mm at Mt Bogong, and 3800 mm at the river channel rather than material supplied Mt Kosciusko. Summer and autumn are generally from the surrounding catchment surfaces. The drier but occasional intrusions of moist air can composition of sediment being transported is result in thunderstorms bringing brief periods of now much finer than previously transported, intense rainfall. Natural river flows decline to an with coarser materials being trapped behind annual minimum in late summer and autumn. many of the upstream dams and weirs. This Just under half the discharge of the River change in the nature and calibre of sediment Murray originates within 500 km of its source. being transported has been demonstrated to Flows in the headwater tract of the Murray are have a profound influence on instream and also augmented by diversions from the Snowy floodplain ecosystems in the Murray system Mountains Scheme (Table 3.1). (e.g. Walker and Thoms 1993; Thoms 1995). Annual net evaporative losses from Lake Hume and Lake Dartmouth average 90 GL and 3.1.3 Hydrology 0 GL, respectively (supplied by MDBC). The major diversion from the headwaters of the A broad overview of the hydrology of the River River Murray occurs from the Tooma River. As part Murray is presented here. The primary aim of the of the Snowy Mountains Hydro-electric Scheme, report as a whole, with respect to hydrology, is to flow in the Tooma River at Tooma Dam is diverted demonstrate and examine the change in the flow out of the catchment to Tumut Pond Reservoir to regime from natural conditions. The reader is be used for the generation of electricity in the referred to Crabb 1997 and Walker and Thoms Snowy-Tumut Development. In an average year, 1993 for more detailed assessment of the effects 295 GL is diverted from the Snowy River to the of regulation on the River Murray. More detailed Tumut River while 865 GL is diverted into the hydrological analysis is presented in the relevant Swampy Plain River and the River Murray. Thus
TABLE 3.1 Sources of streamflow at Albury (Inflow figures are the average modelled current condition inflows from 1891 to 1992; supplied by MDBC)
Catchment Mean Annual Contribution (GL) Comment Snowy Mountains 570 Net contribution of the Hydro-electric Scheme Scheme to the River Murray Mitta Mitta River to 930 Lake Dartmouth Hume unregulated 2610 The unregulated catchment inflow of the Murray and Mitta Mitta rivers Kiewa 610
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the average net gain in flow to the River Murray diversion from the River Murray (see Table 3.3). from the Scheme is 570 GL. Annual diversions for The total area irrigated in the River Murray water supply purposes from streams upstream of Valley (including land irrigated from the Albury are small, being about 33 GL on average. Goulburn River) is greater than 700,000 Downstream of Albury in the Riverine Tract hectares and 90% of this irrigated land lies there are six main tributaries to the Murray (see between Yarrawonga and Swan Hill. Table 3.2). Much of the Riverine Plains is suited Changes in the annual monthly pattern of to irrigation which principally occurs by gravity flows and annual flows with regulation of the
TABLE 3.2 Major tributaries in the Riverine Tract of the River Murray (Diversion and inflow figures are the average modelled current condition values from 1891 to 1992; supplied by MDBC)
River Distance from Current Mean Comments Tributary Junction Mean Annual to River Murray Annual Diversion Mouth (km) Contribution (GL) (GL) Ovens 2027 1600 20 Broken Ck 1773 > 45 Goulburn 1728 1530 2100 Diversions are for the entire Goulburn, Broken, Campaspe and Loddon systems Campaspe 1712 180 Campaspe River at Rochester Loddon 1462 235 Includes minor tributaries in Torrumbarry system Billabong 1284 220 Includes some inflows Creek (Wakool Junction) from the Murrumbidgee system via Yanco Creek. Billabong Creek enters the River Murray via the Edward & Wakool Rivers
TABLE 3.3 Water diversions in the Riverine Tract (Diversion figures are the modelled current condition annual diversions from 1891 to 1992; supplied by MDBC)
Albury to Wakool Total Water Use Diversion as a % Junction Including (GL/year) of Total River Murray and Edward and Wakool Lower Darling Diversion New South Wales 1800 42% Victoria 1300 30%
river is shown below (Figure 3.2). Current high flows within a year have been greatly annual flows at Albury are greater than natural altered. Median monthly flows show that (unregulated) for 55% of years. Flows between seasonality of flows has been shifted with 5000 and 8000 GL/year occur less often, while exceptionally high summer flows and greatly flows larger than this have remained reduced spring flooding. unchanged. While it appears that years of major Further downstream at Yarrawonga, current flooding have only slightly decreased, median annual flows are greater than natural for only monthly flows reveal that the timing of these 8% of years. For 90% of years, current annual
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FIGURE 3.2 Difference between natural (unregulated) and current flows in the River Murray at Albury and Yarrawonga and in the Edward River at Deniliquin (supplied by MDBC)
Median Monthly Flows River Murray at Albury Distribution of Annual Flows River Murray at Albury 800.0 20000 700.0 18000 Current Natural 600.0 16000 14000 500.0 12000 400.0 10000 300.0 8000 200.0 6000 4000 Flow (GL/year) Flow (GL/month) 100.0 2000 0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% % of Years Flow is Less Than Value Median Monthly Flows Distribution of Annual Flows River Murray Downstream of Yarrawonga River Murray Downstream of Yarrawonga 1200.0 20000 18000 1000.0 16000 800.0 14000 12000 600.0 10000 8000 400.0 6000 4000
200.0 Flow (GL/year) Flow (GL/month) 2000 0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% % of Years Flow is Less Than Value Median Monthly Flows Edward River at Deniliquin Distribution of Annual Flows Edward River at Deniliquin 1300.0 10000 9000 250.0 8000 200.0 7000 6000 150.0 5000 4000 100.0 3000 2000
50.0 Flow (GL/year) Flow (GL/month) 1000 0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% Current Natural % of Years Flow is Less Than Value
flows are much lower than natural. For flows although to a lesser extent. Flows at Edward greater than 13,000 GL/year, natural annual River have been impacted in a similar manner flows were slightly greater than present. Median to those recorded at Yarrawonga. monthly flows show that the reduction in In the Mallee Tract, there are two significant annual flows is likely to be due to current tributaries, both entering from New South Wales regulated flows being kept at fairly constant (Table 3.4). The Murrumbidgee River and its levels and the absence of spring flooding that tributaries drain central and southern New South would have occurred naturally. As at Albury, Wales. Rainfall in the headwaters of this group of current summer flows are greater than natural, rivers is generally reliable and the Murrumbidgee
TABLE 3.4 Tributaries in the Mallee Tract (Inflow figures are the modelled current condition annual flows from 1891 to 1992; supplied by MDBC)
River River Mean Comment Distance to Annual Junction (km) Flow (GL) Murrumbidgee 1236 1180 The Snowy Mountains Hydro-electric Scheme is responsible for an average net transfer to the Murrumbidgee River of 555 GL/year from the Snowy and Tooma catchments Darling 825 2270 Inflow to Menindee Lakes storage
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TABLE 3.5 Water diversions in the Mallee Tract (Diversion figures are the modelled current condition annual diversions from 1891 to 1992; supplied by MDBC)
Wakool Junction Total Water Use Diversion as a % of Total River to SA Border (GL/year) Murray/Lower Darling Diversion New South Wales 77 2% Victoria 291 7%
River at Gundagai has never ceased to flow over compared with natural unregulated flows. the period of record. The Darling River and its Annual flows at both sites are always less than tributaries drain the northern regions of New natural and significantly so for most of the time. South Wales and southern Queensland. Flows in Although median monthly flows maintain this group of rivers are highly variable and the seasonality, flows are diminished under current Darling River ceases to flow during drought conditions all year round, with spring flooding events. Water diversions in the Mallee Tract are most impacted. The disparity between natural shown in Table 3.5. and current flows is greater at Wentworth. Also The change to the annual monthly pattern of worth noting is the shift in peak high flows flows, and to annual flows for the River Murray from October under natural conditions to in the Mallee Tract is shown in Figure 3.3. In September for current regulated conditions. This contrast to the flow regime from Albury to shift becomes more pronounced downstream. Yarrawonga and in the Edward River, the The lower Darling River has no tributaries, seasonality of flows at Euston and downstream however there are two major anabranches that at Wentworth return to a natural pattern. carry floodwaters. Talyawalka Creek leaves the However, the volume of water is greatly Darling near Wilcannia and returns downstream reduced under current regulated conditions of Menindee. The Great Anabranch starts
FIGURE 3.3 Difference between natural (unregulated) and current flows in the River Murray at Euston and Wentworth (supplied by MDBC)
Median Monthly Flows Distribution of Annual Flows River Murray Downstream of Euston River Murray Downstream of Euston 1300.0 40000 Current Natural Current Natural 1600.0 35000 1400.0 30000 1200.0 25000 1000.0 20000 800.0 15000 600.0
Flow (GL/year) 10000
Flow (GL/month) 400.0 200.0 5000 0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% % of Years Flow is Less Than Value Median Monthly Flows Distribution of Annual Flows River Murray Downstream of Wentworth River Murray Downstream of Wentworth 250.0 40000 Current Natural Current Natural 35000 2000.0 30000 1500.0 25000 20000 1000.0 15000
Flow (GL/year) 10000 Flow (GL/month) 500.0 5000 0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% % of Years Flow is Less Than Value
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upstream of Menindee and flows for 150 km rapidly from this peak during February and south and joins the Murray downstream of March, to be held at a constant low flow until Wentworth. Water is diverted from the lower October. Again, the current annual flows are Darling for a variety of purposes (Table 3.6). much less than natural flows for 98% of years. The extent of change to the annual monthly In the Lower Murray Tract, downstream of pattern of flows and to annual flows for this tract Wentworth, there are no significant tributaries. is shown in Figure 3.4. Monsoonally-influenced However, there are major groundwater inflows flooding in the Darling River caused two flood and large volumes of water are diverted for peaks under natural conditions, peaking in irrigation, domestic and industrial use (Table 3.7). March and September. Current conditions have Some idea of the change to the annual totally erased the natural pattern and peak flows monthly pattern of flow, and to annual flows, now occur once in December–January. Flows fall can be gained from Figure 3.5.
TABLE 3.6 Water diversions from the lower Darling River (Diversion figures are the modelled current condition annual diversions from 1891 to 1992; supplied by MDBC)
Reach Total Water Use (GL) Diversion as a % of Total River Murray/Lower Darling Diversion Menindee to Burtundy 14 0.3% Lock 10 Pool 65 1.5% Great Anabranch 136 3.1% (including Tandou) Total Lower Darling 215 5.0%
FIGURE 3.4 Difference between natural (unregulated) and current flows in the Darling River at Burtundy (supplied by MDBC)
Median Monthly Flows Distribution of Annual Flows Darling River at Burtundy Darling River at Burtundy 160.0 12000 Current Natural Current Natural 140.0 10000 120.0
8000 100.0
80.0 6000
60.0 Flow (GL/year) Flow (GL/month) 4000 40.0 2000 20.0
0.0 0 0% 20% 40% 60% 80% 100% J F M AJJASODM N % of Years Flow is Less Than Value
Modifications to the natural flow regime at are always lower than natural annual flows. the South Australian border and at the Another impact of regulation most apparent Barrages resembles those at Euston and at the Barrages is the shift in the peak high downstream at Wentworth. Summer flows and flow. At the Barrages this shift is from October spring flooding are both greatly reduced under under natural conditions to August for current conditions. Also, current annual flows current conditions.
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TABLE 3.7 River flows and water diversions from the Lower Murray Tract (Diversion figures are the modelled current condition annual diversions from 1892 to 1992; supplied by MDBC)
South Australia Total Water Use (GL) Diversion as a % of Total River Murray/Lower Darling Diversion Category Irrigation 470 11% Urban and Domestic 140 3% Total 610 14%
FIGURE 3.5 Difference between natural (unregulated) and current flows in the River Murray at the South Australian border and at the Barrages (supplied by MDBC)
Median Monthly Flows Distribution of Annual Flows River Murray at South Australian Border River Murray at South Australian Border 2000.0 40000 Current Natural Current Natural 1800.0 35000 1600.0 30000 1400.0 1200.0 25000 1000.0 20000 800.0 15000 Flow (GL/month) 600.0 Flow (GL/year) 10000 400.0 200.0 5000 0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% % of Years Flow is Less Than Value
Median Monthly Flows Distribution of Annual Flows River Murray at Barrages River Murray at Barrages 1800.0 40000 Current Natural Current Natural 1600.0 35000 1400.0 30000 1200.0 25000 1000.0 20000 800.0 15000 Flow (GL/month) 600.0 400.0 Flow (GL/year) 10000 200.0 5000 0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% % of Years Flow is Less Than Value
3.1.4 Water Quality current velocity, due to the widening of the river channel, and saline groundwater in-flows (see Nutrients and Turbidity section 9.2). However, during significant inflows from the Darling River, there is an abrupt Nutrient concentration (nitrogen and increase in turbidity in the lower River Murray. phosphorus) and turbidity increase along the Increases in turbidity moving downstream may length of the river, though not in a consistent also be attributable to bank collapse. manner (Mackay et al. 1988). Turbidity, for There is a great deal of anecdotal evidence, example, increases at a near-exponential rate though unfortunately no published scientific between the headwater storages (Dartmouth and analysis, suggesting that summer turbidities in the Hume dams) and the confluence with the Murray were once considerably lower than they Wakool River. It then decreases across the are now. Increased summer flows due to irrigation Mallee Plain as a consequence of decreasing demand have escalated over the past four decades,
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and this could prevent suspended sediment suspended solids (which is flow dependent) and settling from the water column during summer. It probably more importantly, in-flows from is therefore feasible from a scientific viewpoint, tributary streams and irrigation drainage water. that summer turbidity is now much higher than in There is little information on the specific pre-regulation times. The consequences of reduced catchment sources of turbidity in the River water clarity in summer on benthic algal growth Murray, other than the percentage of the load and invertebrate grazers are potentially quite that can be ascribed to the main channel and its serious, though difficult to quantify without tributaries, the Goulburn and Wakool Rivers further scientific studies. being major sources upstream of the Darling Total phosphorus concentration changes confluence (Mackay et al. 1988). However, in along the river in a similar fashion to turbidity the Murrumbidgee River catchment, the (though increasing only linearly in the upper majority of the sediment and nutrient load River Murray, and decreasing only a little contributed to the river has been shown to through the Mallee Plains) with the Darling come from gully erosion (sub-surface soils) in River also being a major downstream source. the catchment, being influenced by tree clearing Median total phosphorus concentration in the during the time of European settlement (Olley Darling is naturally very high at 0.310 mg/L. 1996). Pesticides and nutrients may also move Filterable reactive phosphorus (FRP; the most with surface sediment from agricultural areas bio-available form) is very low along the Murray, and this may influence downstream water due presumably to its high reactivity with biotic quality, especially where there is riparian or (algae, bacteria, plants) and abiotic particles, floodplain farming (i.e. near the river) as in the until the Darling confluence. Recent research by lower River Murray (Murray and Philcox 1995). Hart et al. (1996) has shown that a significant percentage of the FRP in the Darling is present Influence of Dartmouth and Hume Dams in the colloidal rather than truly dissolved form. on Downstream Water Quality This probably decreases its bio-availability. Reduced nitrogen (N) species (organic N Releases from headwater storage dams are compounds and ammonia) show the same trend. known to cause both substantial decreases in However, oxidised nitrogen (nitrate) behaves downstream water temperature, and increases in somewhat differently being comparatively high nutrient load and concentrations of natural in the Hume Dam (perhaps due to nitrification toxicants, such as hydrogen sulphide and heavy of ammonia released from bottom waters, see metals (see Allen 1995; several papers in Ward ‘Hume Dam - Nutrient Loads’ below) and and Stanford 1979; Petts 1992). These changes decreasing rapidly along the River Murray, lead to negative impacts on fish, either through probably due to uptake by plants and algae. the direct effects of low temperature on fish Dissolved silica (actually silicic acid) is a key behaviour and reproduction, or through negative nutrient for diatoms. Its concentration is highest metabolic effects of low oxygen or high hydrogen in the Headwater Tract and then decreases sulphide and/or heavy metal concentrations. The rapidly until it reaches very low levels in the increased nutrient loads from the anoxic Riverine Tract, presumably due to uptake by hypolimnion (bottom coldwater layer) can also diatoms, especially Aulacoseira. Silica lead to increased algal growth downstream, concentrations then increase significantly particularly of epiphytic or benthic algae. downstream of the Wakool and Murrumbidgee The water quality changes are brought about confluences, and increase dramatically as a by thermal stratification in the reservoir during consequence of in-flows from the Darling River. the spring and summer. This common behaviour Most of these nutrient ions show elevated of deep reservoirs leads to anoxia in the low concentrations in wet years compared with dry temperature hypolimnion, and the release of years (Mackay et al. 1988). This, not ammonia, orthophosphate, hydrogen sulphide surprisingly, is a consequence of the increased and dissolved heavy metals (especially iron and run-off from the South Eastern Highlands and manganese) from bottom sediments. increased in-flows from tributary streams. Unfortunately, most dams are built with only one It is apparent that these key water quality or perhaps two outlet points, which are normally parameters are influenced by river regulation in placed deep in the reservoir wall so as to allow the broader sense through a combination of water withdrawal during periods of reservoir instream resuspension and sedimentation of draw down. This is the case for both Dartmouth
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and Hume dams. These outlets tend to either be i.e. with no seasonal variation (see Figure 3.6). in the hypolimnion or metalimnion (temperature Because of the 4500 ML regulating pond transition zone between the surface and bottom immediately downstream, during summer layers), in which case they tend to withdraw discharges (200–500 ML/day) the cold water mostly anoxic bottom water or a mixture of may be stored for sufficient time to raise the epilimnetic and hypolimnetic waters. Hume water temperature to that close to ‘natural’ Dam is considerably shallower than Dartmouth (i.e. what would be expected for the unregulated Dam and, as is discussed below, has important river). However, during major summer releases implications for the year to year variation in the residence time in the regulating pond is too water quality released from Hume Dam. short to allow heating. This was the situation in the summers of 1982–83, 1987–88 and all Dartmouth Dam – Temperature summers from 1990–91 to 1996 (end of records Dartmouth Dam is meromictic (permanently – see Figure 3.7). The effect is to greatly truncate stratified) with the epilimnion (surface warm the ‘natural’ temperature range in the Mitta water layer) depth being typically 5–15 m in Mitta River downstream of the dam. More summer and 40–50 m in winter (the full supply importantly, the summer temperature only depth is 161 m). Ebsary (1990) demonstrated briefly rises above 16˚C, a temperature noted to the impact of Dartmouth Dam on water be crucial for the breeding of native fish species, temperature in the Mitta Mitta River between such as Macquarie Perch, and rarely above the the time of construction in 1977 and reaching optimal temperature of 20˚C for Murray Cod full capacity in 1990 (the reservoir did not fill (see section 3.2.5). Based on simulated until 1990 though major releases commenced long-term modelling data, Ebsary predicted that during the 1982 drought). Ebsary showed that major coldwater releases would occur in from 1984 onwards, the temperature of the 11–23% of months (October–April inclusive). water released from Dartmouth Dam was mostly However, Ebsary failed to take into account the constant around 9–11˚C throughout the year, seasonal timing of these low temperature events.
FIGURE 3.6 Comparison of Mitta Mitta River temperatures at Colemans and temperatures of Dartmouth Dam release
Reservoir Release Murray-Darling Basin Commission 19 Dec 90 Stream Temperature at Colemans Dyresm Outlet Temperature 25
20
15
10
5 20000 10000 Temperature (degrees C) Temperature 0 0 Release (ML/day) 1980 1981 1982 1983 1984 25
20
15
10
5 20000 10000 Temperature (degrees C) Temperature 0 0 Release (ML/day) 1985 1986 1987 1988 1989
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FIGURE 3.7 Temperatures of Dartmouth Dam release into the Mitta Mitta River between January 1975 and June 1996
30 Tallandoon 2 per Mov. Avg. (Tallandoon) 25
20 C ° Temp 15
10 Jan 87 Jan 96 Jan 75 Jan 76 Jan 77 Jan 78 Jan 79 Jan 80 Jan 82 Jan 83 Jan 84 Jan 85 Jan 86 Jan 88 Jan 89 Jan 90 Jan 91 Jan 92 Jan 93 Jan 94 Jan 95
5 Jan 81
Unfortunately, they always occur in spring–summer, Dartmouth Dam – Nutrient Loads a time crucial for fish and invertebrate breeding. The The release of anoxic bottom waters is predicted effects of this are discussed in section 3.2.5. to greatly increase the downstream load of During times of riparian release, there is a ammonia and orthophosphate during the spring confounding factor in that the regulating pool and summer. Croome and Welch (1988) showed strongly stratifies (Welch 1984) and the outlet that in December (not the time of maximum valve from the pond (above Colemans) is from stratification) the hypolimnion concentration of the bottom of the weir wall. Furthermore, the filterable reactive phosphorus was 5–10 times height of the pond varies greatly, or at least it that in the epilimnion and metalimnion. They has done in the past; this may change with did not take measurements of ammonia, but increased usage of the hydro-electricity plant. recent work in reservoirs of south-east The consequence is that the temperature of the Queensland suggests that the trend will be the water being discharged from the pond can vary same for ammonia (Jones 1997). The exact dramatically over short time periods. For concentration of nutrients being removed example, in the space of a month from depends on the depth of the off-take relative to December 1989 to January 1990 the the bottom of the chemocline (the depth where temperature at Colemans went from 11˚C up to dissolved oxygen concentration goes to zero); 20˚C and back down to 14˚C (see Figure 3.8). ammonia and phosphate concentrations Irrespective of any absolute temperature increase below this depth. Interestingly, nitrate decreases caused by the dam, the variance and concentrations increase in the epilimnion irregularity of spring–summer riparian flow (Croome and Welch 1988), possibly due to the temperatures below the regulatory pool are entrainment of ammonia rich water from the unacceptably high. The short time at any one hypolimnion, and rapid nitrification. temperature must play havoc with the behaviour The withdrawal ‘envelope’ downstream from and metabolism of fish and other animals, is the release valve is about ± 10–15 m from the likely to lead to greatly increased mortalities of level of the off-take. During summer 1996, the fish eggs or larvae, and have deleterious impacts anoxic, nutrient-rich zone commenced at EL on the rate of microbial and detrital processes. 422 m, which was right on the high level off-take
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FIGURE 3.8 Effect of regulating pond on the Mitta Mitta River temperatures at Colemans between May 1987 and April 1990
Dyresm outlet temperature Stream temperature at Colemans 30 28 Upper envelope of mean air 26 temperature at Dartmouth 24 22 20 18 16 14 12 10
Temperature (degrees C) Temperature 8 6 4 10000 2 5000
0 0 Release (ML/day) 1987 1988 1989 1990
(HLOW – EL 424 m). Under high discharges, the Heywoods Bridge over two four-year periods in released water would contain a mixture of the early 1990s and 1980s respectively. Note nutrient-rich bottom water and nutrient-depleted that in most years (when the dam is reasonably surface water. Thermocline tilting and seiching full) the effects of the deep water off-take are would effect the releases on a day to day or week two-fold. Firstly, the time of maximum water to week basis. Under low discharges the temperature is off-set by about 2 months from thermocline tends to compress the withdrawal January–February to March–April. Secondly, the envelope from above, therefore proportionately maximum surface temperature is up to 4–6˚C more nutrient-rich water would be released. lower than the maximum surface water of the dam – the upstream temperature data from Hume Dam – Temperature Dartmouth Dam (at Hinnomunjie) indicate that Hume Dam, being considerably shallower than this is very close to the temperature that would Dartmouth Dam (approximately 40 m deep at full be expected in the unregulated upper River supply – EL 192 m) is monomictic (stratified but Murray system. The offset in temperature mixing completely once a year). Moreover, the maximum is a consequence of the fact that dam is in some years drawn down considerably, as during spring and summer the water column in for example during the 1982 drought when it the dam is thermally stratified, with overturn reached a depth of approximately 20 m. The dam occurring in April (Brymner 1985). has two outlets at similar depths: one at EL 163 m for the hydro-electric plant and one at EL 158 m Hume Dam – Nutrient Loads for irrigation releases, with the hydro-electricity The impact of releases from the dam on outlet having a capacity of 17,000 ML/day and downstream nutrient concentrations can be being the preferred release point. Depending on the assessed from the MDBC database for Heywoods level of supply in any particular summer, Hume Bridge. Unfortunately there is no ammonia data Dam would be expected to discharge water across a (only TKN which includes a large organic N range of temperatures and nutrient concentrations. component), but the nitrate data show a very Figures 3.9a and b show the temperature at interesting annual cycle with a peak the surface of Hume Dam and downstream at concentration in November–December, and a
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FIGURE 3.9 Temperature of surface water in Hume Dam and in the River Murray at Heywoods Bridge for the periods (a) January 1991 – January 1996 and (b) July 1981 – July 1985
30 Hume Dam 25 Heywoods
20
15
10
5 Jan 91 Jul 91 Jan 92 Jul 92 Jan 93 Jul 93 Jan 94 Jul 94 Jan 95 Jul 95 Jan 96 30 Hume Dam Major draw down 25 Heywoods due to drought
20
15
10
5 Jul 81 Jan 82 Jul 82 Jan 83 Jul 83 Jan 84 Jul 84 Jan 85 Jul 85
FIGURE 3.10 Concentration of TKN and nitrate in the River Murray at Heywoods Bridge between January 1980 and January 1997
1.5
TKN NO3+NO2 1.25
1
0.75 conc. mg/L 0.5
0.25
0 Jan 81 Jan 82 Jan 94 Jan 88 Jan 85 Jan 87 Jan 89 Jan 92 Jan 93 Jan 83 Jan 84 Jan 90 Jan 95 Jan 97 Jan 96 Jan 91 Jan 86 Jan 80
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minimum in autumn–winter (at almost along the River Murray as pointed out in undetectable levels), see Figure 3.10. Note the section 3.1.2. The river flows through a wide absence of the late spring peak during the valley floor trough and the floodplain is drought year of 1982. Two possible explanations dominated by a scroll pattern produced by scroll for this annual nitrate cycle can be suggested, bars and floodplain ridges and contains and both may be important depending on the substantial areas of counter-point bars. The River time of year. The first is that the nitrate is Murray in this region (downstream of Albury to carried into the dam during winter–spring Lake Mulwala) also has an anastomosing pattern run-off. The second explanation, which is (multiple channels separated by floodplain). consistent with recent work on three south east Floodplains represent an oscillating boundary Queensland reservoirs (Jones 1997), is that between aquatic and terrestrial systems. Biota that during dam surface layer deepening and reside on floodplains have evolved to be able to cope overturn in the late autumn, ammonia-rich with both wet and dry conditions. The composition water is brought into the surface layer. This is of a community at any point on the floodplain is then converted to nitrate by nitrifying bacteria strongly influenced by periods of wetting and in the water column. The lag in response from drying, the duration of individual events and the overturn to complete nitrification time of year they occur. Furthermore, the pattern of (approximately 6 months) is longer than in wet and dry conditions explains the mosaic of Queensland (2 to 3 months) but this may be vegetation groups seen in some parts of the explained by the lower winter water floodplain (e.g. black box, river red gum, grassland temperatures, 9–11˚C compared to 16–17˚C. and wetland assemblages at Barmah-Millewa), and The filterable and total phosphorus data why changes to the hydrological regime, whether shows much greater variance, but there is some management-induced or as a result of long-term evidence to suggest a bi-phasic discharge of climate change, will result in disruption of these phosphorus; the first evidence corresponding to patterns (Hillman and Bren 1996). the winter–spring run-off period, the other The aquatic component of a floodplain evidence to phosphate-rich water from the ecosystem (billabongs, anabranches and hypolimnion of Hume Dam. backwaters) is also inexorably bound to river flow Possible management actions to deal with pattern. Again, the biota are typically adapted to the downstream water quality problems caused periods of wet and dry, either by being able to by Dartmouth and Hume Dams are discussed in survive desiccation or by actively recolonising detail in section 4.4.1. However, the underlying newly inundated areas. Billabongs usually are aim is, wherever possible, to release surface extremely productive ecosystems (Boon et al. water from the dams during the spring and 1990). Their biodiversity is also very high with a summer, except during times of major strong bias towards the microscopic size (Cranston cyanobacterial blooms in Hume Dam. and Hillman 1992; Hillman 1996). It is postulated that this massive biomass of microscopic organisms 3.2 RIVER ECOLOGY represents an important food source for the larvae of riverine fish, such as Murray Cod and Golden 3.2.1 Floodplain Ecology Perch, and that the connection of riverine and floodplain waters during the breeding period is an The Nature and Ecological Function of important factor in native fish recruitment. This is Floodplains yet to be proven scientifically, although the successful recruitment of several of these species Floodplains are temporary storage areas of coincides with years of substantial flooding. The alluvial material adjacent to the main river off-stream waters also provide preferred and channel. They are a major ecological feature of specialised habitats for many species, e.g. the the lowland rivers in the Murray-Darling Basin. Flat-headed Galaxias and freshwater Catfish. In Floodplains are formed by a complex interaction any case, the water returning to the river from of fluvial processes. Their character and billabongs as high flows subside are likely to bring evolution is essentially a product of the quantities of organic material and biota. morphology of the valley in which they are The mosaic of habitats, both terrestrial and situated, and sediment transport and hydrologic aquatic, is a major contributor to the high processes. These features change along the river, biodiversity found in floodplains. High hence there are a diversity of floodplain ‘types’ biodiversity is a common feature of ecotones
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and in this case is augmented by the • reduced total volume of flow due to abstraction; juxtaposition of different habitats; for example • reduced velocity, increased depth and removal increased density of non-aquatic bird species of drying cycles upstream of locks and weirs; through access to temporary and permanent waterbodies (Parkinson 1996). • modified day-to-day variation in flows (rates of Junk et al. (1989) formulated the general model rise and fall) (for example see Figure 4.3a); and of river and terrestrial interaction in the ‘flood–pulse • depressed summer water temperatures concept’. This model emphasises the lateral downstream of major storages. movement of materials between channel and floodplain during over-bank flows and highlights These changes are discussed in detail in Chapter the significance of nutrient and sediment transport 4 and Part II. The first three changes are the most to the floodplain and the return of organic material, important in terms of floodplain ecology. As with such as litter, to the river. This carbon input is seen geomorphic factors, flow management effects are as a major energy source supporting the riverine not uniform through the system and food web through decomposers, invertebrates to consequently any attempt to redress detrimental fish and higher organisms. Robertson et al. (1996) effects will need to be made on a regional reach estimated the significance of various carbon sources basis. Even within reaches the effect of flow to rivers under Australian conditions and suggested modification is extremely variable depending in that ‘even relatively modest floods may transfer part on the ‘sill height’ (or flow required to fill a quantities of floodplain carbon that are significant’ wetland) of individual wetlands and the height relative to carbon produced within the stream (or river discharge) at which individual points on channel. They also identified flow, land the floodplain become inundated. In the Hume management and floodplain connectivity as three of Dam to Yarrawonga Zone, operation of Hume the major factors which govern carbon fluxes in Dam has increased or decreased the frequency, shifted the seasonality of inundation, or had no floodplain rivers, and these issues are central to a effect depending on the river height at which number of the themes developed in this report, individual wetlands fill (Hillman 1995). Further especially in Chapter 4. downstream, where abstraction becomes the overriding management effect, the outcome for Anthropogenic Changes floodplains and their interaction with the mainstream is much less complicated. Here the European settlement has wrought substantial loss of high flows and moderate floods means a change to the Murray Darling system which is reduced frequency of inundation for a substantial impinging on its ecology. The changes result part of the floodplain. Restoring a more natural from modification of river hydrology and from balance of flooding and drying along the river is a land management practices on the floodplain. major challenge for successful environmental flow management of the River Murray. Flow Management The River Murray is managed largely as a rural Levees water supply system, primarily for irrigation Although levees are often part of land (see section 3.1.3). In broad terms, this means management activities on floodplains, their effect maximising the availability of water in summer is essentially hydrological. Constructed to protect and autumn and capturing and storing water at various land management activities which other times. Reservoir design, operation of cannot tolerate inundation, they interfere regulating devices, and abstraction of water for completely with the natural interaction between off-stream use, all result in ecologically river and floodplain, destroying linkages and significant changes to flow patterns, as outlined thereby crippling the various ecological functions in sections 3.1.3, Part II of the report and described in this chapter. In essence their effect Appendix 3, including: can override potential flow management issues. • reduction in frequency of small to Levees, in some form, occur along much of the medium-sized floods; Murray. Many were originally constructed prior to Federation in 1901 in response to the large floods • unseasonal shift to summer high flows – between 1870 and the end of the century, and are winter low flows below large storages and thus based in a different philosophy of resource upstream of major abstraction points; management and (one might hope) federal
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cooperation. Their maintenance and and in particular are systematically denuded of enhancement have continued until recent times fallen timber and dead trees. and in some cases (such as urban and dairy Socio-economic constraints obviously production areas in the lower parts of the preclude the reinstatement of floods to Murray) they are now virtually essential. In other floodplains containing urban development, cases, however, they are more a matter of though pressure should be put on local convenience and a means of shifting the problems government authorities to resist ‘reclaiming’ of flooding to someone or somewhere else. floodplain land (however securely) for human settlement. Other situations, though not so clear- Land Management cut, also militate against reinstatement of floods. Although outside the scope of this report, These include intensive agricultural operations in management of floodplain land is a significant which all or nearly all of the enterprise is resource management issue with major impacts situated on the floodplain. This situation exists on floodplain and river ecology. These are: on the lower River Murray and on a significant part of the Mitta Mitta floodplain. To various • the terrestrial ecosystem of the floodplain is extents inundation is incompatible with these modified in such a way as to make it less uses and is likely to produce river pollution able to respond to inundation and less able problems (nutrients, pesticides, dead livestock) to contribute to the river ecosystem as a greater than any benefit accruing from the whole during inundation; flooding of these highly modified areas. • floodplain wetlands are destroyed, or modified structurally or hydrologically in a 3.2.2 Vegetation and Plant Community way that reduces their ecological function Ecology individually and as part of the floodplain–river ecosystem; and Habitats for plant communities of the River Murray are the riverbed, riverbanks (including • the patterns of land use, to varying degrees, in-channel benches), and floodplains with their are incompatible with inundation, and may associated wetlands and anabranches. These are create socio-economic pressures against described below in terms of their dominant reinstating flood regimes. ecological characteristics, their key plant species These considerations, particularly the first two, and characteristic vegetation. are important when assessing the benefits of changes to the flow regime and should be Riverbed identified as priority areas for research in restoration ecology; an area of research neglected For plants, the riverbed is a difficult growing to date in Australia (see Chapter 5). Restoration environment. In upland areas, particularly below of key components of the floodplain ecosystem Dartmouth Dam, river flows are turbulent would need to be part of an environmental flow resulting in mechanical damage, uprooting and package in many parts of the Murray-Darling abrasion. The substrate is typically rocks and Basin. Key components would include: boulders, and the river a series of waterfalls, riffles and pools. It is only in these pools and at • anabranches and wetlands likely to be in stream edges that soil accumulates sufficiently contact with the river during high flows; for plants to develop roots. However, repeated • a riparian strip and islands and benches in small ‘flashy’ floods are a recurrent disturbance the main stream (which provide habitat, for plants. In this environment, river (or stream) snags etc.); and plants tend to be short-lived, fast-growing herbs and forbs, tolerant of the cool climate. • currently forested areas including those used In lowland areas downstream of Hume Dam, for passive recreation. the riverbed consists of gravels, sands and clay, In the last instance, it should be noted that all of which are either unstable or inhospitable superficially similar areas of floodplain forest growing sites for plant roots. Pockets of can be very different in terms of ecological unconsolidated silt may accumulate at the base value. Many forests are heavily, if episodically, of the banks or behind obstructions, such as grazed (limiting regeneration and native snags, creating small vegetation patches in understorey), have significant weed infestations, transient habitats. Plants establishing in these
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habitats typically do so from fragments of stem have no understanding of factors controlling or rhizome and may become a biodiversity these processes and the species population feature, as in the Chowilla Anabranches. For dynamics. Knowledge of species ecology is plants submersed at the bottom of the river, woefully defective in many key areas and is growth is possible only when water is shallow hampered by lack of historical information. and/or clear. When water is deep or turbid, light penetration is reduced to only a few centimetres, Riverbank thus for plants on the river bottom, respiration will exceed photosynthesis and no growth will Sills, bars, slopes, benches and vertical faces are occur. Deeper water during the growing season, all part of the riverbank habitat. This and increased summer flows and turbidity topographic micro-relief means the riverbank exacerbates this. Mechanical disturbance and can be a more diverse habitat than the riverbed. damage occurs here also, as it does in upland In upland areas, the near bank may be steeply areas but the return time, frequency and sloping, often on rock or boulders, and duration is over a longer time scale. Carbon overhung by myrtaceous shrubs making this supply is a further growth-limiting factor. inhospitable to plants. Further downstream this Submersed plants must rely on carbon from the hard substrate is replaced by alluvial material, water column but gas diffusion is much slower typically a heavy, grey clay, sometimes slightly in water than in air. Species differ in their ability sandy and the shrubs are replaced by trees. to use bicarbonate as a supplementary source of This is a stressful environment. It alternates carbon. Pollution and excessive algal growth can from being under water and affected by velocity cause large and sustained changes in pH, and and abrasion to being above the waterline, can therefore affect plant species abundance. exposed to desiccation and grazing. Thus the The bed of lowland rivers may be species abundance and composition of plants on the poor due to a difficult growing environment. The bank is determined by how various factors two main species in the River Murray are both interact: substrate (bank shape, soil types); submersed species: ribbon weed, Vallisneria, and water availability (water regime, pattern of the less common curly pondweed, Potamogeton submergence and exposure, rainfall patterns, crispus. In some areas, floating-leafed pondweed depth or height above mean water level); stress Potamogeton tricarinatus may be important. (includes shading by overhead canopy); bank Although this species can form dense patches, orientation; and disturbances factors (such as only rarely are the patches extensive. Anecdotal velocity and current effects, trampling, stock evidence tells us that the abundance of these grazing, bank instability) (Roberts 1994). beds has grossly changed in this century. For How these environmental factors interact is example, river plants have disappeared from beginning to be described (Frankenberg and middle river zones of the Murray, although beds Tilleard 1991; Roberts 1994; Blanch and Walker of all species persist near Albury and in the 1997) but as yet not with any predictive power. lower Murray. This loss is not unique to the What is clear, however, is that river flow is only River Murray but is known also for the Darling part of what determines species abundance and and Lachlan rivers (Roberts and Sainty 1996, composition. For example, transient pulses of 1997). Several reasons for the losses have been water, such as rain rejection flows, water the proposed (Sainty and Jacobs 1990) but the most banks albeit briefly. This is enough to create an likely is an interplay between disturbance by opportunity for ruderal and stress-tolerant species carp (Roberts et al. 1995), growth limiting to establish above the waterline and this occurs conditions due to high turbidity (Blanch et al. widely in the absence of perennial species. Thus 1996b) and deep water conditions during the in summer and autumn, patches of late growing season. The relative importance of rapidly germinating, fast-growing disturbance and growth-limiting conditions desiccation tolerant annual herbs such as Centipeda probably varies along the river, and over time. spp., Alternanthera, Chenopodium pumilio, and The resilience of species like ribbon weed and Boerhavia are common on exposed riverbanks pondweeds is due to their flexible regeneration downstream of diversion weirs. However, if strategies. They have the capacity to regrow disturbance is light, then perennial grasses and from perennating organs or meristems buried in sedges dominate and annuals are excluded. The the substrate, to form new beds from washed-in perennials are arranged according to species plant fragments, or to establish from seeds. We tolerance to drought and prevailing water regime,
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and form characteristic ‘bands’ or zones. then the reverse is true. This is modified by In river reaches downstream of the major geomorphic history such as natural levees, diversion structures (see section 3.1.3), the bank defunct channels and old meanders. Elevation face is flooded in summer but exposed during changes across the floodplains are rarely more autumn-winter rather than during than a few metres and are often much less; these summer-autumn, as would have been the case small changes have a disproportionate effect on naturally. This is a shift in season and only a plants because of their link to flooding frequency. few species can tolerate these conditions, Water availability includes groundwater which is e.g. Phragmites and short-lived annuals. Yet, possibly an even greater managerial challenge to Phragmites can only survive on very steep banks measure and control. Subsurface geomorphology where it is protected from grazing (Frankenberg is complex and cannot be predicted from surface and Tilleard 1991), highlighting again that for features. Groundwater is fresh in the middle many plants and nearly all plant habitats, River Murray but in the lower Murray it is saline, abundance and species composition are sometimes exceptionally so, as for example under outcomes of both land and water practices. parts of the Chowilla floodplain. Consequently In the middle and in many lower parts of the only species tolerant of saline water, such as river, the riverbank vegetation is marked by black box (Jolly 1996) and river coobah, or long-term changes. Most noticeable has been a species which have the capacity to 'shandy' general loss of perennial plant cover particularly different sources, such as creek-side river red of grasses and sedges; Phragmites, for instance, is gums (Thorburn et al. 1994), can grow on the a widespread species with a broad longitudinal large floodplains of the lower River Murray. range which appears to be in decline although There is also a longitudinal water availability pockets of favourable growth do occur. gradient. The climate becomes drier as the River Significant losses of Acacia dealbata and small Murray flows west. A moisture index, analogous shrubs, such as Leptospermum and Gynatrix, have to the ratio of actual evapo-transpiration to pan occurred in the middle areas. evaporation but calculated as a form of water Concurrent with such losses, there have budget (Specht 1990), shows that the driest part been gains but mainly of non-native species. of the River Murray floodplain lies between Deliberate plantings of exotics, such as willows Euston and the Murray Gorge. Soil types and and Lippia for bank stabilisation, have spread surface topography break up both lateral and downstream with water transported propagules, longitudinal gradients to create patches with as have some key weed species, e.g. golden different species and structure. In the Barmah dodder and burrs (Xanthium spp.). However, as Forest, shallow depressions support moira grass, most of the gains are exotic species, the Pseudoraphis spinescens, and aquatic herbfields. increases do not balance the losses. River On the Chowilla floodplain, lignum or sedge sections which have been ponded now have dominate such depressions. higher and more stable water levels, creating an environment which is more suitable for wetland Consequences of Regulation species. Resulting edge vegetation is a combination of old and new elements (see ‘New The lateral and longitudinal water availability Assemblages on the Murray’ below). gradients provide a means for understanding the consequences of river regulation and diversions Floodplain Vegetation and Water Availability on floodplain vegetation. Water diversions reduce the frequency and duration of flooding Water availability is a key factor in determining downstream (see sections 3.1.3 and 4.1.3, and plant distribution on floodplains. This is influenced Part II). This drying of the floodplain stresses the by the flooding regime of the river, depth to perennial species, such as river red gum, black groundwater and the key climate factors of rainfall box and river coobah, as it effectively increases and evaporation. Water availability operates over the drought incidence to which they are two gradients, lateral and longitudinal. exposed. Shifts in seasonal pattern from winter Water availability varies laterally across the to summer flooding has the two-fold effect of floodplain. The flooding regime is determined by creating stress or excluding one group of species elevation on the floodplain. If the river is incised, whilst creating an opportunity for another then elevation may increase laterally away from species, which is often invasive or exotic. the river to the floodplain edge; but if perched Examples of such species pairs are Moira grass
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and river red gum in the Barmah Forest (Bren Chowilla and Lindsay Island. However, even in 1992), Lippia and Paspalum on the Gwydir these lower areas, small dense forest-like floodplains, possibly even Typha and Phragmites. patches occur in favourable circumstances such None of these replacements can be attributed to as backwaters or inside meander scrolls. a single factor, but all involve water-flow Herbs, grasses and sedges dominate the river management and an aspect of land red gum understorey, the exact composition management. The consequences are, typically, a changing with flood frequency and other change in vegetation structure and a shift in expressions of water availability. In the some aspect of floodplain production. Extending Barmah-Millewa area, the understorey shifts the time period between floods (Department of from tussock sedges, Carex spp., in the wetter Natural Resources and Environment, Victoria, areas to tussock grasses, such as Paspalidium, and unpubl. data) not only creates a drought finally to wallaby grasses, Danthonia, in the drier situation for perennials, it tests and possibly areas. Few understorey species occur along the exhausts the long-term viability of seed banks. full length of the lowland River Murray; most are Shortening flood duration may curtail replaced by equivalent species. Thus in the upper completion of life cycle as no resources are and middle Murray the small tree, Acacia dealbata, allocated to reproduction or propagules. sedges, such as Giant Rush (Juncus ingens), and Although increased drying is the main herbs, such as river buttercup, are replaced impact of regulation on the floodplain, increased downstream by river coobah, spiny sedge and wetting can occur and will also have an adverse blue rod (Smith and Smith 1990, 1991). effect. These over-waterings are much more Only a few other native tree species, such as localised, being associated with weir pools, rain Acacia salicina, Acacia dealbata, Exocarpus spp., and rejections or river reaches between supply and Eremophila spp., are associated with the Murray irrigation off-take. Typical symptoms are stands floodplains. One tree which is not an understorey of dead red gum trees, as in Lake Mulwala or to tall river red gums is river coobah, Acacia backwaters upstream of Lock 6; stands of stenophylla. This small tree can form open cumbungi, as in Millewa Forest; and expanding mono-specific woodlands in the lowland areas Juncus ingens, as in Barmah Forest. such as on the Chowilla floodplain. Introduced species of willows (Salix spp.) are widespread. Species and Assemblages Salix x rubens occurs only in the upper river zones but Salix babylonica is found through most lowland The vegetation of the Murray valley is dominated river zones, particularly where summer flows are by woodlands and forests of just two species: high or water levels are stable. Thus willows river red gum, Eucalyptus camaldulensis, and black dominate in regulated sections or in weir pools. In box Eucalyptus largiflorens. The distribution of the lower Murray, willows were once planted as these corresponds to the two water availability levee markers. They now dominate long reaches gradients – lateral and longitudinal described of riverbank, excluding native species. above. For the distribution of other woody Black box woodland fringes the river red species in the Murray valley see the riparian gum dominated floodplain of the middle river vegetation study (MPPL 1990). zones and Riverine Tract, and becomes River red gum has higher water progressively more important further requirements than black box and is less drought downstream, and dominates the Chowilla tolerant. It occurs in the more floodplain. Black box is virtually absent from the frequently-flooded, wetter and lower areas and Murray Gorge. Like river red gum, black box on more permeable soils along the Murray. In responds to the water gradient. On the Chowilla the upper and middle river zones of the Murray, floodplain it decreases in height and vigour from particularly in the Riverine Tract, river red gum 25 m tall close to the river to only 5 m on the forms extensive open woodlands or dense highest and driest parts of the floodplain. Black mono-specific stands. In the Barmah–Millewa, box woodland is generally open with a sparse Pericoota and Werai forests, river red gums low chenopod shrubland as understorey. The reach 20–35 m, even 40 m tall. However, in the predominance of chenopod species and the lower river zones of the River Murray and in occurrence of samphires in the understorey is a the Mallee Tract reduced water availability response to the generally more saline conditions. restricts river red gums to a fringe beside the The dominance of river red gum and black main river channel, and anabranches such as at box is broken by landform variations such as
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creeklines, hollows and swales where water that have been done, coupled with what is ponds and increases the length of flooding known of growth ecology and reproduction, show inundation. Taking advantage of this are that species are responding to different aspects of vegetation types such as moira grass plains, flow regime. It is these differences, as much as milfoil herbfields and sedgelands in the river red environmental tolerances, that determine the gum areas, and lignum shrublands, sedges and outcome between co-existing species to river coobah amongst the black box. For a good environmental change. Two examples of species general description of floodplain vegetation and growth show how subtle environmental matching for a summary of species composition see Smith needs to be and how easy it is to disturb: and Smith (1990) and MPPL (1990) respectively. • The sedge, Bolboschoenus medianus, shows how depth must be considered in a scale Links with Water Regime commensurate with the study species and how inappropriate depths can interfere with Key descriptors of the flow regime recognised by species resilience. This species requires the Panel for ecological assessment are: total shallow (30–50 cm) flooding in discharge, flood frequency, drought frequency, winter–spring to maximise tuber and flow duration, seasonality and sequencing. development and so enhance its future However, the hydrograph character is also resilience; over-topping minimises tuber important. Key attributes of the hydrograph development. Similarly, flowering, and hence important for ecological analysis are: rates of the opportunity for seed set, only occurs if rise, rate of fall, flood duration, flood peak, flood flood recession is slow, around 2 cm/day; minimum and variability (Thoms et al. 1996; flowering is unlikely at 10–15 cm/day Richter et al. 1996). (Blanch et al. 1996a). Water regime for plants differs in two respects from river flow regime as described above. Some • Spiny mud grass, Pseudoraphis spinescens, plants experience water regime as depth, usually shows an opportunistic response. It regrows as depth x duration, and for some plants velocity rapidly after flooding, whether in June, July is significant (as it is with fish). Depth and velocity (Ward 1991) or October (Jane Roberts, pers. tend to be site-specific and are interpretations of observation) with extensions of 9–10 mm/day flow regime linked to topography. even in winter. This growth trait is beneficial Water regime is not a single attribute for rapidly rising water levels. concept; plants, like other floodplain species, are adapted to a composite of characteristics. New Assemblages on the Lower Murray Maintenance of the adult population (i.e. growth and vigour) relies on a different set Some plant assemblages on riverbanks and on of attributes than for initiating and for floodplains are ‘new’. In many cases these are a completing the reproductive phase. Enough is result of changing the river environment now known of the dominant shrub and woody through regulation, and is most obvious in the species to venture a generalisation in regards to weir pools where conditions generally more the long-lived floodplain species of floodplains favourable to plants exist. Locks and weirs keep in semi-arid Australia. Flood and drought river levels relatively constant through the frequency, plus the sequencing and seasonality, growing season and the width of the river define the flow regime for maintaining the means edge velocity is comparatively low. The adult. In contrast, hydrograph attributes, such as environment is predominantly lacustrine (lake rate of rise and fall, duration and size of flood like). The riverine elements are restricted to peak as well as seasonality, determine successful winter–spring, leaving summer, which is the transition from seed to established seedling. main growing season, as the time of relatively Species specific studies of wetland and stable characteristics. Within weir pools there is floodplain plants have been lacking in Australia; a shift from lower reaches immediately above and the alternative, using generalisations or the weir where water levels are most stable, to conceptual models which link plants to water becoming gradually more variable in the middle regime, whether by guilds or by growth forms, is and upper reaches of the weir pool. very much needed. More studies would help to An analysis, the first of its kind, of riverbank tease out some ecological patterns. In the and edge plants in one of the weir pools of the meantime, results from the few species studies lower Murray found that plants with
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competitive traits occur in the main weir pool Tract) between Yarrawonga and Swan Hill. whereas in the tailwater area, plants that Further downstream, peak abundances occur in dominate are adapted to disturbance and winter and spring. Under very low flow physically stressful environments (Blanch and conditions, Aulacoseira is lost from the water Walker 1997). In terms of species composition column (Webster et al. 1997), therefore numbers this means the fringing community is decline in the lower parts of the river and during dominated by wetland rather than by riverine droughts. The lack of silicon from catchment species (Suter et al. 1993), for example by run-off may also be a contributing factor to the Schoenoplectus validus, Triglochin and Typha. These absence of these diatoms during droughts. low-flow or wetland species have established Bormans and Webster (1993) have demonstrated, amongst, possibly even out-competed river-edge via modelling, that Aulacoseira grows as it is plants, such as Phragmites, so that now the edge carried along the faster flowing mid-section of the vegetation is a mix of residual riverine species, river before being lost by sedimentation further introduced species and species associated with downstream in the slower flowing, less turbulent stable water levels (Walker et al. 1994). These areas. Other common diatoms of the Murray and new assemblages tend to defy ready lower Darling rivers are Nitzschia (several species), classification and interpretation. Rhizosolenia and Asterionella, although Asterionella appears to be absent from the lower Darling River 3.2.3 Phytoplankton and Benthic Algae (Sullivan et al. 1988). Relatively common green planktonic algal The abundance of planktonic (free-floating) and species in the Murray are the desmids, including benthic (attached) algae in the River Murray, as Ankistrodesmus, Actinastrum and Scenedesmus. These in all large rivers systems, is influenced by water are found year round in low abundance along the temperature, turbulence, flow rate, residence length of the river. Another common planktonic time, nutrient availability and turbidity (through green alga in the lower River Murray is the its effect on both underwater light field and filamentous species Planctonema lauterbonii. The nutrient availability). The presence of these Euglendoid flagellate algae Euglena and parameters vary along the length of the River Trachelomonas are common, as are the flagellates Murray, and throughout the year. Mallomonas and Synura, both of which can cause Phytoplankton will grow and be advected taste and odour problems in drinking water supplies. with the river current unless trapped in eddies Of most concern, from a human health and or backwaters, or lost from the water column by economic perspective, is the abundance of the sedimentation. They may also develop in potentially toxic cyanobacterium Anabaena floodplain billabongs and lagoons following circinalis during low flow conditions in the weir inundation by the river, often to massive pools of the Mallee Tract, and lower Darling and proportions (Boon et al. 1990). Phytoplankton Murray rivers. This cyanobacterium produces algae are normally microscopic single cells, or potent neurotoxins known as Paralytic Shellfish aggregated into multicellular filaments or Poisons (PSPs) or saxitoxins. The neurotoxins colonies; the latter being particularly common can kill stock animals if consumed in large for the cyanobacteria (blue-green algae). amounts from a surface scum, or may be Benthic algae grow attached to solid surfaces in bio-accumulated by freshwater mussels in the the river channel, including the surface of river (Negri and Jones 1995). Closely related sediment, snags and aquatic plants. They are species A. sprioides and A. flos-aquae, though not composed of microscopic single-celled neurotoxic, may impart unpleasant earthy or (e.g. diatoms), filamentous (e.g. cyanobacteria) musty odours in river water. The presence of and colonial forms, and larger more complex these, and other cyanobacteria (e.g. Microcystis, multi-cellular algae (e.g. chlorophytes). Aphanizomenon, Anabaenopsis and Oscillatoria The planktonic algal population of the Murray (Planktothrix)) can lead to restrictions on is comparatively low in diversity, being dominated recreational usage of river waters, and the by a few species of diatom, of which Aulacoseira introduction of expensive water treatment (Melosira) granulata is the most common (Sullivan facilities in riverine towns and cities. et al. 1988). It is a filamentous species found Studies on the Murrumbidgee River (Jones throughout the river system, with peak 1994; Webster et al. 1997) have shown that abundances occurring in spring, later in summer Anabaena begins rapid, exponential growth and autumn in the mid-Murray section (Riverine with the onset of persistent (day and night)
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thermal stratification in weir pools. Maximum physical changes of slope, topography and flows, densities can be reached within 14 days. there have been numerous anthropogenic Stratification is mostly controlled by the river alterations to the Murray‘s behaviour. These flow rate, solar insolation and wind speed across include the impoundments, extractions of water, the water. The latter two factors are largely alteration of flow patterns both in time and uncontrollable. Management can control space, removal of natural habitat and cyanobacterial blooms in the River Murray by construction of weirs (as described in maintaining minimum flows high enough to section 3.1.3) as well as a variety of agricultural stimulate mixing and therefore the prevention inputs. All these have an impact on the aquatic of cyanobacterial growth. Recent work by fauna along the entire length of the river. Bormans et al. (1997) has shown that Although the River Murray and its major wind-mixing is more important than flow in tributaries are vitally important, both controlling stratification and cyanobacterial ecologically and economically, it was not until growth in the lower River Murray, below 1977 that the aquatic macroinvertebrates of the Lock 1. In this section of the river, the channel river were systematically recorded (Walker and is very wide and unprotected along the banks. It Hillman 1977). Subsequently, the former River is, therefore, susceptible to wind-induced Murray Commission (now the Murray-Darling mixing by comparatively strong winds during Basin Commission) established a water quality summer low flows, unlike many other parts of monitoring program which included chemical the Murray-Darling system. (started 1978), macroinvertebrate and Planktonic and benthic algae are important phytoplankton (both started in 1980) food sources for zooplankton and monitoring along the length of the river from macroinvertebrates, and their abundance and Jingellic in the upper catchment to Lake species composition may have cascading Alexandrina near the Murray mouth in South influences along the food chain. Loss of benthic Australia. Bennison et al. (1989) documented algae due, for example, to increased water the results of that monitoring for the years turbidity may lead to a decline in the abundance 1980–1985, and showed the changes in the of benthic macroinvertebrates and planktonic macroinvertebrate communities along the crustaceans, especially benthic grazers such as length of the river. These results are still the shrimps and prawns, themselves important food most comprehensive for the river, although sources for native fish. For example, in the lower additional work has been completed in the River Murray where turbidity is increased upper catchment (Blyth et al. 1984; Doeg 1984 dramatically by the inflow of Darling River and Koehn et al. 1995) and in the lower River water, macroinvertebrate populations are Murray and Darling River (Walker 1985, 1996; decreased in size by a factor of ten from Walker et al. 1994, 1995). upstream (see section 4.4.2). On the other hand, The study by Bennison et al. (1989) assuming sufficient light, instream productivity demonstrated five distinct macroinvertebrate would be expected to increase as nutrient communities along the length of the river from concentrations increase downstream, for Jingellic to Tailem Bend. The upstream sites of example, below point source discharges such as Jingellic and Tallandoon were dominated by sewage treatment plants or below the confluence fauna typical of upland rivers where there were of tributary streams. This can lead to the broad ranges of habitats and high species proliferation of benthic algae, especially large richness of mayflies, stoneflies, caddisflies and filamentous green algae such as Spirogyra and other groups usually found in fast flowing Cladophora which may shade out benthic diatom upland rivers. Many of the species found at species, themselves an important component of these sites were restricted to the upland area. the biofilm that forms on submerged surfaces. Immediately below Hume Dam and Yarrawonga Weir, the fauna was again distinctive with low 3.2.4 Invertebrate Distribution and Ecology species richness and abundance. The communities were distinctly different from the In a river system which flows over 2500 km upper river zones and also the middle river from its headwaters to the sea, it is expected zones. They concluded that this community was that there will be significant changes in the consistent with the effect of river regulation and macroinvertebrate communities along the impacts of impoundments. The presence of length of the river. In addition to the natural impoundments causes a disruption to the
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continuum of the river, as discussed in abundances have increased 2–3 fold (Suter section 3.2.1, and so alters the source and type 1996). Abundances upstream at Lock 9 have not of organic material which is an important food altered and have remained similar to previous supply for the invertebrate communities. years, suggesting that the turbidity of the water The middle parts of the river from during the spring-autumn months is important Torrumbarry Weir to Lock 9 (river zone 4), and in determining the abundance of the including the Murrumbidgee River at Balranald invertebrate community. and the Darling River at Burtundy, have a The macroinvertebrate communities of the distinct faunal community. In contrast to the river clearly show a response to the alteration of upper river zones, this stretch of river is environmental condition and a reduction in the dominated by the Dipterans (midges) and complexity of the habitats available for the stoneflies, while mayfly species are greatly fauna along the river’s length. The upper reduced in number. This community group is catchment streams have fauna which require again rich in species and also in abundance. clear, fast flowing water, a wide variety of The lower River Murray has two distinctive habitats together with summer temperatures faunal communities, with elevated numbers of sufficient for them to complete their life cycle crustaceans and significantly fewer insects and reproduce. In the middle river zones, compared with other sites. In addition to this natural flow changes lead to a different faunal change, faunal abundances in South Australia community adapted to lower velocities, higher are an order of magnitude lower than in the temperatures and seasonal fluctuations of water middle zones of the river. The upper portion level. In this stretch of river the importance of from near Renmark to Morgan had a distinct logs, large woody debris and submerged and faunal community which differed from the emergent macrophytes for the provision refuge community near Tailem Bend. The site near and a stable substrate for food cannot be over Lake Alexandrina was distinctly different in its emphasised. It is likely that the faunal composition to all other river sites, with fauna communities of the middle zones would have suggesting these areas experience higher extended into South Australia, probably down salinities than upstream sites. to below the Morgan-Blanchetown area, where The reduction in faunal abundance in the higher salinities and estuarine conditions lower River Murray was considered to be the occasionally would have influenced the faunal result of long periods of high turbidity communities. Today the high level of regulation experienced in South Australia due to the and varying source waters have reduced the operation of Lake Victoria and the high complexity of habitats and extent of aquatic proportion of the turbid Darling River water vegetation communities in the South Australian stored in Lake Victoria (Bennison et al. 1989). reaches, so the macroinvertebrate community During the study period (1980–1985) the here differs from those upstream. Darling River was the major source of water to It is clear that the introduction of South Australia, and, in providing entitlement impoundments has affected nearby faunal flows, the Darling River effectively contributed communities both during construction (Blyth highly turbid water, not only when in high flow et al. 1984) and by causing reduction in summer in spring, but via Lake Victoria throughout the temperatures due to hypolimnion releases during major growing period of summer and into the irrigation season (see section 3.1.4, and Doeg autumn. As a consequence, it was suggested 1984; Koehn et al. 1995). As a result of the latter, that food and habitat availability for the many of the macroinvertebrates have been unable invertebrates was reduced, thus reducing the to reproduce successfully or complete their life abundance in South Australia. cycle. In addition, Dartmouth Dam and Hume Continued monitoring by the MDBC in the Dam have created a disjunction in the continuum late 1980s and early 1990s has shown that high of the river. Alteration in the flow characteristics flows from the River Murray and and regulation in the lower river with weirs, and Murrumbidgee systems (as opposed to the provision of highly turbid water from the Darling Darling), and the provision of lower turbidity River via Lake Victoria in summer and autumn water from Lake Victoria during has affected the South Australian invertebrate summer-autumn months, have resulted in community (Bennison et al. 1989). lower summer turbidities in South Australia. Walker (1990) and Geddes (1990) both Consequently the macroinvertebrate indicate the decline of two key river species in
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South Australia, both of which were found in the lower river zones. A further group inadequately sampled by Bennison et al. (1989). of species which require access to marine waters The freshwater river mussel (Alathyria jacksoni) to complete part of their life cycles are found in has been replaced in weir pools by the the lower zones of the river. These species billabong/floodplain mussel (Velesunio ambiguus) include the Short-headed Lamprey, Pouched as a result of a flow regime changing the flowing Lamprey, Short-finned Eel, Common Galaxias, river to a series of pools (Walker 1990). Similarly, Small-mouthed Hardyhead, Tupong and Estuary Geddes (1990) noted that the Murray crayfish Perch. The taxonomy of several groups, including (Euastacus armatus) once common in South the Hardyheads and Gudgeons, have undergone, Australia prior to the mid 1950s, is now virtually or are undergoing, revisions. These revisions, extinct downstream of Mildura. The alteration in along with other taxonomic work, could result in flow regime is again implicated. There may be the determination of further species. many more changes in species composition or Species range in size from the Murray Cod, loss of species, but without prior information it is which is Australia’s largest freshwater fish difficult to demonstrate. Evidence from aboriginal (recorded up to 113 kg), to smaller species middens suggest that smaller gastropod snails are weighing only a few grams. It has been also reduced in abundance in the highly recognised that most native species have suffered regulated section in South Australia. major declines, particularly in recent decades. Other changes are not as distinct, mainly There is concern about the conservation status of because we have very little information on several species including the endangered Trout communities of macroinvertebrates in large, inland Cod. Commercial fisheries for other species, such rivers. However, there are a number of threats to as Murray Cod, Golden Perch, Silver Perch and the macroinvertebrate communities, all of which Catfish, have all suffered major declines over the are associated with reduction of habitat complexity. past 50 years. Recreational angling has become For the macroinvertebrates it is difficult to precisely more popular in that time, and many of these define a flow regime that will benefit their native species are keenly sought. Of the community. Any regime which promotes the introduced species, Carp and Mosquitofish are extent of the macrophyte communities, the the most widespread, with the former currently development of biofilms, and the reduction of receiving the most public attention. The sedimentation and erosion, and returns a seasonal introduced Salmonid species are restricted pattern, will be of benefit. In addition, increasing mainly to the upper river zones (above Lake the complexity of habitats in the river by adding Mulwala) and are keenly sought by anglers. snags, fluctuating water levels and increasing Six native species are considered to be macrophytes will benefit the biota as a whole. threatened or rare (Wager and Jackson 1995), including the Trout Cod which is now considered 3.2.5 Fish Distribution and Ecology to be critically endangered (National Trout Cod Recovery Team, unpubl. data). The natural range The fish fauna of the Murray-Darling Basin is of this species is now restricted to about 120 km relatively depauperate by world standards, of the River Murray, immediately downstream of containing only 33 native species, several of Lake Mulwala. The capture of Trout Cod is now which are restricted to the lower river zones and prohibited in both New South Wales and Victoria. associated with marine or estuarine reaches. This The state of fish habitats and threats to the can be compared with the more than 1300 fish fish species vary along the river. There is a need species described to date for the Amazon Basin to recognise more subtle but cumulative effects, (Cadwallader and Lawrence 1990). Of the such as reductions in fish habitats or water recorded native species, 26 are found in the river quality parameters, against critical effects such as zones of the Murray considered in this project, water temperature and barriers to fish passage. together with 10 introduced species (Table 3.8). The majority of the species are widespread along Importance of Water Temperature the river, although some have distributions which grade toward either the upper or lower The native fish present in the River Murray could zones. More upland species include Macquarie be described generally as a ‘warmwater’ species Perch, Two-spined Blackfish and the Mountain assemblage. Water temperature is important for Galaxias, all found in the Mitta Mitta River, the functioning of fish species, particularly for whilst species such as the Bony Herring are only reproduction, optimal functioning and growth.
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TABLE 3.8 Freshwater fish species of the River Murray (supplied by John Koehn)
Common Name Scientific Name Comment Native species River Blackfish Gadopsis marmoratus minor angling species Two-spined Blackfish Gadopsis bispinosus Broad-finned Galaxias Galaxias brevipinnis recent intro. into basin Flat-headed Galaxias Galaxias rostratus threatened sp. Mountain Galaxias Galaxias olidus Murray Cod Maccullochella peelii peelii important angling species Trout Cod Maccullochella macquariensis endangered species Golden Perch Macquaria ambigua angling species, comm. Macquarie Perch Macquaria australasica threatened sp. angling Silver Perch Bidvanus bidvanus angling, threatened sp. Southern Pygmy Perch Nannoperca australis Australian Smelt Retropinna semoni Catfish Tandanus tandanus angling, declined Bony Herring Nematalosa erebi Southern Purple-spotted Gudgeon Mogurnda adspersa threatened sp. Western Carp Gudgeon Hypseleotris klunzingeri Midgeley’s Carp Gudgeon Hypseleotris sp. Lake’s Carp Gudgeon Hypseleotris sp. Flat-head Gudgeon Philypnodon grandiceps Dwarf Flat Head Gudgeon Philypnodon sp. Crimson-spotted Rainbowfish Melanotaenia fluviatilis fluviatilis Murray Hardyhead Craterocephalus threatened sp. stercusmuscarum fluviatilis Fly-specked Hardyhead Craterocephalus stercusmuscarum fulvus Darling River Hardyhead Craterocephalus anniculus Agassiz’s Chanda Perch Ambassis agassizi threatened sp. Introduced species Brown Trout Salmo trutta angling Brook Trout Salvelinus fontinalis angling Rainbow Trout Oncorhynchus mykiss angling Carp Cyprinus carpio pest Tench Tinca tinca Roach Rutilus rutilus Goldfish Carassius auratas Redfin (English perch) Perca fluviatilis angling Mosquitofish Gambusia holbrooki
For example, warmwater fish have specific restrict the success of spawning, and may also temperature requirements for breeding which have detrimental effects on metabolic function are generally higher than 16˚C, but which vary and growth rates. This is highlighted in the upper from species to species. The release of cold water zones of the river with high volume irrigation from low level storage outlets poses a major releases from the low level outlets of Dartmouth problem to warmwater fish assemblages (see Dam and Hume Dam. Three species of warmwater section 3.1.4). In particular, such releases may native fish (including endangered Trout Cod,
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TABLE 3.9 Spawning likelihood at Colemans and Tallandoon in the Mitta Mitta River for Murray Cod, Trout Cod and Macquarie Perch between 1968 and 1993 (from Koehn et al. 1995) Colem = Colemans site, Talldn = Tallandoon site.
Murray Cod Colem. ? ? ? X X ? X X X ? X X X X X Talldn. ? ? ? X X X X ? ? X X X X
Trout Cod Colem. ? ? X X X ? X X X X X X
Macquarie Perch Colem. ? X ? X X X X X X Talldn. ? X X X X X X X
Season 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93
= Spawning likely to occur X=Spawning unlikely to occur ?=Insufficient data/unsure
Macquarie Perch and Murray Cod) have River. By contrast, conditions for the spawning of disappeared from the reaches of the Mitta Mitta Murray Cod were considered to be suitable for River downstream of the Dartmouth Dam. It is between 70 and 100% of the seasons between likely that recruitment of these species has not been 1968 and 1978, before the dam construction (no possible due to coldwater releases during the seasons were considered to have provided spawning season of this species (Koehn et al. 1995). unsuitable conditions although insufficient data Using available temperature data, Koehn et al. made determinations unsure for 30% of seasons). (1995) made an assessment of the likelihood of Trout Cod had likely successful spawning the successful spawning of Murray Cod, Trout temperatures in only five seasons, with only one Cod and Macquarie Perch for each season from of these occurring since 1986, and none since 1968 to 1993 at both Colemans and Tallandoon. 1989. Macquarie Perch may have been able to As Trout Cod were only previously found to be reproduce in most seasons prior to 1986, but not in the reaches close to and upstream of since 1989. Colemans, an assessment for that site only was Water temperature is not only important for the made for that species. spawning of fish, but also for the development of As Table 3.9 shows, there have been decreased larvae and their general growth and survival. Each opportunities for the spawning of these warmwater species has upper and lower critical temperature fish species following the operation of Dartmouth tolerances (see Koehn and O'Connor 1990), with Dam in 1980, with little likelihood of successful optimal temperatures within this range. Optimal spawning for any of these three species during the temperatures are difficult to determine exactly and past 5 years to 1993. The effects of the changed hence only indicative ranges are known for many temperatures are most pronounced closest to the of our native fish species. dam and so each of these species has had more However, changed temperatures conditions in the opportunities for successful spawning in the lower river below the dam have favoured species such as reaches. The species most affected was Murray Brown Trout, with the greatest increases in numbers Cod, which requires the highest spawning being at sites closest to the dam wall. Optimal temperature of about 20˚C. This species was likely temperatures of 4–19˚C for Brown Trout and to have had opportunities to spawn successfully in 10–22˚C for Rainbow Trout are reported in only 3 seasons (20%) in the upper reaches and five Cadwallader and Backhouse (1983). With upper seasons (33%) in the lower reaches of the Mitta critical temperatures of 19–30˚C for Brown Trout Mitta River since the dam has been in operation. (Cadwallader and Backhouse 1983), these conditions None of these occasions have been in the past nine would have limited production of this species in the years at the upper sites. It is not surprising river prior to the dam. The current temperature therefore that there has been no evidence of regime is much more favourable to this coldwater recruitment for this species in the Mitta Mitta species, especially during the warmer months.
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Migration fragmentation. Larvae can also be transferred into irrigation channels where they are unlikely to Fish are mobile creatures and have a need to survive (Koehn 1996). move widely throughout the river system. Whilst it has been known for some time that the adults Ecology of species, such as Golden Perch, can migrate over 1000 km (Reynolds 1983), it has only Reproductive strategies vary between species, recently been discovered that large numbers of with some species such as Golden Perch and juvenile fish of species, such as Silver Perch also Silver Perch producing up to 500,000 eggs which move upstream (Mallen-Cooper et al. 1996). are laid in the water column and left without Species, such as Murray Cod, which were care by the parents. In contrast, species such as previously thought to be sedentary have been the River Blackfish lay only a few hundred eggs shown to undertake relatively large migrations which are adhesive, laid on a wood substrate, (Koehn 1996). Whilst these species may be able and subjected to parental care. A similar strategy to survive and reproduce even if such is employed by the Southern Purple-spotted movements are not able to take place, their Gudgeon which may spawn several times in a ultimate survival and distribution over the longer season, producing varying numbers of eggs each term may be detrimentally affected. Many other time. The spawning of most species in the River species, especially those in the lower zones of the Murray occurs in late spring or summer. river, have parts of their life cycle which must be Certain fish species rely on flooding for completed in saline water (normally the sea) and successful reproduction. Many fish populations hence have to migrate to complete their life have evolved to exploit the high food availability cycles. Some of the major migrations which are on floodplains in times of floods. During flooding, recognised are understood to occur in a particular nutrients accumulated on the floodplain are season, e.g. for spawning, and may often be released and support an increase in production of affected by changes to flows. However, our aquatic plants and invertebrates which provide a understanding is not complete for all movements. rich food source for juveniles (Gehrke 1991). Hence fish passage should be available to all Reduced flooding together with diminished peak species throughout the year. levels significantly decreases the juvenile habitat Aggregations of fish migrating upstream often available, and thus the number that can be occur immediately downstream of barriers (weirs supported. A further complication is the and dams) making these fish very susceptible to clearance and replacement of native vegetation capture by anglers. Illegal fishing also occurs to with flood intolerant exotic crops and pastures. varying degrees along the river. Under such conditions, floodwaters rapidly Increases in water levels, both large and small, become de-oxygenated as microbial communities can stimulate the movement of fish decompose the flooded vegetation. This may (Mallen-Cooper et al. 1996). Reductions in adversely affect the survival of certain fish flooding may restrict such movements, as might species, particularly when movement to other constant flow levels. The limiting of cues and the areas is restricted (Gehrke 1991; McKinnon & barriers to movement may affect spawning Shepheard 1995). success and the distribution of species. Sudden Fish diets also vary between species and reductions in flow levels can also lead to the between life stages. Many of the Murray species stranding of fish. could be viewed as opportunistic feeders, with Another form of movement which is often diets varying with availability of prey. There is forgotten is the drift of larval fish. This has the little information to quantitatively link food purpose of recolonisation and distribution of supply in the river system with species offspring, and can be affected by altered flow abundance. Most of the species are visual feeders rates and impounded waters. High irrigation and may be affected by increased turbidities flows during early summer may now mean that which decrease visibility. A critical need for food larvae are carried greater distances than would supply in the form of microorganisms is likely to have occurred naturally. Impounded waters can occur at the time of first feeding when larvae trap larvae and prevent their distribution need an almost constant food supply to maintain downstream. Such effects on the overall structure energy levels. Such food may be provided from of fish populations is unquantified but it is likely plankton blooms which can occur on floodplains to result in some form of population during high spring flows.
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Habitat 3.3 CONCLUSION
Fish numbers are often related to the amount of Large semi-arid lowland river systems, like the habitat available. Of course there must also be River Murray, typically have a highly variable access to this habitat which can be blocked by flow regime. The river channel and floodplain barriers. Snags or large woody debris are the morphology are configured to accommodate major form of structural habitat in lowland rivers variable flows, and the plants and animals are and are widely used by many species. The use of also adapted to such natural variation. However, this habitat has long been recognised, but recent this has been suppressed by water resource research has shown that its importance has development and a more stable regime (i.e. less probably been underestimated (Koehn 1996). variability over the daily, seasonal and Snag removal has been widespread throughout inter-annual scales) is now imposed. As a the river, leading to a major loss of fish habitats. consequence, the River Murray is undergoing a Snag numbers are now considerably less than series of compensatory adjustments. Some of those which occurred naturally. Snags are used these adjustments are still not complete – 60 as home sites for territory formation, predator years after the construction of the first regulating avoidance and prey detection. They offer structures. Importantly, some native species are protection from high water velocities, and are adapting to the new flow regime while others sources of food and spawning sites for many are in decline. This chapter has illustrated a river species. The number of snags needed to maintain system undergoing significant environmental fish populations has not been fully elucidated; changes – changes that are detrimental to the however, more snags would provide more overall ecological integrity of the system. habitat for more fish. Throughout this chapter, the Panel has Many of the off-stream habitats, such as outlined management actions that in the Murray billabongs and wetlands, have also been system are threatening the key components of degraded, removed or made inaccessible. river–floodplain ecosystem health – diversity, This can lead to the loss of species which linkage and functioning – together with specific depend on these habitats in these areas. examples of their impacts. This information is Habitats in the form of pools and scour holes now encapsulated in Tables 3.10 and 3.11. can also be lost through infilling by Together, these tables effectively summarise sedimentation. Removal of snags and bank ecosystem response to the range of flow, river erosion can lead to a more uniform channel and relevant land management practices. In without a diversity of habitats. Variations in particular, Table 3.10 shows that there are many depth and velocities are also important to aspects of flow management that reduce the provide the suitable habitat for all species variability and seasonality of the natural flow throughout their life cycle, and the presence of regime and have major ecological impacts. These snags promotes such habitat diversity. issues and threats are discussed in Chapter 4. The management of fish habitats and fish populations is somewhat dependent on the biological requirements of the species. Whilst much of this knowledge is incomplete, the strategies needed for most species is sufficiently well known to make sensible management decisions. The diversity of species requirements and the linkages between habitats must be recognised. Some management actions, such as providing fish passage or appropriate outlets for natural water temperatures, involve financial and engineering decisions only – they will not impact adversely on water allocation or future river operations. Others, such as the provision of water and the protection of catchment processes, are more complex and rely on an integrated approach with many agencies and land managers.
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TABLE 3.10 Flow management activities that threaten the key components of river–floodplain ecosystem health – habitat diversity, natural linkages and metabolic functioning
Flow Management Ecosystem Health Observed and Predicted Ecological Impacts Action Component Constant flow for Habitat Diversity Erosion and incision of riverbanks sustained periods Change in bed morphology Reduced range of bank habitats May favour exotic species Metabolic Reduced epiphytic productivity Functioning Change in vertical temperature profile Loss of cues for invertebrate reproduction probable Loss of cues for fish movement Loss of cues for plant reproduction Unseasonal flow Habitat Diversity Habitats not available when needed Degradation of habitats through death of native species, promotion of invasive species and lack of regeneration of native species Natural Linkages Linkage is made ineffective – timing of flow triggers is critical as they often work in association with other triggers such as increasing temperature or daylight hours Changes in transport of particles and biotic material Metabolic Reduced light penetration through increased Functioning turbidity and depth which affects instream productivity Change in sediment transport regime Change in vertical temperature profile Lack of oxic processes if natural drying out phase of floodplain is prevented Increased Habitat Diversity No natural drying out leading to a loss of minimum flow ephemeral habitat Loss of instream habitat diversity necessary to maintain biological diversity May favour exotic species Degradation of natural low flow channel shape Changes to instream habitat Metabolic Reduced benthic productivity if too deep Functioning Lack of oxic processes if natural drying out phase of floodplain is prevented Decreased Habitat Diversity Reduced and even lost species requirements for frequency of reproduction and regeneration flooding periods Loss or redistribution of habitats Natural Linkages Reduced input of organic material from floodplain to river resulting in reduced fish reproduction success Metabolic Reduced exchange of organic carbon, nutrients, Functioning sediment, etc., between floodplain and river Reduced duration Habitat Diversity Critical life cycle stages cannot be completed individual floods Reduced recharge of floodplain and/or bank Changes to natural rate of morphological change Rapid rates of Habitat Diversity Bank slumping causing erosion and loss of habitat, rise of fall while resulting siltation smothers and degrades habitat Stranding of biota and/or habitats leading to deaths Weir pools Habitat Diversity No natural summer drying out leading to a loss of ephemeral habitat Loss of instream habitat diversity necessary to maintain biological diversity May favour exotic species Degradation of natural low flow channel shape Alteration of instream habitat Metabolic Stratified pools develop anoxic bottom water Functioning (if overflow weirs) Thermal stratification favours cyanobacteria
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TABLE 3.11 River and land management activities (other than flow management) that threaten the key components of river–floodplain ecosystem health – habitat diversity, natural linkages and metabolic functioning
Ecosystem health Management Observed and Predicted Ecological Impacts component Action Habitat Diversity Grazing of riverbanks Reduced ability for regeneration of native vegetation No refuges for biota Reduced snag input to river Value of bank as habitat for invertebrates and fish is lost May cause changes in vegetation, species and structure Increased bank instability Clearing of riverbank Value of bank as habitat for invertebrates and fish is lost Loss of snag input to river Promotion of exotic Habitat value doubtful, particularly where willows riparian vegetation replace native vegetation Loss of snag input to river in the future If canopy density is changed, then micro-climate and habitat changes will impact invertebrates Changed in-channel morphology and therefore habitat Removal of snags Loss of structural habitats Loss of breeding sites for some fish species Loss of macroinvertebrate habitat Loss of channel diversity Loss of diversity of water depths and velocities Recreational boating Bank erosion resulting in siltation that has a smothering impact on habitats Re-suspension is some areas Loss of in-channel benches Aggregate extraction Change in bed morphology Natural Linkages Levees Disrupt linkages Floodplain Devalues linkages – remaining linkages are less effective development as development reduces area and quality of habitat Removal of snags Loss of habitat and food substrate on floodplains, and input of this to the river Various land use May lead to water quality changes that affects practices linkages – saline intrusions and blackwater build up to form a barrier to fish movement Culverts and Disrupt linkages regulators Devalue linkages – remaining linkages are less effective as development reduces area and quality of habitat
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Ecosystem health Management Observed and Predicted Ecological Impacts component Action On-stream storage Linkage is made ineffective – timing of flow triggers structures is critical as they often work in association with other triggers such as increasing temperature or daylight hours Results in serial discontinuity (the river upstream of the structure is separated from the river downstream) which prevents fish movement and invertebrate drift, and obstructs the transfer of instream material downstream Metabolic Increase in diffuse Increases severity of algal blooms Functioning nutrient sources May shift balance between allochthonous (eg terrestrial leaf input) and autochthonous (within stream) carbon inputs to the river, with resultant changes to biotic community structure Diffuse toxicant Agricultural and industrial chemicals can have sources toxic effects Erosion and sediment Increased turbidity and siltation input from catchment Absorption and transport of pollutants modification Clearing of riverbank Reduces input of organic material, including leaves and snags Alters type and timing of carbon inputs, if bank replanted with non-native vegetation or shift in vegetation types occurs Increased sediment input Removal of snags Loss of biofilm area and productivity for macroinvertebrates Temperature of Decreases productivity especially if cold water being releases released in summer Modified cues for native fish breeding May favour exotic fish and other fauna Reduces opportunity for completion of life cycle as there are critical thresholds for hatching, germination etc. Can alter overall productivity Hypolimnetic Potential effect on downstream productivity due to (bottom)releases increasedconcentrations of nitrogen, phosphorus, from storages iron, magnesium Potential effect on downstream microbial processes Aggregate extraction Interrupts sediment transport
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4–COMMON ISSUES & MANAGEMENT ACTIONS
The conclusion of the previous chapter The Panel also identified other issues of importance from an ecological perspective. highlighted the management actions While they are discussed in some detail in this which are having a significant impact on chapter, management actions to address these the health of the River Murray and lower particular issues are presented in the specific river zone sections of Part II. This is because Darling River, and summarised the types these issues tended to be localised and specific of impacts observed. to certain river zones.
The river zone assessments in Part II of this report 4.1 CHANGES TO FLOW REGIME detail where these impacts are occurring. Figure 4.1 summarises the most serious ecological issues The major feature of the hydrology of the River occurring in each of the river zones and shows the Murray is the variability of flow (see Chapter 3). change in ecological issues as you progress The physical and ecological character of the river downstream. It can be seen that many of these has evolved as a result of this variability. issues are common to a number of zones. Hydrological variation influences biological diversity The ecological issues common to multiple zones in rivers by controlling key habitat conditions both are discussed in this chapter in terms of the in-channel and on the floodplain. Indeed, many changes to the natural flow regime and observed species use certain variation in flows as cues for impacts on the components of ecosystem health. Generic management solutions to improve river breeding and migration. Reduction of flow management and operation, and thus reduce the variability disrupts these cues with detrimental major ecological impacts, are recommended here. impacts, for example, to native fish populations. These solutions were developed for application in Flow variability operates at a range of scales. the River Murray or anywhere that one of these For example, flow fluctuations occur on a daily issues occur. These management recommendations basis while seasonal variability in the River Murray are applied to relevant river zones in Part II. leads to low summer flows and high winter–spring FIGURE 4.1 Relationship between major ecological issues and river zones
NEW SOUTH WALES SOUTH AUSTRALIA Constant flows r Unseasonal flows g rive ZONE 6 Reduced occurance of floods in w ls lo oo Increased risk of algal blooms F m f p s o Impeded fish passage n fro rie Lake e Victoria tio s ra a Alte to
Hay Euston M Balranald u r r u m b i d g e e R i v e M r u r r a y R Yass i v e ADELAIDE r ZONE 5 Swan Hill Wagga Wagga CANBERRA
Deniliquin r
e
v
i R
ZONE 4 e
e g
Murray Mouth d i
Albury b
m u r r VICTORIA Echuca u M
ZONE 3 ZONE 2 ZONE 1
Negative effects of weir pools Increased risk of algal blooms Impeded fish passage Reduced habitat Reduced snags Unseasonaly low water temperatures Increased turbidity Constant flows Unseasonaly high flows Reduced flooding Reduced floodplain
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FIGURE 4.2 Aspects of the flood pulse that may be ecologically important. A = duration of zero flow (and often, discontinuity of channel pools); B = amplitude of falling limb ("drawdown"); C = interval since last flood peak; D = amplitude of rising limb; E = duration of rising limb; F = duration of falling limb; G = interval since last flood minimum; H = slope of rising limb; I = slope of falling limb. Modified from Boulton et al. (1998).
C
Bankfull stage
H D I
B Discharge EF
G A
Time floods. Similarly, variability occurs over annual and baseflows rather than naturally elevated flows. inter-annual time scales, the latter demonstrated The major effects of extended periods of by the 1974 flood and 1982 drought. To elucidate constant flow are: hydrological variability it is useful to consider an • loss of stimulation for fish movements; idealised hydrograph (Figure 4.2) depicting flow • reduction in the range of riverbank habitat and events at a given point. This hydrograph depicts a bed habitat (i.e. reduced wet/dry area) and sequence of flood pulses (changes in river stage or • instability of the river channel banks and height) which may be characterised by their subsequent reduction of in-channel amplitude, duration and the slope of their rising and complexity and habitat diversity due to falling limbs. A sequence of flood pulses constitutes erosion and sedimentation. the flow history of that point of the river. This can The Panel acknowledges that it is not possible to be generalised statistically to describe the flow mimic perfectly the natural variability in the regime of part or of the entire river using the central flow regime whilst at the same time providing tendencies of mean amplitude and duration, and irrigation or entitlement flows from storages. the mean frequency of daily, monthly and annual However, the Panel believes that some variation flows. A reduction in flow variability at any scale can be introduced on the pattern of high can have an important impact on the river summer flows and low winter baseflows. This ecosystem. The Panel aimed to return variability to would have benefits in reducing the likelihood the River Murray at the scales where it was seen to of bank erosion, introducing some lost be affected to such a degree as to negatively impact invertebrate and small fish habitats, and in ecosystem health. The ecological issues shown in encouraging greater biofilm development by Figure 4.1 include changes to the variability at a increasing the range of riverbank that is being range of scales. These are discussed below. alternately wetted and dried.
4.1.1 Constant Flows RECOMMENDATION G1
A number of river zones are currently suffering Releases at a constant discharge should be from the influence of stable water levels for long avoided. Instead releases should mimic a natural periods of time, due to operating the river at rainfall event in the catchment by using a step constant flows (Figure 4.3a and b). Constant function to vary flows as shown in Figure 4.4. The flows cause a loss of flow variability at a daily flow would rise over two days and then recede and monthly scale. (The issue of stable water gradually. A suitable time scale for this variation levels in weir pools is treated separately.) would be 2 weeks giving two peaks per month. Periods of constant flow occur during the The amplitude of the variation around the desired summer months when the river is used to level should be ± 20% (in terms of river height, provide water for irrigation and to carry the not flow, with respect to bank height at the point South Australian entitlement; they also occur to of release) truncated where necessary by a lesser extent in winter with extended periods of minimum and channel capacity flows.
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FIGURE 4.3 Impact of flow regulation in the River Murray at (a) Torrumbarry and (b) Euston
30000
1908 - Pre-Regulation 25000 1994 - Post Regulation
20000
15000
10000 Torrumbarry Flow (ML/day) Torrumbarry
5000
0 1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec Date 80000
70000 1908 - Pre-Regulation 1994 - Post Regulation 60000
50000
40000 Euston Flow (ML/day) 30000
20000
10000
0 1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec Date
FIGURE 4.4 Diagrammatic graph showing variation in flow according to a step function via adjustments to releases from storages on the River Murray
Alternative flow distribution
12000 Rise corresponds to a day or two after a rainfall event 10000
8000
6000
Flow (ML/day) 4000
2000
0 Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9 Day 10 Day 11 Day 12 Day 13 Day 14 Day 15 Day 16 Day 17 Day
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The Panel recommends that flow variations 4.1.2 Sustained Unseasonal In-channel Flows could be re-established using a step function with two peaks per month for constant flow Sustained unseasonal flows refer to extended periods. The step function is intended to periods of flows which are unlike the natural mimic how natural flows normally increase flow regime at that particular time of the year. rapidly following rainfall and then gradually The most obvious is sustained high flows over taper back to baseflow. This is only a first summer, and sometimes into autumn, when the attempt at dealing with this issue and the natural flows would have been dropping. Also Panel expects that this could be refined on the significant is the constant low baseflows in basis of scientific assessment (adaptive winter (see Figure 3.2). Figure 4.5 demonstrates management strategy) and as river operators the change that has occurred to seasonality at become adept at operating under these Albury, which is just downstream of Hume Dam. ecological guidelines. High summer flows not only affect the Modelling undertaken by the MDBC (see instream environment but also riparian and Appendix 3, Section 1: 1a, 5, 7) indicates that wetlands areas. Increased flow durations, as a this rule would, in general, provide an adequate consequence of water resource development, level of supply. The ecological benefits need to have resulted in these areas being continually be substantiated. In some cases, there may need wet. The result is a mismatch between available to be an upper limit to ensure that channel habitat and biological needs, and lack of capacity is not exceeded. Low levels should not appropriate life cycle cues for a number of fall below the recommended minimum. In species of fish, plants and macroinvertebrates. recommending this introduction of variation in The effects of unseasonal high flows are not river height, the current rules which govern the well understood and need to be quantified. In rate of rise and fall should be adhered to many cases, the effects are not immediately (e.g. the six inch rule below Hume Dam). obvious. For example, increased flows in late Specific applications of this rule are given for spring and summer due to irrigation demands individual river zones in Part II. coincide with spawning and hatching of many
FIGURE 4.5 Impact of flow regulation in the River Murray downstream of Hume Dam at Albury
60000
50000 1918 - Pre-Hume Dam 1994 - After Hume Dam 40000
30000 Albury Flow (ML/day)
20000
10000
0 1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec Date
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TABLE 4.1 The impact of current development on the frequency of occurrence of the 10, 20 and 50 percentile natural conditions flood flows for consecutive stations down the River Murray1 All values are obtained from an annual series of the maximum monthly flow in the year (expressed as a mean daily flow) for natural and 1994 conditions simulated over 100 years (supplied by MDBC).
Station Flow (ML/day) Exceedance under natural Exceedance under 1994 conditions (% of years) conditions (% of years) Albury 32,000 50 18 47,000 20 5 55,000 10 4 Yarrawonga 42, 000 50 18 62,000 20 7 78,000 10 4 Torrumbarry 43,500 50 12 53,500 20 7 57,000 10 3 Swan Hill 27,000 50 14 31,500 20 5 32,500 10 3 D/S Wakool Jn 43,000 50 15 85,000 20 7 112,000 10 4 Euston 65,000 50 13 115,000 20 6 142,000 10 4 Wentworth 70,000 50 13 135,000 20 5 165,000 10 3 Flow to South 63,000 50 13 Australia 119,000 20 5 141,000 10 3
native fish. Unseasonally high flows mean storages every year are adequate to sustain the increased larval drift. Increased travel distances aquatic biota. At best, such releases must restrict mean the larvae are carried further than would the amount of habitat available to all aquatic have occurred naturally. Although the overall species. Restricted habitat is an issue because consequences are not exactly known, enough organisms confined to this limited habitat can be evidence of negative ecological impacts exists expected to be vulnerable to disturbance, such and therefore this issue must be addressed. as fishing, and to density-dependent processes, In the River Murray, unseasonal high flows such as competition and predation. are caused by the need to provide irrigation water and the South Australian entitlement. 4.1.3 Reduction in the Occurrence of Floods The Panel accepts that it is unable to recommend changes that will alleviate this for In nearly all river zones, the frequency of low to the instream environment other than the medium flood flows has been reduced under incorporation of some variability into the flow 1994 conditions (1994 conditions are used as a regime, as described above. benchmark of current regulated conditions by Unseasonal low winter flows occur in several the MDBC) and some flows are now one-third river zones when storages are being filled with to one-quarter of their natural occurrence, see winter flows. There has been no attempt to Table 4.1. This has had differing impacts on determine if the low flows released from channel and floodplain habitats, including
1For the River Murray, there is a strong linear relationship between the maximum monthly flow in the year and the peak daily flow in the year. This relationship gets stronger further downstream with correlation r square values increasing from 0.85 at Albury to 0.97 at the South Australian Border. For mid range floods, the peak daily flow at Albury and Yarrawonga is roughly twice the mean flow in the maximum month. At sites downstream of the Barmah/Millewa Forest, the peak is typically 25% greater than the mean.
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instream benches, anabranches, flood runners recommending this principle, the Panel merely and wetlands connected to the river at various indicates that the occurrence of flooding below river heights (see section 3.2.1). Each habitat this level is deemed to be totally unacceptable. It requires a specific flow regime to maintain its is expected that monitoring of habitat natural functioning. Of particular importance inundation will refine this level in the future. are the frequency, season and duration of In addition, flood flow durations which are inundation as well as the drying or antecedence too long or too short should be avoided, such as period. All of these factors are important for at half or twice the natural flood flow. It is also triggering aquatic plants to grow or germinate. important that these high flow events continue Flows at a certain river height are important to occur at natural times, generally from late for the input of organic matter to the river and winter to late spring depending on the natural for maintaining habitat diversity. During floods, pattern for that river zone. river and floodplain are linked allowing the Low to medium flood flows have been transfer of carbon and nutrients from the significantly modified throughout the entire River floodplain to the river and vice versa. These Murray system and do not meet the 50% rule linkages also provide opportunities for (see Table 4.1). However, the effect of regulation macroinvertebrates to breed, water plants to on flow regimes varies considerably between river regrow and fish to move into previously zones. Whilst in areas upstream of major unavailable habitats including anabranches, diversions (e.g. upstream of Hume Dam and flood runners and the floodplain itself. Access to between Hume Dam and Yarrawonga Weir), the these habitats allows fish to utilise floodplain total quantity of water carried annually by the production (food) and breeding resources. In river has not significantly changed with particular, floods themselves may provide cues regulation, the main effect is one of altered for fish breeding and/or resources for successful distribution of flow in time. Although this results larvae survival (see section 3.2.5) as well as a in a range of changes to hydrological regimes on flourish of canopy growth in trees and the the floodplain, the specific consequences are opportunity for seedlings to establish. Where dependent on the level at which each part of the the occurrence of floods has been reduced, this floodplain is connected to the river. Areas can have significant effects on aquatic food inundated at/or below normal irrigation releases chains and fish recruitment. For example, the will be flooded for longer periods and with successful recruitment of Murray Cod appears to changed seasonality. For areas flooded at flows be greatest in years when significant flooding higher than normal irrigation releases, occurs. Some changes to the flow regime are modification of the frequency, duration and possible without major impact on current uses seasonality of flows will occur. The size of the and the Panel recommends that, for each extreme (1 in 100 year) flood event is likely to habitat type, the following principle should be have been reduced only slightly by river used as a general rule. regulation. However the expected frequency of all other flood flows has been diminished 3–4 fold The operational target for flooding should be (Table 4.1). This reduction in flooding frequency that any given flood should occur no less than has reduced the integration between the river and 50% (half) as frequently as under natural the floodplain and led to a consequent conditions (as determined from the annual flood degradation of the floodplain ecosystem. series) and that the duration of the remaining The Panel considers that there are three floods should be as close to natural as possible. possibilities for alleviating such problems: • conservation of existing flood events; This is a guideline to assess the severity of change in flood frequency. The basis for this • enhanced watering for some wetlands; and general rule is the experience and knowledge • conservation of a reduced floodplain. possessed by the Panel on Murray and Darling river floodplains and wetlands and is, therefore, The first two are discussed below, the third is an expert opinion. It is important to state that discussed in section 4.3.1 as it is also a habitat although the ‘50%’ rule may be met for conservation issue. particular habitats in some river zones it may not be entirely satisfactory from an ecological perspective (see Appendix 3, Section 4). In
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Conservation of Existing Flood Events Lake Victoria and Menindee Lakes. Modelling undertaken by the MDBC and the Department RECOMMENDATION G2 of Natural Resources and Environment (Victoria) suggests that under some To conserve flood events, abandon the current circumstances, this will have minimal impact on practice of pre-releases. other water users. Operating schedules can be developed which would enhance the watering The practice of making pre-releases from storages regimes of selected wetlands. as a routine flood mitigation measure should be discontinued. Pre-releases attenuate the flood RECOMMENDATION G4 hydrograph, thereby reducing the flood peak and lengthening the duration of the flow event. Hence, A review should be undertaken of the River there is a reduction in the frequency of flooding of Murray as a whole to identify opportunities to relatively high floodplain areas and an increase in increase the watering of targeted wetlands and the duration of flooding in areas that are watered the river zones that would most benefit from this. at below flood peak flows. The combined effect is detrimental to vegetation and macroinvertebrates. Pending this general review, the Panel is aware The Panel is aware that a review is being of specific work looking at the potential for undertaken of operations of major storages enhanced watering at specific locations. For where pre-releases are made, namely at Hume example, work is being undertaken in Victoria to Dam, Dartmouth Dam and Menindee Lakes, develop ways of enhancing the flow regimes of and it specifically recommends the cessation of key wetlands in the mid-River Murray area; and pre-releases which are designed to reduce in South Australia, operational opportunities and downstream flooding in the operation of these ecological consequences have been investigated. three storages. It should be noted that the Panel considers In addition, given that almost every river zone the benefits of this type of enhanced wetland has experienced a reduction in the frequency of watering will be greatest in the river zones less low to medium level flooding (Table 4.1) and that affected or altered by weir pools (upstream of this must have influenced some habitat type or the Murray-Darling confluence) as these can aspect of river functioning, the Panel formulated complicate wetland management in some cases. the following recommendation. Wetlands connected to weir pools are discussed in section 4.5.2. RECOMMENDATION G3 4.2 LINKAGES During the period June to September, when storages are filling, the MDBC should explore the The character of the River Murray is dependent option of passing a percentage (e.g. 10%) of the on ecological connections between the river and inflows, thereby ensuring that the hydrographic its floodplain (lateral linkages), and between characteristics (other than volume) of water flowing different parts of the river itself (longitudinal into storages are not altered and that releases reflect linkages) (see section 3.2.1). Under a natural flow the arrival and timing of these in-flowing waters. regime fish, for example, use the lateral linkages The intention of this recommendation is to to access three levels of the floodplain: anabranch introduce more flow variability by attempting to channels, flood runners and the floodplain itself. pass a percentage of inflows. Longitudinal linkages would have occurred at nearly all flow levels, with fish moving upstream Enhancing the Flood Regimes of Specific Areas and downstream except in extreme low flows of Floodplain during drought years, when the river would have been restricted to a few deep pools. Water held in storage can be used to enhance Maintaining these connections and their the water regimes of targeted wetlands by temporal character is essential for the health of releasing a relatively small quantity of water the river. Present ecological research is unable to timed to augment (raise) the peak of an existing quantify how many or which parts (if any) of flood event; it could also be used to extend a these connections can be sacrificed without natural event, if this were needed. This strategy major consequences. It is likely to be several is possible with water from Dartmouth Dam, years before river ecology is understood well
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enough to do this. In the meantime, the Panel This unseasonal flooding degrades these wetland has taken a precautionary approach of areas by causing tree death and invasion of conserving as many of these linkages as is cumbungi, by interfering with the natural feasible. However, it should be noted that while wetting/drying cycles and by disrupting nutrient some linkages are affected by water resource cycling processes. This issue is discussed in more development and regulation, many others are detail in section 8.2. threatened by land practices. Two management options that could address unseasonal wetland flooding are: 4.2.1 Unseasonal High Flows Providing • reduce summer flows to below the threshold Artificial Linkages likely to affect wetlands. The scope for this will have to be established on a case-by-case basis; and Unseasonally high river flows in summer alter the timing of the river channel and floodplain • build regulators on wetland inlets and linkages, as many wetlands, anabranches and exclude high summer flows. flood runners are connected to the river at these The use of regulators to exclude water from flow levels. Consequently, the natural drying wetlands and allow the re-establishment of phase of these areas during summer may be natural wetting/drying cycles is common in a reduced or eliminated, causing significant number of wetland areas associated with the ecological changes. Sustained high summer Murray. The Panel insists on caution in using flows, resulting from operating the river to meet this option. Regulators conserve some of the irrigation requirements and the South value of a wetland as habitat but effectively Australian entitlement, mean that in many destroys its natural linkages with the river. The sections wetlands are now permanently presence of a regulator interferes with the input inundated. Pressey (1986) categorised wetlands of organic material from the wetland into the affected in this way on the River Murray, river. Similarly, regulators prevent fish passage including those linked to weir pools, as between river and wetland, thereby reducing ‘hydrological category 1’. These account for the habitat value for fish (and other mobile 11% of all wetlands by number and an biota) of both anabranches and flood runners, impressive 35% by wetland area. However, this and the wetland itself, and compounds the is likely to be an underestimate of artificially problem of inadequate or broken lateral inundated wetlands as another category, linkages. There may be a need to examine ‘hydrological category 2’ must include some regulator design and management to ensure wetlands of concern here. The inundation of that these issues are accommodated. similar areas by weir pools is considered The Panel accepts that the use of regulators separately below in section 4.5.2. to prevent unseasonal flooding of wetlands may The problem of unnecessary linkage is be appropriate to conserve a few specific particularly evident in river zone 4, the wetlands. However, the full implications of Barmah-Millewa-Gulpa Forest area, with the building regulators should be examined. combination of the Barmah Choke area (where the river channel restricts the volume of flows RECOMMENDATION G5 that can pass downstream), high river flows and rain rejection events. Rain rejection events A set of ecological, engineering and hydrological occur when water is released for downstream guidelines for the use of regulators to exclude use from upstream storages just prior to rain in high summer flows from wetlands be the downstream catchments and so the release developed; the ecological criteria for developing becomes excess to downstream requirements. the guidelines should be based on an assessment Given that the river is run at bank-full capacity of the impact of altered linkages (two-way) for most of the summer, excess flows due to between floodplain and river, and local and rain rejections spill into wetlands where poor regional benefits or disbenefits. drainage results in unseasonal flooding. Rain rejection events exceeding the channel capacity 4.2.2 Barriers To Fish Passage of the River Murray in the Barmah Choke area have occurred 19 times between July 1986 and It is important for native fish to be able to move January 1996, and most of these occurred freely along the river throughout the year as between December and February (DLWC 1996). well as into anabranches, floodplain channels
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and onto the floodplain itself (see section 3.2.5). movements across the floodplain. Further, unless This movement can be obstructed by barriers to high flows over-top such structures, fish can fish passage. Such barriers can be both small become stranded as water levels recede. and large structures. Large dams, weirs, locks, There are many instances where regulators, levees and road crossings all act as congregations of fish occur immediately blockages. Continuity is an issue; providing downstream of barriers (Figure 4.6). These fish passage past one blockage is only successful up are extremely susceptible to fishing pressure to the next barrier. Barriers apply in both (both legal and illegal) and to predatory birds. A directions, upstream and downstream, as well as clear example of this is the outflow from Lake on to and off the floodplain. Victoria to the Rufus River. Such congregations Recent research has shown that although are also evident at other barriers, including there are peak times for fish migration, fish move those with fish passage devices. up and down the river in all seasons and out Two of the major structures on the River through anabranches and flood runner channels Murray have recently had fish passage devices onto the floodplains during floods. Murray Cod, installed on them; a fish ladder at Torrumbarry Trout Cod, Golden Perch and Silver Perch are all Weir and a fish lift at Yarrawonga Weir. The species that use anabranch channels during the original fish ladder at Torrumbarry (Figure 4.7) spawning season. Fish movements can include was shown to pass considerable numbers of fish, small and large migrations as well as the drift of including large numbers of native fish, notably larval fish. The capacity to move is crucial to fish Golden Perch and Silver Perch. Over 20,000 recolonisation after disturbance, breeding and native fish moved through the Torrumbarry recruitment of larvae. The presence of regulatory fishway in its first 2.5 years of operation structures along the river prevents such (Mallen-Cooper et al. 1996). Of particular movements and so is of major concern. Smaller interest were the large numbers of juvenile fish regulatory structures on flood runners and moving upstream. Their movements coincided effluents block fish movement through channels with changes (often small) in water levels. and onto the floodplain, while the presence of Reconstruction of the Torrumbarry Weir in 1997 other small structures such as culverts and levee provided an opportunity to install a new fish banks on the floodplain can constrain ladder, with monitoring of fish passage still to
Figure 4.6 Typical barrier to fish passage.
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Figure 4.7 Ladders assist migration of fish.
commence. The fish lift at Yarrawonga Weir also fully or partially open; and passes large numbers of fish. These particular • removal of some barriers. devices have been designed for native fish although, whilst considerable numbers of fish RECOMMENDATION G6 have been shown to have passed through them, their overall effectiveness (in terms of the The Panel endorses the aims of the MDBC Fish percentage success of those seeking passage and Management Plan (Lawrence 1991) and those achieving passage) has not been fully recommends the following: evaluated. Older fish ladders on locks in the • Year round fish passage suitable for all native lower Murray, such as Lock 6, have low fish species be provided throughout the river. effectiveness and need replacement if they are • A review of blockages on watercourses and to function as effective fish passage devices. In impediments to water movement across practice, most weirs provide no fish passage floodplains. except when overtopped or removed (which is • The introduction of planning controls on increasingly rare due to reduced flooding). floodplains to prevent and/or remove However, flows during overtopping may be blockages of watercourses and anabranches difficult to negotiate or even hazardous for small that change the movement of water across fish. The cost of installing fish passage is high and through floodplain systems. which means operational options must be • The effects of barriers on the downstream considered. movement of fish be investigated. The lack of continuously available fish passage through the weirs in the lower Murray 4.3 HABITAT section and the lower Darling River is a serious issue. It is not compensated for by over-topping The integrity of riverine ecosystems relies on the flows, which provide only limited and physical, chemical and ecological components of intermittent opportunities for movement. the ecosystems. The health of the river requires Possible options to deal with this are: that these are appropriately balanced and • installation of fish passage devices; ‘available’ when required. Habitat requirements • use of locks for fish passage by leaving them even for a single group, such as fish or plants,
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are complex. Understanding and catering for a Murray floodplain are now dedicated to uses suite of species is therefore particularly which permanently alienate the floodplain from challenging. It will never be possible or even the river in a functional ecological sense, but worthwhile to document habitats requirements wishes to emphasis that this only increases the for all species. At best, generalisations must be importance of the remaining floodplain areas made and constantly refined in light of better (see section 3.2.1). knowledge concerning the critical pathways and keystone species for river health. Habitats are RECOMMENDATION G7 dynamic, changing through time; they include the water itself, structural features (including Identification of floodplains or floodplain parts biota), and soils. These connect to the river at with high functional value for special protection. different times within a flow regime and when Development of an ecological guide and they connect, which may be only temporarily or ecological priorities amongst land management intermittently, then these become relevant to policy instruments likely to be used for all the river. In dry periods, this relevance is easy to remaining floodplains. overlook especially when the focus is on water That any future development on any part of management and on flow regimes. The Panel the floodplain be such that it does not further recognised that structural and spatial aspects of alienate any of the floodplain, that land uses are habitat, namely floodplain, riverbanks and snags compatible with ecological functioning of the require special attention in the River Murray. floodplain and that flooding and flowpaths are not further impacted. 4.3.1 Conservation of the Floodplain 4.3.2 Reduction of Snag Numbers The effective floodplain (on a whole river basis) is and will continue to be reduced in size and Snags and woody debris are the most important extent as there are areas where it is not possible structural habitat in lowland rivers. Snag to increase flooding frequency or restore flood removal has occurred on a large-scale in almost peaks. A significant loss of off-river habitat has all river zones of the River Murray but already occurred for freshwater fish. This places particularly downstream of Barmah. This a special value on those remaining floodplains represents a loss of critical habitat for aquatic to support and provide the necessary range of biota, such as fish and macroinvertebrates, as habitats, and organic and nutrient inputs. well as a substrate loss for biofilms. Within the Therefore, the condition or ‘quality’ in terms of river, snags add habitat diversity by river health of remaining floodplains has an accumulating debris and creating diverse water enhanced importance. It will be necessary to depths and velocities. Above water, snags are explicitly conserve the ‘river-important’ values habitats for other biota, both aquatic and of this reduced floodplain area to ensure the terrestrial, provide a perching place for birds, a healthy functioning of the river. feeding platform for water rats and a resting In river zones where the major part of the place for tortoises. They are important not only floodplain has been alienated, for example in the main channel of the River Murray but through land use or levees, such as the lower also in flood runners and anabranches, and Mitta Mitta River or drained areas in South during floods on the floodplain. Their Australia, the value of the remnant effective importance is only beginning to be floodplain areas must be recognised by local and acknowledged, belatedly, given the extent of the State Government planning authorities, desnagging programs of the past. particularly in land management. Examples of The degree of habitat loss is conveyed by activities focusing on floodplain functioning are early descriptions of snags as ‘standing up like controls on grazing and wood collection to soldiers’ or ‘those in the upper Murray as a conserve the organic debris on the floodplain, forest’. Systematic removal began in the 1850s and planning controls to prevent blockages to in the lower river zones of the River Murray for water flow and fish passage. Where whole paddle-steamer access and huge numbers were floodplain rehabilitation is not possible, the value eliminated. These numbers alone suggest almost of discreet functional units, such as anabranches 5 million snags have been removed, roughly and islands, should not be overlooked. equivalent to 2500 for each river kilometre. This The Panel accepts that parts of the River is an enormous loss of habitat.
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Excerpts from historical records of boats RECOMMENDATION G8 dedicated for snag removal show the enormous number removed for navigation purposes. The introduction of a policy to protect existing Examples of these excerpts (drawn from snag populations in terms of their number, size Treadwell and Koehn 1997) include: and position. This should cover alterations to and removal of snags and wood from the river, • The Melbourne removed 3000 trees in the anabranches, channels and floodplain. This first nine months of 1888 and removed policy should be assessed to determine the 300–400 snags per month, over a 49 year changes to snagging and resnagging over time. operating period. Nearly 1.8 million snags In key areas for native fish conservation and were removed. restoration, an appropriate density and • The Industry removed 3 million snags distribution of snags be reintroduced. Key areas, between 1911 and the late 1960s. methods and implementation issues should be • Two thousand tons of logs were removed determined by the MDBC Fish Management from against Torrumbarry Weir in 1939. Committee with funding to be provided through • A total of 24,500 snags were removed Murray-Darling 2001. between Albury and Yarrawonga as recently Revegetation and protection of riparian zones along the River Murray be made a as 1976 and 1987 for the MDBC. priority for funding under Murray-Darling 2001. As a general guideline and pending better information, the Panel considers that: 4.4 DISRUPTION OF METABOLIC FUNCTIONING An adequate density of snags is about 12 large snags from native trees (mostly river red gum) The Panel examined several water quality per 100 m of bank. aspects, in particular temperature and turbidity. As research proceeds, it is likely that the This figure was derived from two sites, Bruce’s essential habitat aspects of water quality will be Bend and downstream of Wakool Junction. Snag refined. For example, absence of knowledge and orientation and location within the channel is historical data meant the Panel was unable to important for their usefulness as habitat, consider water quality characteristics such as pH and alkalinity. Salinity was not specifically therefore the practice of dragging snags to the considered; the Panel recognises that long-term river edge and then realigning them with the increases will change river functioning. At flow may not meet habitat needs, for example in present it appears that salinity fluctuations are a providing velocity shelters. Similarly, lopping part of the natural variation in the water quality branches high in the water column reduces their of the river. Further, current salinity value as habitat for fish and some birds, and management rules appear to be protecting the prevents smaller woody debris from being river sufficiently, although monitoring is trapped. This trapping helps in the processing of ongoing. Given that the animals and plants are organic matter, by attracting macroinvertebrates, adapted to this variation, salinity is not and it provides a habitat for smaller fish. In considered to be a major ecological issue for the addition, the finer branches can reduce the river. However, it may be an issue for the ecology of some inundated wetlands (and is velocity of the water and its erosional capacity. certainly a major problem for human water use). Three steps and three timescales are necessary to rectify the loss of snags. These are: 4.4.1 Unseasonally Low Water Temperatures • protect existing snags; • reinstate snags in key areas; and Because water lower in storages is cooler and has a • revegetate and protect riparian zones to different water quality, releasing water from low provide snags in the long-term. level off-takes sends this bottom water downstream
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TABLE 4.2 Optimal spawning temperatures and times for major fish species in the Mitta Mitta River (from Koehn et al. 1995)
25°C
Golden Perch
20°C Murray Cod
Trout Cod Carp, Goldfish Macquarie Perch Blackfish 15°C Australian Smelt
Redfin
B-f Galaxias
10°C Brown, Rainbow Trout
Mountain Galaxias
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(see section 3.1.4). Temperature depression of species but also for their general metabolic several degrees can occur for a few hundred functioning and growth rates. It is also likely that kilometres as a result. This effect is particularly true the temperature reduction has influenced some for large deep storages, such as Dartmouth and species of aquatic macroinvertebrates, reducing Hume Dams, so river zones immediately their ability to complete life cycle stages during the downstream of these, such as zones 1 and 2, are summer months. most impacted (see Section 3.1.4). Lower Possible effects of the coldwater releases on temperatures have two types of effects. Firstly, the ecology of Hume Dam itself and the effects of metabolic rates and biochemical rates are reduced, cold water released into the Murray upstream of affecting all processes within the river, such as Hume Dam from the Snowy Mountains scheme photosynthesis, degradation, respiration and should also be investigated. Further, the effect of bacterial production. Consequently, there is an low level storages, such as the Menindee Lakes overall reduction in all forms of productivity. Scheme or Yarrawonga Weir, has not been Secondly, some fish species require temperature considered. There are suggestions that the lower thresholds to be reached to trigger spawning (see Darling River is periodically impacted in some Table 4.2). Lowering river temperatures is likely to way but this needs confirmation. favour introduced fish. This has already occurred in The only way to address temperature the Mitta Mitta River as a result of the construction depression in the River Murray is to ensure that and operation of Dartmouth Dam (Koehn et al. water is released at more natural (higher) 1995), with the loss of three warmwater native fish temperatures during the spring–summer season. – Murray Cod, Trout Cod and Macquarie Perch – This can be done by releasing water from the and a four-fold increase in Brown Trout, an epilimnion of the storages. As a general rule introduced coldwater species. Water temperatures during the summer months, this would be are vital not only for the successful breeding of fish within approximately 10 m of the surface.
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Options to rectify the problem of low construction of the Menindee Lakes in the 1960s, temperature releases, specifically from which extended the period during which Darling Dartmouth Dam are: water supplies the lower Murray, and Lake • build a new variable level off-take; Victoria in 1929, which can store turbid water • introduce stop-logs on the existing off-take from the Darling River and release it over an to raise its height by 31 m above current extended period. In recent years, River Murray height; flows have been higher and Murray water has • make the surface layer deeper using aerators; been stored in Lake Victoria. Consequently the and turbidity downstream was considerably lower. • introduce a siphon over the dam wall to Higher turbidity may decrease the risk of provide a mix of surface and sub-surface water. algal blooms (although this is uncertain for The effectiveness of destratification, sometimes buoyant cyanobacteria) but it also has seen as a fifth option, has been examined in detrimental impacts on other instream field trials at Dartmouth Dam (Welch 1984; productivity. Turbidity limits the growth of Croome and Welch 1988) where it was found to submersed plants, such as Vallisneria, and have little success. These studies may not be restricts the growth of benthic algae biofilms to conclusive as, at least in the second study, the a narrow subsurface range; both these are much destratification system was not switched on more vigorous and abundant during periods of until December after the reservoir was already lower turbidity corresponding to Murray water strongly stratified. Modelling studies (Ebsary (see section 3.2.3). Macroinvertebrate data 1990) indicate the thermocline of Dartmouth (Bennison et al. 1989) indicated that in the past Dam can be lowered considerably, however the the abundance of macroinvertebrates was predicted increase in water temperature was reduced by a factor of 10 compared with sites only 60C and the cost was very high (in excess upstream of Lake Victoria. However, in recent of $3000 per day in 1990). years, there has been a 2–3 fold increase of the An examination of the stop-logs option (see macroinvertebrate numbers. This correlated to Appendix 3, Section 1: 2) indicated that their reduced summer–autumn turbidities due to the use on the existing off-take would only be influx of low turbidity River Murray flows (see effective in 20% of months. The manipulation Figure 4.8). Abundance increases have been of stop-logs on the existing structure would be mainly in the crustaceans, especially the shrimp difficult when the outlet works were below the (Paratya australiensis) and prawns water and could involve significant recurrent (Macrobrachium australiense) (see Figure 4.9). costs. The most effective option is the Increased invertebrate numbers would provide a installation of variable level off-takes in both greater food supply for fish. However, the Dartmouth and Hume dams. Such an off-take is increased turbidities result in decreased visibility operated with great success on the Thomson and is likely to be detrimental to the visual River Dam in Victoria. feeding fish species, which form the majority of species in the river. Sedimentation can smother RECOMMENDATION G9 fish eggs and blanket habitats. It is recognised that under natural conditions Installation of variable level off-takes for prior to regulation, the Darling River would Dartmouth and Hume Dams be given serious have provided high turbidities during its consideration. A study should be commissioned flooding in early spring, but without Menindee to determine: costs and benefits, effectiveness, Lakes and Lake Victoria, periods of high which river reaches are most impacted, and turbidity would not have extended so long into which have best potential for recovery. the growing season nor into the summer and autumn as they do today. The only way to deal 4.4.2 Increased Turbidity During Summer with this increased turbidity is to increase the euphotic zone of the river in South Australia by In the lower River Murray, turbidity levels during introducing some variation in weir pool heights summer depend very much on whether the over the summer. water has come from the Darling River or the River Murray. Turbidities are higher when water is sourced from the Darling. Turbidity in the lower River Murray has also been affected by the
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Figure 4.8 River Murray macroinvertebrate numbers and turbidity at Murtho in South Australia
1400 400 Turbidity 1200 350 Macroinvertebrate Numbers 300 1000
250 800 200 600 150 Turbidity (NTU) 400 100
Numbers per Artificial Substrate 200 50
0 0 J A J O J A J O J A J O J A J O J A J O J A J O J A J O J A J O J A J O J A J O J A J O J A J O J A J O J A J 1980 1982 1984 1986 1988 1990 1992
Figure 4.9 Abundance of (a) the shrimp Paratya australiensis and (b) the prawn Macrobrachium australiense in the River Murray at Murtho South Australia
30
P.australiensis 25
20
15
10
Numbers per Artificial Substrate 5
0 J AJOJAJOJ AJOJAJOJAJOJAJOJ AJOJA JOJAJOJAJOJ AJOJAJO JAJ 1980 1982 1984 1986 1988 1990 1992
50
45 M.australiense
40
35
30
25
20
15
10 Numbers per Artificial Substrate 5
0 J AJOJAJOJ AJOJAJOJAJOJAJOJ AJOJA JOJAJOJAJOJ AJOJAJO JAJ 1980 1982 1984 1986 1988 1990 1992
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RECOMMENDATION G10 zones of the river, and distinguishing between the initial trigger(s) and the maintaining In years when Darling River water is the conditions, is an essential prerequisite for predominant supply to South Australia, weir developing appropriate solutions. pool heights should be manipulated over a range of 30 cm (i.e. the target river height RECOMMENDATION G11 ± 15 cm) in a month. An investigation into the triggers and sources of This will also have the benefit of introducing turbidity in the middle zones of the River some variation for the maintenance of bank Murray be undertaken, especially for the vegetation. Upstream of the Murray-Darling summer period. Attempts should be made to Junction, the evidence for increased turbidity in identify and analyse historic turbidity data sets. summer is less substantial although the public believe the river to be more turbid. The difficulty 4.5 EFFECTS OF WEIR POOLS of accepting observations in this case are that most observations relate to a particular period of Weir pools are features of the middle and lower time, whereas a long time frame is needed to river zones of the River Murray and the Darling establish trends as many catchment and water Great Anabranch. Their density is such that in the management issues affect sediment transport. lower River Murray, the stream becomes a series In the riverine environment, periods of high of continuous weir pools and has the flow now occur in summer and this would characteristics of a flowing river only in high compound the effect of turbidity. Higher flows. Where weir pools are close together within summer flows result in the river flowing deeper a river zone, or if a weir pool is particularly large and more strongly than in pre-regulation within a zone, then the whole ecological conditions, reducing the opportunity for character of the river zone is changed. Particularly particles to settle out of the water column. As a significant are changes to habitat, because in each result, the riverbed may receive less light, be weir pool a section of river is held behind a cooler and darker, and possibly remain anoxic retaining structure. This changes the predominant longer now than under pre-regulation characteristic of the river from being lotic (flowing conditions. Consequently, benthic processes may water) to being lentic (standing water). The be significantly affected. In particular, benthic retaining structure maintains high and stable algae productivity may be greatly reduced water levels within the channel above the during what would naturally be the peak structure rather than fluctuating flows as in the growing season. This would have cascading food natural situation. The result is that the river web impacts (see section 3.2.3). system has now become a highly modified habitat Resolving the question of the origins of with certain ecological characteristics and turbidity for the Riverine Tract in the middle consequences (see Table 4.3), discussed below.
TABLE 4.3 Ecological characteristics and consequences of weir pools considered under the three key principles of Ecological Variability (EV), Linkages (L) and Habitat (H)
ECOLOGICAL CHARACTERISTICS AND CONSEQUENCES EV L H Unseasonal and protracted wetting in low level wetlands x x Maintaining a head of water that raises groundwater levels x x under nearby floodplains Increased risk of algal blooms within the weir pool in summer x x Lack of drying in low level wetlands connected to the weir pool x x Sedimentation within weir pools x x Changing the character of the river to a series of linear pools x x x Loss of bank habitat due to permanent wetting x x Loss of in-stream structural heterogeneity x x Loss of primary productivity (plant and attached algae) due to x x reduced and static euphotic zone Bank instability downstream of the locks x x
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4.5.1 Constant Water Levels in Weir Pools stratification of the weir pool. These flow targets are based on observations at three river sites Weir pools, with constant water levels which over the past two years (Jones 1997). Physical often stratify, have an increased risk of algal modelling of the weir pool and river blooms, particularly in summer. A number of hydrodynamics may in the future lead to river zones were identified as being at high risk modifications to these interim flow targets. from algal blooms, particularly in the lower Murray. In this area, this risk is increased by the 4.5.2 Effects of Weir Pools on Connected pool/lake nature of the weir pools. Two options Wetlands for dealing with this are: • providing pulsed flows of 10,000 ML/day for The presence of a weir pool affects the wetlands 2–3 days every 10–12 days to mix the weir connected to it. Surface water connections pool; and mean these wetlands now have the same water • establishing a higher minimum baseflow regime as the river and so are inundated more over the summer months, when there is a frequently and for longer than natural. higher risk of algal blooms. Subsurface water connections mean that river Providing pulsed flows was modelled by the zones affected by weir pools develop shallow MDBC (Appendix 3, Section 1: 15, 16) for river groundwater tables in the adjacent floodplain. zone 4, for Euston to Wentworth and Barmah Although this appears to provide well-watered to Torrumbarry, but was not considered feasible growing conditions for woody riparian because of the delivery time between the vegetation, the consequences of the restricted storage and the downstream weir pool where root zone and saturated soils are that mature the pulse was needed. Therefore, the second trees are readily affected by wind, and option is considered to be more feasible. windthrow is common in well-watered anabranches and weir pools. There are also RECOMMENDATION G12 likely to be substantial changes to understorey communities (see section 3.2.2). For prevention of cyanobacterial problems in The Panel urges caution in managing the the Alert Level 3 range (> 15,000 cells/ mL for water regime (including the adoption of depth integrated samples), river flows should recommendations to enhance watering of not drop below 4000 ML/day for periods floodplain areas in section 4.1.3) of these exceeding one to two weeks during the period wetlands as it is unlikely that they require November to April for the weir pools along the further watering. An understanding of the River Murray between Torrumbarry Weir and influence of groundwater on these wetlands and Wellington that have been identified as high risk the implications of increased watering on for algal blooms. groundwater tables should be considered before For prevention of cyanobacterial problems in increasing the flooding regime. the Alert Level 2 range (2000–15,000 cells/mL for depth integrated samples), river flows should not RECOMMENDATION G14 drop below 8000 ML/day for periods exceeding one to two weeks during the period November to An investigation into the dynamics of the April for the weir pools along the River Murray groundwater table under the floodplain of the between Torrumbarry Weir and Wellington that lower River Murray, below Wellington, prior to have been identified as high risk for algal blooms. any enhanced wetland watering initiatives. The study must include both short-term and RECOMMENDATION G13 long-term responses to river flooding and weir pool manipulations in order to understand To reduce the risk of algal bloom in the Darling consequences, including the likely fate of the River and Great Anabranch, a minimum remnant woody vegetation on various parts of baseflow be provided during January, February the floodplain. and March of 500–1000 ML/day for periods of greater than one week. The Panel is particularly concerned about wetlands which are now permanently flooded The provision of these flows should reduce the due to weirs. Such wetlands and the high level risk of algal blooms by minimising persistent benches within the river, need to be drawn
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down periodically to restore their natural a single year and this contributed more than ecosystem function. This could also benefit bank 800,000 tonnes of sediment to the river. Abrupt and riparian vegetation. To do this would require bank failure is associated with rapid falls in manipulating weirs. Weir operations have water levels after weir reinstatement. Falls of up already been investigated in South Australia. to 2.5 m in four days are known to have Options to deal with weir pool connected occurred below some locks. wetlands are: Modelling undertaken by the MDBC • lower weir height to a level that would drain indicates that weirs do have a significant or at least draw down wetlands in late influence on the rate of water level fall winter/early spring for two (2) months; downstream of the main channel locks and • use locks to introduce some variability in weirs. This influence is demonstrated in river height by leaving them open. This Figure 4.10 which shows the modelled impact could have fish passage benefits; of the weir at Lock 6 on the recession of the • remove weir gates or controls for 6–8 weeks 1981 flood. The weir prevented the river prior to a flood and reinstate after flood. A channel from slowly draining as the flood trial at Lock 8 is suggested; receded, making the recession steeper than it • delay weir reinstatement and then fill weir would otherwise have been. This effect is pool slowly. This would reduce the likelihood cumulative and becomes more significant as the of excessive downstream drawdown and flood recession moves downstream. The flood subsequent bank erosion; and recession can be made steeper still if weir pool • reinstate a section of river by permanently level is not maintained as the weir is reinstated. removing a weir; Lock 8 is suggested as it Figure 4.11 demonstrates the impact on would have less impact on users than any downstream levels of refilling a weir pool that other weir. has fallen below its desired level. Sudden refilling can result in sharp decreases in the flow RECOMMENDATION G15 downstream and, correspondingly, sharp falls in water level. This impact can be reduced if the In the immediate future, weir pools be drawn down rate of refilling the weir pool is slowed. to the lowest possible level for two months in late winter/early spring to provide the opportunity for RECOMMENDATION G16 drying of adjacent floodplain wetlands and high level benches of the main channel. If the level upstream of a weir falls below pool Removing weir gates for 6–8 weeks prior to a level during weir reinstatement, the pool should flood and reinstall after flood as an experiment not be refilled immediately but should be filled to demonstrate the benefits of reintroducing gradually over the next week to reduce rapid wetting–drying cycles, and as an opportunity to falls in water level downstream. document costs and benefits relating to various MDBC strategies. Reviewing the function and utility of all weirs in the lower River Murray and lower Darling River using a cost–benefit approach, with a view to the complete removal of some weirs.
4.5.3 Bank Instability Downstream of Locks
In some areas, but especially the lower Murray Tract, severe bank erosion occurs downstream of weirs or locks. The primary bank erosion mechanism is abrupt bank failure, which results in large quantities of eroded sediments entering the river in a short time. Over 35% of the total bank length between Locks 3 and 4 is actively eroding and 75% of this occurs within the 30–40 km reach downstream of each lock. More than 2–3 m of bank retreat has been recorded in
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Figure 4.10 Impact of Lock 6 weir on the recession of the 1981 flood
20.0
19.5 With Lock 6 Weir Without Lock 6 Weir 19.0
18.5
18.0
17.5
17.0 Level downstream of Lock 6
16.5
16.0 1/11/81 6/11/81 11/1/81 16/11/81 16/11/81 16/11/81
Figure 4.11 Impact of refilling Lock 6 weir pool, both at and below its desired level, on the recession of the 1981 flood
20.0
19.5
19.0
18.5
18.0
17.5
17.0 Level if Weir reinstated at Pool Level Level downstream of Lock 6 16.5 Level if Pool missed and susequently refilled Corresponding upstream Water Level 16.0 1/11/81 6/11/81 11/1/81 16/11/81 21/11/81 26/11/81
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5 – REFLECTIONS ON THE PROCESS
Some of the present woes of the The macro view is imperative if long-term river function is to be maintained. There are areas and Murray-Darling system arise from the reaches within the River Murray system which fact that it straddles four States. Past make an important, distinctive and irreplaceable contribution to the entire river’s health. These need independence of action – a valuable to be formally recognised and their functions fillip to progress under some protected (analogous to protecting aquifer recharge zones). If not protected, then, by an attritional circumstances – has resulted in process known as ‘Tragedy of the Commons’, the intrastate resource demands taking very functions of these key areas are in danger of being gradually eroded and possibly lost forever. At high precedence over considerations of present such areas are known only through the system as a whole. regional studies and not through whole river studies, so an important step is to confirm such The situation is exacerbated by the fact that the findings. Two such areas requiring further study are: ‘scientific community’ has been slow to understand the ecological dynamics which support the • The Great River Red Gum Forests of the functioning of the river as a living ecological system Middle Murray. By their location and natural and thereby the natural forces essential to sustain flooding patterns, Pericoota, Werai and Gunbower its quality as a human resource. (Even now we are forests as well as Barmah-Millewa Forest probably only beginning to grasp the all-pervading have similar and combined roles that sustain the significance of high variability to our natural river in the middle reaches and further systems (‘The Australian Factor’) Although lack of downstream. Only some of these roles have been ecological knowledge remains a limiting factor in studied. Roles already known for the Barmah- developing ‘indefinitely’ sustainable resource Millewa Forest and hence likely to apply to other management practices, as a community we forested regions are: understand enough to be sure that management of • breeding sites for fish, waterbirds and other the system must acknowledge the interdependence species; of its parts and that short-term, small-scale gains • dissolved and particulate carbon export; and may be fatal to the system as a whole. Throughout • sedimentation traps. this Scientific Panel process, the Panel attempted to step back and identify bigger issues at the whole • The Lignum Swamps in the Lower river scale and areas where current knowledge was Wakool and Edward Rivers. The ecology of limiting. These are discussed below. these areas is little known and appreciated but, undoubtedly, when flooded they function as highly 5.1 MACRO VIEW OF THE RIVER productive shallow ephemeral wetlands, favoured by many waterbirds, colonial nesters and waders, The approach used in this Scientific Panel study and are symptomatic of the ‘Boom and Bust’ pulse was to consider river functioning initially through of healthy inland rivers. These areas fall outside the site inspections then by reach. Similar reaches were normal research scheme because they are flooded then combined to give large river sections (referred by medium to high level flood events and so their to as river zones), based not just on their bio-physical condition and status is not well understood. Threats attributes but influenced by present condition to such areas are clearing; retaining their function and dominant water management style. The relies on maintaining the original vegetation cover interdependence of these river zones must be quasi-intact over great areas. emphasised and although the current reductionist approach has helped in analysing urgent issues, The Panel’s three principles for ecosystem health such issues cannot be solved in isolation. Any (see section 2.1.1) emphasise habitat diversity, change in management must be assessed in terms river–floodplain linkages and metabolic of its effects both upstream and downstream. This functioning. These can be applied to the whole process of persistently stepping back to consider the river. When taking a macro view of the river, river and its floodplains at an ever-increasing scale longitudinal processes must also be considered and is essential. At the moment, the knowledge to do the principle of connectivity must be included. this effectively at a whole river scale is lacking, yet This immediately highlights that there are two this is essential. major disconnections along the River Murray:
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1. Isolation of upper from middle River Murray. surprisingly difficult to transfer technical Hume Dam has effectively isolated the information across disciplinary boundaries. This middle and lower reaches of the Murray situation raises important issues. from the upper reaches. The main issue is the need to express 2. Isolation of lower Darling from the middle hydrological information in ecologically and upper reaches. The Menindee Lakes significant ways. Hydrology, relative to other storage scheme has been in operation since disciplines, is data-rich. Data are usually 1960 and in that time the lower Darling has summarised into forms including flow duration been re-connected with the middle and curves and return frequency graphs to provide upper river only once. useful comparative information for other Consequences of disconnection are many. hydrologists. However, as with all summary Species accustomed to using the river as a techniques, information is lost and much of this highway, up or down, can no longer do so. For lost information may be of particular ecological the many species that make longitudinal significance. Questions of variability, seasonality, migrations for breeding purposes, this means precedence (pattern over extended time scales) that population renewal is being curtailed. and rate of change are all likely to have an Similarly, isolation by man-made structures can impact on the riverine ecosystem but are largely isolate populations. Although this is not likely to lost or ill-expressed in current hydrological result in chronic in-breeding it may carry some summaries. Value would be gained by other penalties. Finally, disconnections are often assembling a small team which included associated with changes in water management, appropriate hydrological, ecological, and such as above and below a major weir system statistical expertise to develop methods that like Menindee Lakes. This results in adjacent produce ecologically orientated summaries of river reaches acquiring distinctive ecological existing hydrological data. characters, as different water management styles As a corollary of this requirement, is the need are imposed onto adjacent and otherwise similar to identify the changes to hydrology brought reaches. A suitable goal for long-term about by flow management and to produce management would be to minimise these generalisable descriptions of these changes. Such ecological jumps across disconnections, by descriptions can be aimed at estimating the minimising their ecological differences. degree of change relative to the ‘pre-regulation’ state and thereby provide a means of ranking or 5.2 KNOWLEDGE GAPS AND FURTHER comparing various sites within and between RESEARCH river systems. This approach would provide an index of modification. Alternatively, the aim The Panel is aware of two current consultancies might be to emphasise the various types of which, among other things, are identifying hydrological change resulting from flow knowledge gaps in relation to environmental management by grouping and classifying sites flows in the Murray-Darling Basin. There is no using multivariate techniques. intention to duplicate that effort here by Whether indexes or classifications are attempting another exhaustive evaluation. produced it would be desirable to base new Rather, the intention is to add a few developments, as far as practicable, on purely observations arising directly from the current hydrological data (at least non-biotic) so that study. Many of the research needs identified they could be used as ‘independent variables’ in arise directly from the recommendations made measuring the effect of management-induced in this report. These recommendations were change in aquatic ecosystems. The development made based on both first principles (some of of ecologically relevant hydrological variables, which lack specific data and/or application related to management changes, would have within Australia) and the Panel’s experience. the additional benefit of providing the opportunity for ‘management experiments’. 5.2.1 Linking Hydrology and Ecology Current management activities throughout the Murray-Darling Basin provide a potential Even with the goodwill and mutual respect spectrum of hydrological ‘classes’ which could between Panel members, which prevailed form the basis of comparative ecological studies. without exception throughout this study and in This type of work should form an important the preparation of the report, it has been part of an adaptive management strategy.
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Difficulties in linking ecosystems to and should form part of the development of hydrological change result from lack of data multi-purpose management plans which rather than problems of interpretation. For incorporate downstream resource security and example, what is the significance of unseasonal environmental and other issues upstream and flows for ecology in arid variable flow regime downstream of the Menindee Lakes. environments or the significance of large floods for fish? Linking biological data and ecosystem 5.2.3 The Role of Temperature in the function to hydrological variables in the field Functioning of the Instream Environment presents substantial difficulties of scale and other confounding factors. Progress in this area Release of bottom water and the consequent requires either large-scale research or effect of lowering water temperatures in the well-designed experimentation (or a receiving river is not just an issue associated combination) to produce generalisable with the River Murray but with all rivers information. This is a priority area of research. affected by large storages. The temperature Lack of this information will, for some time, depression recorded in the Mitta Mitta River is remain the limiting factor in maximising the substantial and, according to similar studies ecological benefit from ‘environmental’ flows. done on the Macquarie River, may extend downstream quite considerably. 5.2.2 Management of Menindee Lakes Based on first principles of temperature and Lake Victoria determining the rate of reactions, a 100C drop during the most active biological season should Both Lake Victoria and the Menindee Lakes reduce productivity and limit metabolic have the problem of primarily supplying water processes. However, the effect is unlikely to be across State boundaries (see section 11.1). They uniform as species vary in their temperature are also broad shallow basins in hot dry areas responses. Habitat heterogeneity and diversity with the concomitant risk of high evaporation also mean that species exposure to low losses – a problem they share with many large temperatures will vary, and there may be privately owned off-river storages. temperature refuges. Suggested studies on the Cultural heritage issues at Lake Victoria effect of temperature are: currently over-ride other management • growth of ribbon weed Vallisneria americana, considerations and prevent planning for its use the most widespread submersed river plant (in addition to storage) as a buffer to create in the River Murray; ecologically sound flow patterns. For example, Lake Victoria could be used to extend the • an assessment of how temperature duration of floods in the lower River Murray. In depression has shifted spawning contrast, Menindee Lakes present an opportunity opportunities away from favouring native to develop multi-purpose management regimes fish to favouring introduced species; which could fulfil/enhance a number of • carbon transformations in the river outcomes including water supply and sediments, the role of priming and general environmental goals. Unfortunately the rate questions; and Menindee Lakes fall at the boundary between this study and an earlier study of the • temperature as a determining factor for algal Barwon/Darling by a different Scientific Panel growth and control in species composition. (Thoms et al. 1996) and consequently were not investigated by either Panel. The fact that the 5.2.4 Drying Wetlands Connected to Lakes represent a large evaporative surface Weir Pools provides a significant resource conservation benefit for allowing them to drain, at least when The Panel suggests a trial experiment as a downstream supply security is not threatened. means of determining both the operational Preliminary research also indicates that dry feasibility and ecological benefits of drawing periods (when seasonally appropriate) may down water levels in weir pools on a regular enhance the productivity of the Menindee Lakes, versus irregular or not at all basis prior to spring support native fish recruitment, and possibly floods. Ideally there should be multiple sites, depress populations of European Carp. These chosen so as to cover different river reaches and implications need to be investigated rigorously to address specific questions that might
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influence recovery, including for example time regeneration strategies of floodplain species since installation, draw down history, general (i.e. the relative importance of vegetative versus climate, operational timing. Possible study sites sexual reproduction) and the conditions that are Lock 7, as an example weir in the lower trigger either strategy and that determine their Murray and as a navigational weir, and success, are essential. Work on wetland species Pooncarie as an example of a low-level is already underway. In addition, knowledge of non-navigational weir. It would be important to species ecology is woefully defective in key make this a multi-disciplinary study so as to link areas. For example, are Vallisneria and ecology with water security, benthic chemical Potamogeton crispus setting viable seeds? What processes, water quality and economic costs, are their germination requirements? What is and hence gain scientific community support known of seed viability? These types of research and public confidence. Similarly, wetland and floodplain draining and recovery experiments questions need to be addressed. that investigate microbiology, carbon turnover, etc should be undertaken. 5.2.7 Biodiversity Conservation
5.2.5 Optimisation of Environmental Water The reasons for seeking beneficial conservation outcomes are the same as those justifying Throughout this report, the Panel consistently conservation in general, namely the suggests and recommends increasing flood peak conservation of biodiversity and the retention of height rather than extending flood duration. This all ecosystems for future generations. It is clear is based on the premise that flood frequency that general resource management has resulted in species extinctions locally, and probably needs to be increased and that areas of floodplains completely, and that flow management is need to be rewatered more frequently. Further, it causing major changes in the distribution of assumes that current durations are adequate to floodplain communities (Hillman and Bren water the floodplain and to recharge soils. In fact, 1996). The extent to which these changes will these assumptions are based on best-informed proceed and their significance to conservation guesses, and need to be investigated and (i.e. threat of extinction) is far less clear. This confirmed or substantially rejected. urgently requires investigation. In the interim, Research is needed into the relative merits of local actions at the scale of the individual favouring certain components of the flow system (e.g. regulators on individual wetlands, regime over others; specifically flood peak flood management in Barmah-Millewa) are versus duration. An optimisation or modelling important, although details of such actions are study is needed to investigate the long-term mostly outside the brief of the River Murray effects on woody vegetation and floodplain Scientific Panel. The Panel identified the watertables of an environmental water policy following specific research areas that influence favouring peak flow over duration and of any biodiversity conservation: flow–peak combinations. The study should • monitoring fish passage; include at least two floodplain models, such as • investigating fish disease outbreaks from Chowilla and Barmah, in order to discriminate Lake Victoria; between species effects, subsurface conditions • determining adequate baseflows for the River and broad-scale climatic and local Murray to protect fish, macrophytes and geomorphologic effects. Trees are suggested as habitat; and the key in this study but they should be assessed • ascertaining appropriate snag densities. not just for their intrinsic merit but as significant structural (habitat quality) features; 5.3 POTENTIAL ISSUES FOR FUTURE prediction and long forecasting are needed. MANAGEMENT
5.2.6 Regeneration Strategies 5.3.1 Migration of Abstraction
Autecological studies are lacking for the Trading of licences under the cap agreement dominant and key floodplain plant species in means that the current pattern of abstraction the Murray-Darling Basin. Studies into the throughout the Murray-Darling Basin could
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change without changes in the total amount of 5.3.4 Redistribution of Recovered Water abstraction. A net movement of abstraction upstream results in reduced flows over a greater A number of proposed water management length of the river for the same amount of water changes would, if implemented, lead to use. This may be compensated for by increased increased discharges downstream. For efficiencies (lower evaporation) but this would example, regional river flow increases may need to be accompanied by a concomitant result from regulating wetland inflows, water reduction in allocation. If downstream policy development, or structural changes, movement of allocations was desired, it could be such as the bypass channels at Barmah Choke. possible to offer incentives to achieve this. Historically, increased quantities of water available in any reach have resulted in rapid 5.3.2 Relative Ecological Significance of increases in local utilisation. In some instances, Demand Changes such as efforts to protect Barmah Forest by diverting more water through the Edward Most of this report has dealt with the effects of River, such changes may have increased total changes to water management (mostly abstraction. Likewise, increases to the capacity regarding the operation of regulating structures) of Lake Mulwala, initially as a means of on downstream ecological conditions. However, managing rain rejection flows, have gradually as previously noted (e.g. see section 7.2), the been allocated for consumptive use. This has problems at any site are often driven by lead to increased abstraction and a return of downstream demand. Assessing the relative the problem of managing rain rejection flows. ecological effects of changes in demand (±) at A reasonable outcome of the capping various points in the Murray-Darling Basin may process is the development of policy which provide valuable insights. Such a study/model ensures appropriate redistribution of flow might be valuable in ‘fine-tuning’ arrangements efficiencies and protects water gains resulting for licence transfer, improvements in water from modifications to produce environmental delivery efficiency, etc. It might also help benefits (for example, closing off large address questions such as ‘If we achieve a 10% wetlands during summer flows). Clear policy reduction in abstraction, is the resultant credit protecting all uses, including environmental better ‘spent’ in reducing damaging irrigation allocations, is an essential pre-requisite to flows upstream or increasing the amount of supporting specific radical changes which water passed downstream?’ would increase flexibility in the system rather than simply resulting in additional abstraction 5.3.3 Release and Recapture of and the exacerbation of current problems. The Environmental Flows possibility of developing a channel to bypass the Barmah Choke is a case in point. It is It appears likely that released pulses of water for likely that the operation of such a bypass, environmental purposes (e.g. releasing part of the without jeopardising irrigation allocations, Barmah-Millewa Forest environmental water could introduce flexibility which would allow allocation in large volume pulses to extend for increased flow variability both upstream and natural flooding) from upstream storages could be downstream of the Choke, management of recaptured in downstream storages (possibly inundation at Barmah-Millewa Forest, and previously lowered for the purpose). If the delivery of enhanced environmental flows downstream storages are off-stream, downstream. Absorption of this flexibility in environmental costs would be less. However, it gradually increased allocation downstream should be noted that such pulses may cost the (the historical trend) is not only unacceptable user as movement of water onto the floodplain is, but will result in an environmental disbenefit. in part, a consumptive use. Over-bank flows Further, the establishment of appropriate increase the ratio of surface area to depth and policy will permit the exploration of a wider therefore the potential for evaporation and range of management alternatives movement of water over distant dry areas is likely (e.g. Barmah by-pass and identification of to result in significant groundwater recharge. sacrifice zones).
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PART II – RIVER ZONE ASSESSMENT
Recommendations for issues common to a transitional rivers (Schumm 1988). It has been the subject of comprehensive environmental number of river zones (discussed in studies relating to the impacts of the Chapter 4) form the basis for the construction and operation of Dartmouth Dam (e.g. Doeg 1984; Koehn et al. 1995). Whilst individual river zone recommendations many of these studies investigated the effects on outlined in this chapter. For each river zone the Mitta Mitta River, pre-dam surveys also included terrestrial flora and fauna throughout the following information is provided: the entire Mitta Mitta River valley. • a description of current hydrologic management practices; 6.1 HYDROLOGIC MANAGEMENT
• key environmental values; The river zone is dominated by the operation of • major impacting processes; the Dartmouth Dam which was commissioned in 1979 (see Figure 6.1). The dam is the largest • priority issues; water storage in the Murray-Darling Basin, with • recommendations for future management; and a capacity of 3906 GL, contained by a 180 m high wall, the highest in Australia. Dartmouth • a prediction of the environmental benefits Dam is also the deepest reservoir in Australia that should occur as a result of with a maximum depth of 161 m and an implementation of the recommendations. average depth of 60 m. The dam has a total 6 – ZONE 1 – MITTA MITTA surface area of 65 km2 and extends some 40 km up the Mitta Mitta River valley. Mostly, water is RIVER – DARTMOUTH DAM released through the hydro-electric power TO HUME DAM station which draws water from an offtake that is 62 m below full supply level. However, The Mitta Mitta River between Dartmouth Dam releases are also made from the lower off-take and Hume Dam consists of a 100 km reach (121 m below full supply level) when the which is characteristic of both upland and storage is low. The dam also has a spillway for Figure 6.1 Map of Zone 1: Mitta Mitta River between Dartmouth Dam and Hume Dam
ALBURY Thurgoona
Bethanga
WODONGA Bonegilla
Tangambalanga Tallangatta Yackandandah
Eskdale Dartmouth
N
02030km10
Base map copyright AUSLIG Map compiled by GIS Unit, MDBC
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uncontrolled overflows. Such overflows occurred the knowledge that the inflows to Hume and in July and November 1990, October and Dartmouth dams are highly correlated. November 1992, October 1993 and October to Operators are confident that a substantial November 1996. Downstream of the dam there proportion of the total airspace in both dams is a 4.5 GL re-regulating weir giving operators of can be held in Dartmouth Dam without running the power station limited flexibility to generate the risk of Hume Dam spilling when Dartmouth power at times of peak demand without causing Dam is not full. Dartmouth Dam can therefore large fluctuations in the Mitta Mitta River flow. be drawn down to the airspace target at any The main purpose of the dam is to augment time without running the risk of losing water water supplies to the River Murray system. In resource. The adoption of these targets has dry years, water is transferred from Dartmouth enabled earlier releases to be made at lower Dam to Hume Dam, where it is subsequently release rates and has given operators greater released to meet irrigation demand. Because the flexibility to satisfy other requirements. channel capacity of the Mitta Mitta River Operators of the power station have the (10,000 ML/day) is much less than the right to release water to meet electricity maximum release from Hume Dam demands when the storage gets close to full. In (25,000 ML/day), releases from Dartmouth Dam addition, the Murray-Darling Basin have to be commenced early in the irrigation Commission has a pre-release policy for flood season. This is done on the basis of a forecast to mitigation in the Mitta Mitta River valley. This the end of May which assumes minimum policy involves pre-releasing any water that inflows and peak demands. The extra water that the Commission is certain would otherwise is forecast to be required in Hume Dam is spill before the end of the season. scheduled to be released from Dartmouth Dam Because of the flood routing characteristics during the irrigation season. In the past, the of Dartmouth Dam, which are determined by release was delayed for as long as possible and the hydraulic characteristics of the spillway and then released at channel capacity to minimise the incremental capacity of the dam above the the possibility of making an unnecessary release. spillway crest level, floods passing through a full However, releasing earlier at lower rates has storage are significantly modified. Flood peaks recreational benefits in Hume Dam as well as are significantly lowered and the time to peak is environmental benefits in the Mitta Mitta River. extended in comparison with passage of the For these reasons, airspace target operation was same flood without the presence of Dartmouth adopted for Hume and Dartmouth Dams in Dam. Filling of the dam in winter and spring, in 1993. Airspace refers to the available ‘room’ in a many years, results in a reduction in the storage for inflows to be stored. intensity and duration of downstream flooding. Airspace targets have been developed using Some idea of the impact of the operation of Dartmouth Dam on the annual monthly pattern
FIGURE 6.2 Comparison of natural and current regulated flows in the Mitta Mitta River below Dartmouth Dam (supplied by MDBC)
Median Monthly Flows Distribution of Annual Flows Mitta Mitta River at Colemans Mitta Mitta River at Colemans 160.0 5000 Current Natural Current Natural 140.0 4500 4000 120.0 3500 100.0 3000
800.0 2500
60.0 2000 Flow (GL/month) Flow (GL/year) 1500 40.0 1000 20.0 500
0.0 0 J F M AJJASODM N 0% 20% 40% 60% 80% 100% % of Years Flow is Less Than Value
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Figure 6.3 Change in monthly flow in the Mitta Mitta River downstream of Dartmouth Dam
20th Percentile 9000 8000 Current Conditions 7000 6000 Natural Conditions 5000 4000 3000 2000 Flow ML/day 1000 0 Jul Oct Jan Feb Sep Dec Jun Apr Nov Mar Aug May Month
50th Percentile 9000 8000 Current Conditions 7000 Natural Conditions 6000 5000 4000 3000 2000 Flow ML/day 1000 0 Jul Oct Jan Feb Sep Dec Jun Apr Nov Mar Aug May Month
80th Percentile 9000 8000 Current Conditions 7000 Natural Conditions 6000 5000 4000 3000 2000 Flow ML/day 1000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
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of flows in the Mitta Mitta River can be gained • overbank flooding, when it occurs, is lower from the diagrams in Figure 6.2. and lasts longer due to the attenuating A minimum release from Dartmouth Dam in influence of the dam spillway; and the Mitta Mitta River is maintained at times • water temperatures in summer are cold when transfers to Hume Dam are not required. and variable. These riparian releases vary from 200 ML/day when Dartmouth Dam is less than 60% full to 6.2 ENVIRONMENTAL CONDITION 500 ML/day when it is greater than 80% full. In addition to these rules, there are constraints on There are two distinct riverine sections between the rates of rise and fall downstream of the re-regulating weir and an embargo on releases Dartmouth and Hume dams. The first distinct from the power station when the flow at river section, between Dartmouth and Tallandoon is greater than 10,000 ML/day. approximately 10 km downstream of the Dartmouth Dam has had a significant impact Colemans gauging station, is contained within a on water temperature in the Mitta Mitta River. narrow gorge (valley trough) up to 800 m wide in places. Here the river channel has: The dam has been shown to stratify, with the thermocline generally occurring at depths of • very little alluvial storage within the narrow between 5 and 10 m below the surface from valley floor trough and hence limited October to January, and up to 40 m in floodplain development; August–September (Ebsary 1990). Because the • high stream energies and turbulent first offtake is 62 m below the full supply level, in-channel conditions throughout the section; releases generally occur from below the thermocline, especially during spring and summer • a channel morphology influenced by bedrock (see section 3.1.4 for more details). The outcome constraints and thus the characteristic feature is a significant truncation of the ‘natural’ of this section is relatively narrow turbulent temperature range. The summer temperature ‘runs’ separated by large pools; and only briefly rises above 16˚C, and rarely above • a river bed dominated by bedrock ledges and 20˚C; temperatures which are critical for the cobble, pebble and gravel sized sediment. spawning of warmwater native fish species (see section 4.4.1). These severely reduced water The river system within this section (Dartmouth temperatures occur during the spring and Dam and Colemans gauging station) has vigorous riparian vegetation and abundant summer period, which is the spawning and in-channel and near channel habitat giving the rearing time for most of the native fish species. As outward appearance of relatively good health. flows are high due to irrigation demands, there is However, the macroinvertebrate fauna is low in also little chance for water to warm in the abundance and variety, suggesting poor regulating pondage as residence time is low. instream environmental condition. Another consequence of Dartmouth Dam is that In the second riverine section, downstream of the temperature of the water being discharged Colemans gauging station to Hume Dam, the from the regulating pond can vary dramatically river channel and floodplain are contained over short time periods, see Figure 3.8. within a broader valley floor trough, up to Dartmouth Dam has had a substantial impact several kilometres wide. Stream energies are on the flow regime downstream (Figure 6.3). In relatively lower than upstream of the Colemans this river zone, compared with natural conditions: site and hence the in-channel environment is • there are more prolonged periods of low flow; less turbulent. In-channel environments similar to this section are usually dominated by • there is much less variation in flow from day riffle/pool sequences and point bar systems with to day, with most short duration flows riverbed sediments dominated by gravels. The resulting from rainfall events now harvested; characteristic meandering pattern of the river • the flow regime has a different seasonal channel has produced a complex mosaic of pattern, with more summer and autumn floodplain features, some of which contain large flows and less winter–spring floods; billabongs and backwater areas. The condition of the river in this section is considered to be poor. • the frequency of overbank flooding is Instream environment. Instream habitats, greatly reduced; particularly downstream of the Colemans site, are
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in very poor condition. There is evidence of populations have essentially been from channel degradation throughout this river zone, ‘warmwater’ fauna to ‘coldwater’ fauna. The with the exposure of once buried bridge piers major influencing factor appears to be the and supports and active bank erosion. Riffle/pool releases of cold water from the lower offtakes sequences are not well developed and the over the spring and summer months. The riverbed is relatively flat and uniform. The aquatic macroinvertebrate fauna has also riverbed surface is armoured and imbricated by changed and can now be described as a relatively gravels although beneath this layer the gravel depauperate fauna which is characterised by a sediments are choked with finer sands. coldwater, upland mayfly species. After 20 years Armouring of gravel bed rivers is typical of dam operation, the fauna remains downstream from dams where sustained periods depauperate, of similar composition to previous of high flows occur. Also, Doeg (1984) has studies and with dominant taxa characteristic of documented that the river was subjected to high disturbed ecosystems. This suggests that without levels of sedimentation during and immediately intervention to restore conditions within the following the construction of the dam. This may river, there is likely to be no improvement in the have resulted in the infilling of pools. In addition, fauna in the future (Koehn et al. 1995). the river has been extensively desnagged, further Water quality. The main water quality reducing the amount of instream habitat. problem is the release of cold water from low Riverbanks. There are signs of active bank level offtakes. This has been the main cause of erosion, which appear to be a result of sustained detrimental ecological effects. The release of periods of high flow. Since 1973 there have anoxic bottom waters during spring and been considerable river management works summer is expected to greatly increase the conducted on the river upstream of Colemans downstream load of ammonia and gauging station by the Mitta Mitta River orthophosphate during this time. Under high Improvement Trust: rock beaching of eroding discharges, the released water contains a banks, planting of willows, lopping of mature mixture of nutrient-rich bottom water and willows, planting of native trees on river banks nutrient-depleted surface water. Under low and snag removal (Koehn et al. 1995). discharges the thermocline tends to compress Riparian zone. Riparian vegetation has been the withdrawal envelope from above, therefore virtually cleared via grazing along the entire length proportionately more nutrient-rich water would of the lower riverine section of the river. The be released. The increased nutrient loads from remaining trees do not appear in good condition the anoxic hypolimnion also can lead to and in many cases, native trees have been replaced increased algal growth downstream, particularly by willows. Bank side vegetation is depauperate. of epiphytic or benthic algae. Floodplain. Flow regulation has dramatically reduced river channel–floodplain linkages. In 6.3 THREATENING PROCESSES many cases, these have been blocked, thus reducing the exchange of organic material. The Changes in the natural flow regime and floodplain is heavily grazed and, in some areas, hypolimnion releases from Dartmouth Dam are drained. Irrigation and laser graded pasture have responsible for much of the poor environmental increased on the floodplain. This, together with condition observed in the lower section of this introduced plant species, has greatly modified the river zone. These changes include: floodplain flora and probably reduced the productivity of these systems. Flood frequency and • constant flow levels causing bank erosion, duration have been considerably reduced. The changes in bed morphology and major issue here is the need to reinstate flooding consequent reduction in available instream to inundate some of the flood runners. Further, habitat; selected areas require regeneration, revegetation • unseasonal high flows in summer and and protection from cattle access. As a general autumn within the channel, mainly affecting rule, development of off-river (and off-billabong) fish and macroinvertebrates; stock watering should be actively encouraged. • reduction in inundation of floodplain affecting Biota. Native fish species now comprise less wetland health and floodplain functions; than 7% of the fish biomass in the Mitta Mitta River below Dartmouth Dam and less than 4% • unseasonal low water temperatures in at sites closer to the dam. The change in fish summer and rapid temperature fluctuations
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have virtually eliminated warmwater native 6.5 MANAGEMENT RECOMMENDATIONS fish and have affected macroinvertebrate community diversity; and 6.5.1 Low Water Temperatures – High Priority • potential impacts of hypolimnion releases on RECOMMENDATION Z1.1 water quality immediately downstream of the dam. Releases should be from the surface waters of Other major threatening processes include: Dartmouth Dam, except in periods of major • reduced linkages between floodplain cyanobacteria bloom. wetlands and the river thereby removing Preferably this would be achieved by installation interactions between these habitats and of a variable level off-take for Dartmouth Dam. reducing the input of carbon to the river; A study should be commissioned to determine: costs and benefits, effectiveness and which river • reduction in the numbers of snags, affecting reaches are most impacted and which have the the available critical habitat for fish and best potential for recovery. macroinvertebrates; and • grazing of the riparian zone causing The installation of a variable level off-take for reduction in habitat, carbon inputs to the Dartmouth Dam is viewed as the highest priority stream and shading, as well as increased recommendation for the river zone. If this were bank instability. implemented, then other water quality problems in the area downstream of the dam would also be addressed, including a reduction in downstream 6.4 PRIORITY ISSUES nutrient loads (see for example section 3.1.4). It needs to be recognised that the restoration of a In the opinion of the Panel, the key issues for natural temperature regime is the prerequisite for the the management of this river zone, in priority restoration of native fish and macroinvertebrate order, are: populations. However, once this is achieved, further work will be required to ensure the re-establishment • unseasonally low water temperatures in of native fish. This would require an adequate spring and summer and rapid, extreme assessment of what is required and the development temperature variations; of a management plan which may need to include • constant high flows within the river channel other measures such as fish stocking, restoration of during summer; some instream habitat and possibly revegetation of riparian areas. It must be recognised, however, that • reduction in flooding specifically affecting the restoration of this river zone as a native fish area some features of the floodplain; and may reduce its value as a trout fishery. • reduction in instream habitat (snags and pools). 6.5.2 Constant Flows – High Priority The overriding issue for this river zone is the RECOMMENDATION Z1.2 unseasonally low temperature (i.e. during late spring–summer) of releases from Dartmouth Introduce variability in the current patterns of Dam. This is compromising the biological integrity sustained flow by: of the system. Changes to the flow regime could alleviate enhanced channel erosion and restore • avoiding releases at a constant discharge. some heterogeneity in the bed, recreating pool Instead releases should mimic a natural and riffle sequences and thus increasing instream rainfall event in the catchment by using a habitat. The reintroduction of snags would also step function to vary flows as shown in improve instream habitat. However, these actions Figure 4.4. The flow would rise over two days alone are unlikely to result in the reintroduction and then recede gradually. A suitable time of native fish species and more natural scale for this variation would be 2 weeks macroinvertebrate communities unless the water giving two peaks per month. The amplitude temperature issue is adequately addressed. of the variation around the desired level should be ± 20% (in terms of river height, not flow) truncated where necessary by minimum and channel capacity flows.
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• maintaining the current winter riparian RECOMMENDATION Z1.5 releases of 200–500 ML/day and vary in steps of 50 ML. The re-establishment of flood runners be recognised as a high priority for funding of local • not running at channel capacity for longer groups under the Murray-Darling 2001 program. than 5 days at a time except in dry years (defined by the need to make releases in This will improve the condition of the floodplain. September) when this period could be extended to 3 weeks. 6.5.4 Reduction of Instream Habitat – Medium Priority These recommendations have been modelled by the MDBC (see Appendix 3, Section 1: 1) and RECOMMENDATION Z1.6 appear to be quite feasible. In introducing this variation in the summer months, the Panel Should recommendation Z1.3 concerning the acknowledges that there could be a problem at cessation of pre-releases be accepted, the Panel higher flows given that overbank flows occur at recommends the development of a stream 9300 ML/day. Therefore, for flows near channel restoration plan by the regional Catchment capacity, the Panel considers that the level of Management Authority to restore pool/riffle variation may have to be truncated. sequences and reinstate snags. The Panel considered that the introduction of this type of variability would have some value, It would then become essential to ensure that even if the temperature issue was not addressed immediately. It will reduce the current level of there is some habitat for fish and bank and bed erosion and should create more macroinvertebrates to utilise. Any instream bank habitat for bank vegetation to re-establish. habitat would then be maintained through the In examining this issue, the Panel felt that the flow variations recommended. Instream habitat MDBC should be commended in recognising this rehabilitation should be matched by protection as a problem and developing their winter riparian and enhancement of the riparian vegetation zone. release policy. The current rules for rates of rise and fall were also examined and were endorsed by RECOMMENDATION Z1.7 the Panel. The development of a riparian vegetation plan 6.5.3 Reduction in the Inundation of Some by the regional Catchment Management Features of the Floodplain – High Priority Authority to re-establish riparian vegetation and, where possible, a riparian vegetation RECOMMENDATION Z1.3 corridor along sections of this river zone.
To conserve flood events, the practice of pre-releases 6.5.5 Evaluation of Low Flow Levels – from Dartmouth Dam should be abandoned. Low Priority
Given that this river zone has experienced a Although the other issues are of greater importance, reduction in the frequency of flooding and that there has been no evaluation of the adequacy of the this must have influenced habitat type and river minimum environmental flows which are provided functioning, the Panel formulated the following down the river at low flow times. recommendations. RECOMMENDATION Z1.8 RECOMMENDATION Z1.4 An environmental flow study be undertaken to During the period June to September, when determine the adequacy of current minimum Dartmouth Dam is filling, the MDBC should environmental flows. explore the option of passing a percentage (e.g. 10%) of the inflows, thereby ensuring the hydrographic characteristics (other than volume) of water flowing into Dartmouth Dam are not altered and that releases reflect the arrival and timing of these in-flowing waters.
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7 – ZONE 2 – HUME DAM TO TOCUMWAL
7.1 HYDROLOGIC MANAGEMENT orders are routed upstream allowing for forecast tributary flows and the operation of the small re- Hume Dam and Yarrawonga Weir are regulating storages at Euston and Yarrawonga Weirs. Flows released from Hume Dam take 23 days to the two major regulatory structures reach the Darling River Junction so orders and which affect flows in this river zone releases need to be made in advance. At times, rainfall in the irrigation areas will cause irrigation (Figure 7.1). demand to be significantly less than the original order. If there is insufficient re-regulating capacity in Hume Dam. This is the most important regulating Yarrawonga and Euston Weirs, this reduced demand structure on the River Murray. Work on the dam will result in a small flood or a ‘rain rejection’ flow commenced in 1919, water storage commenced in moving down the river. 1929, and the first stage of construction was In some years, when the volume of water in completed in 1936. In 1936 the capacity of the Menindee Lakes and Lake Victoria is low, water has Hume Dam was 1522 GL. This was increased to to be transferred from Hume to Lake Victoria to 1800 GL in 1949, 2460 GL in 1958 and 3038 GL in supply South Australia’s entitlement flow. Because 1961. The dam has a maximum surface area of of channel constraints between Hume and 20,190 ha with an average depth, when full, of 15 Yarrawonga and through the Barmah Forest area, m and a maximum depth of 37.7 m. Water can be these transfers have to be made in advance. Like the released via the power station which has a capacity releases from Dartmouth, the volume and timing of of 20,000 ML/day, or through four irrigation valves transfers is determined on the basis of a forecast to with a total capacity of 35,000 ML/day. The power the end of May which assumes minimum inflows station and the irrigation valves draw water from and peak demands. between 29.5 and 34.3 m below full supply level. The capacity of the river channel between Hume Dam has a gated spillway with a sill 7.2 m Hume Dam and Yarrawonga Weir is below full supply and these gates are operated so as 25,000 ML/day which limits the releases that can to keep the dam at or below the full supply level be made. There are also constraints on the rate at during floods. which releases from Hume Dam can be changed. Water is released from Hume Dam to meet When flows in the river are less than channel downstream demand. The demand from Hume capacity, the fall in water level at Doctors Point, 16 Dam is determined from orders for water from as far km downstream of the dam and just downstream downstream as the Darling River Junction which of the Kiewa River Junction, is limited to 0.15 are forwarded to the operators each day. These m/day and the rise is limited to 0.30 m/day.
Figure 7.1 Map Zone 2: River Murray between Hume Dam and Tocumwal
Tocumwal
Cobram Mulwala Corowa Wahgunyah ALBURY Yarrawonga Thurgoona Rutherglen Bethanga Chiltern WODONGA Bonegilla Springhurst
Tallanga Thoona N Beechworth WANGARATTA Oxley 02030km10
Base map copyright AUSLIG Map compiled by GIS Unit, MDBC
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The operators are required to provide minimum • utilise any surplus capacity in Mulwala Canal flows of 600 ML/day downstream of the dam, 1200 and the Edward Escape to bypass flow around ML/day downstream of the Kiewa River Junction the forest if it will prevent downstream flooding. and 1800 ML/day downstream of Yarrawonga Weir. The practice of pre-releases occurs from Hume Unfortunately each of these actions has limited Dam. The rate of pre-release is generally restricted to effectiveness and consequently unseasonal flooding the channel capacity of 25,000 ML/day. The upper of the forest still occurs regularly (see section 4.2.1). 0.15 m of the storage is generally held empty to Hume Dam and Yarrawonga Weir have provide some leeway for operating the spillway significantly altered the flow regime of the River gates during floods and to provide limited capacity Murray. Hume Dam catches natural floods in winter for attenuating short duration peaks. and spring and releases water at or near channel Yarrawonga Weir. This weir is located 237 km capacity throughout summer and autumn to meet downstream of Hume Dam and was completed in irrigation demand. Major diversions for irrigation 1939. Its primary purpose is to provide the hydraulic occur at Yarrawonga Weir which reduce the average head necessary for water to be diverted into the volume and the seasonal variation in the flow Mulwala Canal to supply New South Wales, and the downstream. The changes to the flow regime at Yarrawonga Channel for Victorian irrigation. These Albury are highlighted in Figures 4.5 and 7.2, while canals have capacities of 10,000 and 3400 ML/day changes downstream of Yarrawonga Weir are respectively. The Yarrawonga Weir has a capacity of shown in Figures 7.3 and 7.4. In this river zone, 126 GL and a surface area of 4490 ha. It has an compared with natural conditions: average depth of 2.8 m, the maximum being 7.6 m. Releases from Yarrawonga Weir are generally made • flows are much less variable, particularly in through the power station which has a capacity of winter and spring because the smaller floods are 12,500 ML/day. Higher flows are passed through the now stored in Hume Dam; weir’s undershot gates. Because of the need to maintain sufficient • seasonality of the flow has been reversed hydraulic head for water to be diverted, (particularly between Hume Dam and opportunities for using the Yarrawonga Weir to re- Yarrawonga Weir) with low flows in winter and regulate flow are limited. At times of low flow in the high flows in summer and autumn; Yarrawonga Channel, it is possible to operate in the • flow is at or near channel capacity for much top 0.5 m of the weir pool. However, the of the year (especially downstream of requirements to supply higher flows in the Yarrawonga Weir); Yarrawonga Channel and limit erosion in the top 0.05 m of the weir pool, effectively limits operation • average annual flow at Albury has been to around 0.3 m. This level has an effective storage increased because of the additional water capacity of 14 GL. Priorities for the operation of the transferred from the Snowy River; Yarrawonga Weir are to keep the water level within the operating range, to limit the fall in water level • average annual flow downstream of Yarrawonga downstream to 0.3 m/day and to meet the Weir has been decreased because of the water downstream order. The main method for achieving diverted for irrigation; these aims is adjusting releases from Hume Dam. • frequency and duration of flooding in winter and However, it takes four days for a change in the flow spring has been much decreased; at Hume Dam to reach Yarrawonga Weir. During summer and autumn, a key MDBC • opportunity for low lying wetlands to dry out in operational aim is to limit unseasonal flooding in the autumn has been much decreased; Barmah-Millewa Forest, 100 to 200 km downstream of the Yarrawonga Weir. At times of • rain rejection events can cause unseasonal peak diversion, rain rejection events can easily cause flooding during summer and autumn the downstream channel capacity of around 10,600 downstream of Yarrawonga Weir; ML/day to be exceeded. In a rain rejection event, • deep offtakes at Hume Dam mean that the water the normal operation is to: released is up to 4–6˚C lower than the surface • increase flow downstream of the weir to c water of the dam; and hannel capacity; • time of maximum water temperature • catch as much surplus flow as possible in the downstream of the dam is offset by about 2 weir pool; and months from January–February to March–April.
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Figure 7.2 Change in montly flow in the River Murray at Albury
20th Percentile 40000 35000 Current Conditions 30000 Natural Conditions 25000 20000 15000 10000 Flow ML/day 5000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
50th Percentile 40000 35000 Current Conditions 30000 Natural Conditions 25000 20000 15000 10000 Flow ML/day 5000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
80th Percentile 40000 35000 Current Conditions 30000 Natural Conditions 25000 20000 15000 10000 Flow ML/day 5000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
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Figure 7.3 Typical change to the flow regime in the River Murray at Yarrawonga Weir
80000
70000 1923 - Pre-Regulation 1994 - Post Regulation 60000
50000
40000
3000 Yarrawonga Flow (ML/day) Yarrawonga 20000
10000
0 1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec Date
7.2 ENVIRONMENTAL CONDITION bars of various sizes, counterpoint bars, point bars, convex and concave benches, and low amplitude Downstream of Albury, the River Murray flows riffle/pool sequences. across the Riverine Plains. The river is dominated The River Murray through most of this river by a multi-branched channel system and a large zone is a mixed to suspended-load river channel. well-defined floodplain, and can be described as a Hence the character of the river substratum is a meandering anastomosing pattern (multiple mixture of gravely sands grading to fine-medium channels separated by floodplain). These channels sands near the Barmah-Millewa Forest. move laterally across this broad floodplain and have Five sub-reaches have been identified by done so for the last 10,000 years. Rutherfurd (1991) within this river zone; each with There are many different river–floodplain distinctly different riverbed slopes (hence different landforms within this river zone, producing a energy gradients), channel dimensions and river complex mosaic of physical habitat compartments. channel patterns. These sub-reaches are: Anabranch channels associated with the • Hume Dam to Ryan Creek. This sub-reach has a anastomosing river channel pattern dominate the sinuosity of 1.20, contains three historical cutoffs river system. These channels are developed by and a number of abandoned channels but no repeated avulsions (the sudden abandonment of a active anabranch channels. part or all of a channel for some new river course which is at a lower level of the floodplain). For the • Ryan Creek to Corowa Throat. Sinuosity is most part, the main channel of the River Murray is higher in this sub-reach with values up to 2.4. sinuous to highly sinuous while the main There is also a large network of active and anabranch channels are relatively straight. Most of inactive anabranches in this area. the anabranches convey flow at discharges below • Corowa throat. Here the channel is much bankfull levels. Hence, cutoffs, abandoned channels, straighter due to a narrower floodplain. There floodplain ridges and anabranches, at various stages are no active anabranches in this sub-reach. of development, dominate the floodplain. There are also a variety of floodplain surfaces at different • Corowa throat to Lake Mulwala. Sinuosities elevations above the bankfull channel of the River increase in this reach, values up to 2.4 and Murray. The channels contain the following several active anabranches exist on the morphological features: convex and concave scroll floodplain.
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Figure 7.4 Change in monthly flow in the River Murray downstream of Yarrawonga Weir
20th Percentile
60000 50000 Current Conditions 40000 Natural Conditions 30000 20000
Flow ML/day 10000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Aug Mar May Month
50th Percentile
60000 50000 Current Conditions 40000 Natural Conditions 30000 20000
Flow ML/day 10000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Aug Mar May Month
80th Percentile 50000 Current Conditions 40000 Natural Conditions 30000 20000
Flow ML/day 10000 0 Jul Oct Jan Feb Sep Dec Jun Apr Nov Mar Aug May Month
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• Lake Mulwala to Barmah. Sinuosities decrease bank, a sloping mid profile and a steep toe. The and there is also a significant reduction in the upper bank generally has a well-developed erosion dimensions of the main channel. There are notch that corresponds to a constant water level. many distributary channels on the floodplain Similarly the steep bank toe also corresponds to a that convey flow at a range of stages below water level that is held constant for long durations. bankfull. The development of an erosion notch is an important factor influencing the stability of the The environmental condition along this river zone is banks. Furthermore, the bank material is made highly variable. vulnerable to other erosion processes following Instream environment. The instream prolonged wetting, such as slaking where the environment throughout the entire river zone is in strength of the soil bonding between particles is poor condition. There have been 16 cutoffs between reduced with increased wetting. The combined Hume Dam and Lake Mulwala, thus reducing the pressure of modified flows, bank erosion, and available main river channel habitat. Cross-sectional grazing and trampling by stock has resulted in a dimensions of the main channel and major serious depletion of emergent plants, grasses and anabranch channels have also changed substantially native shrubs such as Callistemon – a major agent in through this river zone since the turn of the combating bank fretting from surface wave action. century. Channel changes have been recorded by Riparian zone. The riparian zone would the New South Wales Department of Land and naturally comprise river red gum woodland with a Water Conservation since 1977, with changes in shrub layer and an understorey dominated by widths, depths and cross-sectional area of up to 8, native grasses or sedges. In this river zone, the 23 and 19% respectively in the main channel and native riparian vegetation communities (especially up to 25, 35 and 35% respectively in major the ‘non-tree’ species) are in relatively poor anabranch channels. Bed degradation has occurred condition, often restricted to relatively inaccessible in the River Murray downstream of Hume Dam. In areas. Much of the vegetation has been cleared or some places the riverbed is heavily armoured as a grazed. These riparian vegetation communities have result of this adjustment process. Thus regulated also been affected by a reduction in flooding flows are now incompetent to mobilise this frequency. The loss of vegetation from riverbanks sediment layer. As a consequence of bed erosion also contributes to their instability. near Hume Dam, the riverbed has aggraded further Floodplain. Anabranch channels are an downstream towards Lake Mulwala and this has important feature of the floodplain environment been accompanied by a widening of the river and require flooding for their development. A channel. River channels do change their dimensions reduction in the incidence of flooding reduces the and position on the floodplain naturally. However, probability of anabranch development. However, an rates of change in this reach have increased over increase in flow conveyance in anabranch channels time because of flow regulation, desnagging, a will lead to an increase in erosion. Some anabranch decline in the health of the riparian vegetation and channels are showing signs of active erosion due to waves induced by boat activity in some lower desnagging (in a few cases) and constant high flows, reaches (Rutherfurd 1991). The upstream section of while others have experienced a reduction in this river zone (Albury to Yarrawonga) has been flooding. Decreases in river–floodplain linkages will heavily desnagged, with 24,500 recorded snags reduce the exchange of organic material and is being removed between 1976 and 1987, likely to affect fish breeding, particularly of the significantly reducing instream habitat especially for smaller species. Modification of the floodplain is fish species such as Murray cod. This also has an greatest for the higher flood inundation levels important impact on instream productivity by (infrequent floods) and is least for low flood-prone limiting habitat for instream biofilm development areas (particularly on the inside of bends). This and macroinvertebrate production. inverse relationship between floodplain alienation Riverbanks. These are considered to be in very and flood frequency results in a simple reduction in poor condition along almost the entire length of this effective floodplain size in downstream reaches as river zone. Erosion has virtually eliminated much of well as a reduction in the benefit from the physical bank habitat, such as benches, in floodplain–river interaction during major floods. In certain areas. For example, Rutherfurd (1991) has this river zone, however, it serves to amplify noted bank erosion rates of up to 1260 mm/year at (accentuate) the negative effects of regulation, locations along the Murray with average rates of particularly seasonal shift of floods, by making the bank retreat of 160 mm/year. The riverbanks parts of the floodplain least affected by land use, the typically exhibit a faceted profile with a steep upper most subject to (unseasonal) flooding.
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In this river zone, grazing pressure tends to be • unseasonal high flows (summer and autumn) in determined by stock access. Thus low lying areas are the main channel and in some key anabranches grazed lightly, particularly when access is restricted mainly affecting fish and macroinvertebrates; by anabranches during high flow. However, these areas have been extensively cleared in the Hume • reduction in flooding affecting in-channel Dam to Yarrawonga Weir section. Below benches and anabranches; Yarrawonga Weir, and particularly further • reduced linkages between floodplain wetlands and downstream, stands of river red gum remain, the river, reducing input of carbon to the river and especially on the concave sides of bends, providing a affecting fish passage to the floodplain; and forested ribbon the width of the river’s meanders. Biota. Fish populations in the upper parts of this • changes in summer and autumn water river zone are affected by the reduced number of temperatures in the upper section caused by low snags. Reduced flooding and lowered summer level releases from Hume Dam. temperatures have significant effects on the Other major threatening processes include: spawning and life cycle requirements of warmwater species such as Murray cod. In the lower reach of • reduction in the numbers of snags, particularly in this river zone, fish populations are reasonably the main channel of the upper section of the river healthy due to increased snags providing good zone, affecting available critical habitat for fish and habitat, and because of Lake Mulwala. Lake macroinvertebrates in the upper reach; and Mulwala appears to be an effective nursery ground • grazing of the riparian zone causing reduction in for Murray cod and may assist in warming water habitat, increased bank instability, reduced temperatures. The River Murray, between Lake carbon inputs to stream, and shading. Mulwala and the Barmah Choke, contains excellent native fish populations, including the only remaining truly natural population of the 7.4 PRIORITY ISSUES endangered Trout cod. This section of the river is of great significance to lowland native fish species. In the opinion of the Panel, the key issues for the Macroinvertebrate communities reflect the level of management of this river zone in priority order are: instream habitat, having generally low diversity but with the highest abundances associated with logs • unseasonal high flows; and woody debris. • conservation of the anabranches; Water quality. In addition to the changes in water temperature caused by low level releases from • constant flows; and Hume Dam, changes in nutrient concentrations • changes in the occurrence of floods. have also been observed. Nitrate data from Heywoods Bridge shows a very interesting annual 7.5 MANAGEMENT RECOMMENDATIONS cycle with a peak concentration in November–December, and a minimum in 7.5.1 Unseasonal High Flows autumn–winter (at almost undetectable levels). The filterable and total phosphorous data shows much The Panel accepts that due to irrigation demands greater variance, although there is some cyclic downstream, there may be little opportunity to behaviour with a possible bi-phasic discharge of address this issue or to compensate for its effects in phosphorous. The first phase corresponds to the winter–spring run-off period and the second to this river zone. Any improvement to the current phosphate rich water from the hypolimnion of situation, either by reducing demand or improving Hume Dam (see section 3.1.4 for more details). the capacity to deliver water by other routes, that would result in increased air space in this river zone 7.3 THREATENING PROCESSES should be used for non-consumptive environmental flow management. In the past, any such gain has Regulation results in the following environmental tended to be used to increase water deliveries. Many problems: of the problems in this river zone have resulted from this approach, with the result that flows in this zone • constant flow levels (both high summer and low now occur at or above natural design capacity for base), causing bank erosion to the main channel the entire irrigation season to the detriment of the and anabranches, changes in bed morphology riverine ecosystem and eventually the resource and consequent reduced instream habitat, and users. A change in this approach is corollary to the reduction in range of available bank habitats; water capping process.
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The use of regulators to exclude summer management options. It will be necessary to flooding from wetlands is a possibility but, as examine the rights and obligations of both discussed previously in section 4.2.1, it must be management and landholders as a step towards recognised that this form of management does not developing ecologically sound anabranch always reinstate fish passage and organic inputs to management procedures. the river. Regulators should not be seen as a general solution. Rather, their use may be appropriate in RECOMMENDATION Z2.1 some cases as a conservation measure and to reinstate some broader wetland functions (through The restoration of anabranches be recognised as a natural wetting and drying processes) if early spring priority for funding for local groups under the floods occur. Murray-Darling 2001 program.
7.5.2 Conservation of Anabranches - High 7.5.3 Constant Flows - High Priority Priority RECOMMENDATION Z2.2 The conservation of anabranches will depend to a large extent on the flooding frequency and the Releases at a constant discharge should be avoided. introduction of some variation in the flow regime. Instead releases should mimic a natural rainfall event This would be enhanced by a complementary in the catchment by using a step function to vary program of restoration and land management flows as shown in Figure 4.4. The flow would rise controls including: over two days and then recede gradually. A suitable • the re-introduction of snags into the time scale for this variation would be 2 weeks giving anabranches where they have been removed; two peaks per month. The amplitude of the variation • control of wood collection from the floodplain; around the desired level should be ± 20% (in terms • controls on stock access on the floodplain; of river height, not flow) truncated where necessary • removal of blockages on anabranches that by minimum and channel capacity flows. prevent downstream flows; and • control of pest species, particularly willows. Modelling undertaken by the MDBC has indicated Anabranches, as with other floodplain components, that this could be feasible (see Appendix 3, Section are mostly on private land. This may limit 1: 1a). Some problems arise at higher flows due to Figure 7.5 Hume Dam pre-release operation in 1989
35000 3500 Storage filled but Spill Second Pre-release Limited to 25,000 ML/day 30000 3000
25000 2500
20000 2000
15000 1500
First Pre-release Hume Storage (GL)
10000 1000 Downstream Flow (ML/day)
Downstream Flow 5000 500 Hume Storage
0 0 11/12/89 2/4/89 22/5/89 11/7/89 30/8/89 19/10/89 8/12/8927/1/90
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channel capacity constraints, the need to exceed the Pre-releases from Hume Dam currently occur permissible rate of fall, and the possibility of causing approximately two years in three. They usually spills at Yarrawonga Weir. Given this, the Panel occur during the late filling phase of Hume Dam considers that the amount of variation could be to provide air space to accommodate future flood truncated at flows near channel capacity. events. Figure 7.5 shows how pre-releases in With this rule, there may be a need to introduce 1989 prevented the downstream flow from more variation at low flows. However, this may exceeding channel capacity. The effect of require the river to be run at unacceptably low pre-releases is usually small, increasing the levels, decreasing the amount of available instream duration of channel capacity flows by one to two habitat. Current minimum flows are 600 ML/day weeks but reducing the duration and extent of downstream of Hume Dam and 1200 ML/day at overbank flows. In instances where there is a Albury, and it is considered that the flow should not series of high runoff events in the catchment, fall below these levels. As no environmental flow study has been undertaken, it is difficult to judge pre-releases can extend for as long as six weeks. whether or not these minimum flows are adequate The process does not significantly influence the to provide for aquatic fauna. They are lower than behaviour of very large floods (return frequency the expected winter flows, and even lower than the greater than 1 in 20 years). naturally expected 5 percentile flow for Albury in These recommendations will ensure that the March. The natural 10 percentile flow at Albury for incidence of floods, up to and including the 1 in March is 1400 ML/day. These low flows, compared 20 year event, are not further reduced. For some to natural low flows, are of concern and may specific wetland areas at the downstream end of impose some ecological effects which are not explicit this river zone, some improvement in the without further study. Whilst the Panel considers flooding regime will be experienced, particularly priority should be given to achieving flooding for the river red gum forests and lignum wetlands. this river zone, it considers that the issue of minimum flow levels needs to be investigated and 7.5.5 Low Water Temperatures – the results considered in the context of current Medium Priority recommendations. In examining this issue of flow variability, the Panel also reviewed the current rules Details of factors contributing to temperature governing rates of fall and endorsed the current depression in this river zone are presented in MDBC ‘6 inch rule’. section 3.1.4. For a number of reasons (lake 7.5.4 Reduction in the Occurrence of Floods – depth and flow volume) the summer depression High Priority of temperature in the River Murray immediately downstream of Hume Dam is less severe than in RECOMMENDATION Z2.3 the Mitta Mitta River below Dartmouth Dam, being a decrease of approximately 6˚C. However, To conserve flood events, abandon the practice of there is a 2–3 month offset in the timing of pre-releases from Hume Dam. maximum temperature which could be highly significant for biota. The maximum temperature Given that this river zone has experienced a is significant as a number of native fish species reduction in the frequency of flooding (of floods up require appropriate temperature conditions for to the size of a 1 in 20 year return frequency) and successful breeding. In addition to direct that this has influenced habitats and aspects of river temperature effects on biota, such as fish, the functioning, the Panel formulated the following depressed summer temperatures represent a recommendation. significant reduction in the amount of heat RECOMMENDATION Z2.4 (temperature x time) in the system. As the metabolism and rate of development of The MDBC should explore the option of passing a cold-blooded organisms (insects, crustaceans and percentage (e.g. 10%) of the inflows, when Hume fish) is temperature-dependent, a reduction in Dam is filling, during the period June to September, the amount of heat results in slower growth thereby ensuring the hydrographic characteristics rates and extended development times, which in (other than volume) of water flowing into Hume turn mean longer exposure to predators and Dam are not altered and that releases reflect the possible disruption to the synchrony of arrival and timing of these in-flowing waters. breeding seasons.
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RECOMMENDATION Z2.5
Releases should be from the surface waters of Hume Dam, except in periods of major cyanobacteria bloom. Preferably this would be achieved by installation of a variable level off-take for Hume Dam. A study should be commissioned to determine: costs and benefits, effectiveness and which river reaches are most impacted and which have best potential for recovery.
7.5.6 Evaluation of Low Flow Levels – Low Priority
There has been no evaluation of the adequacy of the minimum environmental flows which are provided down the river at times of low flow.
RECOMMENDATION Z2.6
An environmental flow study be undertaken to determine the adequacy of current minimum environmental flows.
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8 – ZONE 3 – TOCUMWAL TO TORRUMBARRY WEIR, INCLUDING BARMAH CHOKE
Barmah Choke refers to the area of the • during times of flood, distributing floodwaters in equal volumes to New South River Murray, in the vicinity of Wales and Victoria so that neither State’s Barmah, where channel capacity is levee system, fringing the forests, was subject to inequitable water levels. reduced to a maximum of 10,600 Procedures for the operation of the regulators ML/day due to natural channel were developed to meet these objectives in the constrictions. This feature has 1950s and 1960s. However, in more recent times regulator operation to suit forest ecosystem important effects on the water requirements, particularly for waterbird management of the River Murray both breeding, has gained greater importance. Regulators are operated to water or dry out the upstream and downstream of this forests and wetlands, depending on ecosystem point (Figure 8.1). needs. The MDBC has allocated 100 GL/year of water for use within the forests. Some 40 GL/year 8.1 HYDROLOGIC MANAGEMENT of this water is currently being used (1997). Generally, regulators are set to give equal Regulators are structures built at the off-take flow at the peak of a flood or rain rejection to from the River Murray to natural channels to each State forest unless there is interstate prevent unseasonal flooding of the forests agreement at agency level to do otherwise. during the irrigation season. The MDBC is During minor floods and rain rejection events, responsible for the proper operation of when flows exceed channel capacity but regulators. At the time of their construction, overbank flow can be partially controlled, advice some forty years ago, regulators were seen to be is sought from State forest management performing two functions: agencies on the most appropriate parts of the forest to water. Factors influencing regulator • during the irrigation season, preventing the opening include consideration of ecosystem loss of regulated flows from the River needs and the impact of flooding on commercial Murray and unseasonal flooding of the forest and recreational activities. that would cause tree death and permanent There are 28 regulators in this rive zone, inundation of wetlands; and which release water from the River Murray into
Figure 8.1 Map of Zone 3: River Murray between Tocumwal and Torrumbarry Weir
02030km10 Base map copyright AUSLIG Map compiled by GIS Unit, MDBC
N
Jerilderie
Wakool
DENILIQUIN
Finley
Cobram Leitchville Stratherton
Gunbower Katunga Nathalia
Moama Numurkah ECHUCA
Tongala Lockington Kyabram
Rochester Merrigum SHEPPARTON
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RIVER MURRAY SCIENTIFIC PANEL ON ENVIRONMENTAL FLOWS
the Barmah and Millewa forests. Of these, 19 • flow is held constant, at or near channel regulate flow from the River Murray into creeks in capacity, for much of the year; the Millewa Forest in New South Wales and 9 into • average flow at Yarrawonga has decreased creeks in the Barmah Forest in Victoria. Under because of the water diverted for irrigation; regulated flow conditions, only the Edward River and Gulpa Creek offtakes, which divert water • frequency and duration of flooding in winter northwards into the Edward River, are open. The and spring has been greatly decreased; other regulators are closed and water is excluded • opportunity for low lying wetlands to dry out from the forests. When the flow at Tocumwal in autumn has been greatly decreased; and exceeds about 10,600 ML/day, it is no longer possible to exclude water from the forests and water • rain rejection events sometimes cause will flow over the banks. To prevent this from unseasonal flooding in summer and autumn. happening, the forest regulators are opened and water flows through a network of creeks. In 8.2 ENVIRONMENTAL CONDITION Victoria, these creeks return to the River Murray at Barmah Lake while in New South Wales they flow The Cadell Fault has an important influence on the into the Edward River. physical, hydrological and ecological nature of the The Edward River offtake, Gulpa Creek offtake entire River Murray (see section 3.1.1). The and the Edward Escape from the Mulwala Canal are presence of prior and ancestral channels has also used to meet the irrigation demand in the Edward had an important influence on the evolutionary River. The major demand in the Edward River is process of the river–floodplain landscapes in the from Wakool Canal (up to 2400 ML/day). There are Barmah region. In particular, a change in the also requirements to release water into the Wakool, degree of confinement immediately upstream of Yallakool and Niemur rivers (up to 750 ML/day) to the head of the Barmah Fan resulted in a series of supply private diverters on the Edward River (up to avulsions, the present course of Bullatale Creek 800 ML/day) and to meet the minimum flow target and a number of other distributary channels. The of 300 ML/day downstream of Stevens Weir. At change in the degree of confinement caused by the times the Edward River is also used to transfer water backwater of the Cadell Fault changed the River from Hume Dam to Lake Victoria to supply South Murray from a laterally meandering system with Australia. In recent years the releases through the numerous cutoffs and anabranches, as seen Edward River have been reduced to 1600 ML/day to upstream of Tocumwal, to a vertically accreting address concern at the damage being done to the system downstream. Hence, the form of forest along the Edward River by high water levels. river–floodplain physical habitat compartments in To meet demands, the Edward Escape can the Barmah region differs to that found in release up to 2400 ML/day, the Edward River surrounding river zones. A large number of offtake up to 2000 ML/day and the Gulpa Creek channels off-take from the River Murray to flow 350 ML/day. During floods, the Gulpa Creek offtake across the Barmah Fan. The presence of the Cadell is fully opened and up to 2000 ML/day can pass Block has also resulted in the limited channel down Gulpa Creek. Following floods, flow in the capacity within the region of the Barmah Choke. Gulpa Creek is often boosted during December and Hence flows above this level (10,600 ML/d) result January to around 750 ML/day to maintain in flooding of the Barmah-Millewa Forest and sufficient head in the attached Reed Beds swamp to flows through the numerous channel networks of enable the waterbirds to complete their breeding. the Barmah Fan eventually flowing into the The flow regime in this river zone can be defined Edward River system. by the flow at Yarrawonga Weir. The comments The Gunbower and Wakool fans did not made in Chapter 7 about the changes from natural develop from backwater effects, but rather from conditions at Yarrawonga and the observations increases in slope around the northern and drawn from Figures 7.3 and 7.4 apply equally well southern flanks of the Cadell Block. On the to the flows in this reach. southern flank, the modern River Murray is In this river zone, compared with natural contained within an incised section of the ancestral conditions: Goulburn River whereas below Torrumbarry Weir the degree of confinement is markedly reduced. • flows are much less variable, particularly in Tectonic activity and a hierarchical influence of winter and spring because the smaller floods past river channel activity have resulted in a are now stored in Hume Dam; complex modern river–floodplain geomorphology
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in this region. River channel dimensions are greatly experienced in the Edward River and a number of reduced, with the cross-sectional areas ranging anabranches. As a consequence, several anabranch from 300–600 m2. Sinuosities are also reduced systems are eroding their channels. In some areas, (mean of 1.2). In-channel velocities are the size of the floodplain has been reduced by levee consistently higher within the Barmah Choke building. The number of snags has been region in comparison to other gauging stations. considerably reduced in some channels. High rates The condition of this river zone of the River of sedimentation have been recorded by Thoms Murray was considered to be variable, with some (1995) in areas of high to very high flood very poor aspects and other aspects displaying only frequency. This is attributed to poor land and water minor signs of degradation. management practices. High rates of sedimentation Instream environment. As a result of the have the potential to completely alter the naturally limited channel capacities in the Barmah distribution of floodwaters across the floodplain Choke region, in-channel velocities are generally surface and the condition of floodplain habitats. higher and channel morphologies not as complex, Biota. Fish populations in this river zone are thereby limiting the range of habitats available. dominated by carp, although there are significant Because of the importance of this river zone in populations of native species. Carp appear to be terms of water supply downstream, river using the flooded wetlands for breeding. The operations, such as desnagging and dredging, are unseasonal flooding is currently providing excellent common. The dominant instream habitat of this conditions for carp breeding. Native fish species region is probably within the numerous off-take have been shown to use the anabranch and flood channels. The majority of these still contain runner systems when they become available. As abundant habitats. However, several of the larger these channels provide good fish habitats, they channels, including the main channel, do display could be significant in providing habitat and signs of active erosion and a reduction in available spawning needs of these species. The flooded habitat. wetlands may also provide organic inputs to the Riverbanks. The availability of various bank river system, including planktonic food supply for habitats has been reduced significantly in this juvenile fish. Macroinvertebrate populations within region for two reasons. Active riverbank erosion in the main channel were predominantly found on the main channel of the Murray and in many of the few logs present. Their abundance was quite the anabranch channels has removed many low and included species which are generally benches. Also, constant high flows through this found in faster flowing rivers. region permanently inundate those benches that remain. This increased inundation reduces the 8.3 THREATENING PROCESSES function of these features as temporary carbon and nutrient storage areas. The major environmental problems are: Riparian zone. The area consists of the • unseasonal high flows (summer and autumn) Barmah-Millewa Forests which are dominated by which affect wetland health, stands of river red gum with the common rush, macroinvertebrate communities, native fish Phragmites, along the water’s edge. Constant high breeding and recruitment, and favour carp; flows during summer and autumn contribute to tree death. This and the reduced frequency of • reduction in frequency of naturally occurring naturally occurring floods are causing a change in winter–spring floods; the composition of vegetation communities in the • constant flow level within the channels area. causing erosion; and Floodplain. The floodplain areas of this reach have been well studied. The wetlands are of high • reduction in snags. conservation value and have been listed on the Ramsar register because of their large size, high 8.4 PRIORITY ISSUES biodiversity and ecological productivity. Currently, these areas are degrading because of persistent high The unseasonal flooding of wetlands is the most flows during summer and autumn which have significant issue in this river zone. This is disrupted natural wetting/drying cycles. This occurring because the river is used to transport disruption influences nutrient cycling, creates a water for irrigation during the summer months. mismatch between habitat availability and life cycle To meet this need, the river is run at bankfull cues. Unnatural high summer flows are also capacity. Any excess flows that occur as a result of
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rain rejections or higher flows in the Ovens or is required within the system from December 1 to other tributaries (due to rain in these catchments) the end of the irrigation season. Prior to this, floods spill into the forests. There is limited scope to do occur naturally and there may be some value in confine these flows within banks because: letting any excess flows top up or extend those floods. From December onwards, the Panel • channel capacity of the river in the Barmah considered that excess flow would be unseasonal Choke is restricted to 10,600 ML/day at and should be avoided. Tocumwal (possibly less); System flexibility could be provided through a • river zone is run at full capacity during most range of mechanisms including: of the irrigation season; • policy instruments. For example, charging for • it takes four days for a change in release at irrigation water ordered but not used, a system- Hume Dam to reach Yarrawonga Weir; wide reduction in allocation or trading rules developed to protect environmental values; • the rate of flow reduction at Albury is limited to 6 inches a day to minimise bank erosion; and • structural solutions, e.g. building a bypass channel or on-route storages; and • limited capacity within Lake Mulwala is fully committed to storage. • improving efficiency of use on-farm and throughout the system, without reallocating This problem is described more fully in DLWC any of the water saved through such efficiencies. (1996). The other priority issues for this reach are: Some of these mechanisms have been examined • reduced frequency of natural floods; and in DLWC (1996). The Panel considers that it is • constant flows. outside its role and areas of expertise to recommend which of these alternatives should 8.5 MANAGEMENT RECOMMENDATIONS be used. It is the Panel’s role to indicate the level of airspace or operational flexibility 8.5.1 Unseasonal Summer–Autumn Flooding required to protect the environmental values of – Very High Priority this river zone. The Panel requested the MDBC to undertake In this river zone, the low channel capacity of the modelling to determine the level of flexibility Barmah Choke is the driving factor of water required to reduce the level of summer flooding management. It limits the amount of water that in this river zone. The MDBC modelled the can be conveyed downstream to meet the required options where the equivalent of 10%, 20% and demand or to pass on minor flow variations 50% system flexibility was provided during the generated upstream. Current demand is only met summer months only (i.e. the maximum by running the river channel at full capacity for permissible release rate at Yarrawonga was most of the year, i.e. during the irrigation season decreased from 10,600 ML/day to 9600, 8600 from mid August to May. To add additional and 5600 ML/day). The costs of these options in capacity, water is often routed through the terms of lost productivity ranged from $660,000 Mulwala Canal via the Edward and Finley Escapes per year (10% reduction) to $2.6m per year for and, less frequently, through the Yarrawonga a 20% reduction and $11m per year for a 50% Channel and the Broken Creek Escape. In some reduction (see Appendix 3, Section 3 and DLWC years (1 in 10), water has to be transferred from 1996). The effectiveness of these changes in Hume Dam to South Australia, which puts reducing the occurrence of rain rejection floods additional stress on the system. The current has not yet been assessed. situation allows no scope to store and manage excess flows caused by rain rejections and/or RECOMMENDATION Z3.1 higher tributary flows. It is the view of the Panel that the water supply During the period, December 1 to end of the system is overallocated. The Panel examined the irrigation season, Barmah Choke should be run major operating parameters but could find no below channel capacity (i.e. < 10,600 ML/day at flexibility. Therefore, the Panel believes the only Tocumwal) in order to prevent summer flooding. way to manage excess flows in this river zone is to provide some flexibility within the entire system so Rain rejection events would need to cease in order that excess flows can be accommodated. Air space to achieve this recommendation. The Panel notes
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that an extra 20 GL of system flexibility would Walpolla Islands (see Figure 3.1), and the potential capture 64% and an extra 50 GL would capture means of supplying this water. 76% of the rain rejection events. Therefore, whilst preventing rain rejection events could be achieved by 8.5.3 Constant Flows – Medium To Low Priority a range of policy instruments, the Panel encourages the MDBC to put measures in place to achieve 20 GL As discussed in section 4.1.1, the Panel would of system flexibility (e.g. channel capacity or storage normally recommend the introduction of some space), as this will have additional benefits in other variability in the flow pattern to reduce the risk of parts of the River Murray. erosion and to introduce some wetting and drying The environmental effects of any structural of instream habitats. However, given the pressures solution would obviously have to be reviewed. If on this river zone and the need to run the channel flexibility is provided by decreasing allocations, at full capacity, the Panel recognises that this would there would be other environmental benefits across create some difficulty. Therefore, the Panel would the system, particularly in terms of increasing the rather see attention given to the recommendations frequency of flooding during winter and spring. An outlined above. alternative to reducing the incidence of unseasonal flooding is to construct regulators that isolate RECOMMENDATION Z3.4 wetlands in summer and autumn, and allow them to dry out. If the opportunity arises, some variation in flow should be introduced providing this does not RECOMMENDATION Z3.2 increase the risk of summer flooding of wetlands.
The Panel insists on caution in using regulators and 8.5.4 Conservation of Anabranch Channels – recommends that a set of ecological, engineering Medium Priority and hydrological guidelines for the use of The Panel recognises that within this river zone, regulators to exclude high summer flows should be it is possible to improve some areas by a change developed; the ecological criteria for developing in operations within the current allocation these should be based on the impact of altered regime. However, because the entire system is linkages (two-way) between floodplain and river; over-allocated, any improvement in one area will and on local and regional benefits or disbenefits. only occur at the expense of another, i.e. the problem can be shifted around. For example, Refer to section 4.2.1 for a more detailed Appendix 3 (Section 1: 17) gives some options to explanation of this recommendation. improve the Edward River at the off-take but these could well be at the expense of Gulpa or 8.5.2 Reduced Frequency of Winter–Spring Tuppal creeks. It needs to be clearly recognised Flooding – High Priority that there is no real operational flexibility in this river zone. Any changes in operation within the RECOMMENDATION Z3.3 current allocation requirements will result in degradation elsewhere. Therefore one option to A review should be undertaken of the River deal with the problem of summer flooding in this Murray in this river zone to identify opportunities area is to clearly identify the areas of highest to increase the watering of targeted wetlands and conservation value, ensure that these are the reaches that would most benefit from this. protected and explicitly identify those areas that will be affected by the changes in operation as Large volumes of water would be required to ‘zones of sacrifice’. Some of the options identified augment existing floods in order to inundate an in DLWC (1996) fall into this category. adequate amount of floodplain. The Panel notes that the Victorian Department of Natural Resources RECOMMENDATION Z3.5 and Environment is examining ways of increasing the flooding frequency of the Barmah-Millewa A study be undertaken to determine the Forest using the 100 GL environmental allocation comparative ecological values of the wetlands, for this area. This work is investigating flooding and anabranches (including the Edward River) and additional water requirements for Barmah-Millewa creeks and identify areas of high conservation Forest and other downstream floodplains, including value in order to nominate areas which might be Gunbower Forest, Hattah Lakes, and Lindsay and used as zones of sacrifice.
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9–ZONE 4–TORRUMBARRY WEIR TO WENTWORTH
Features in this river zone of the River river. The target minimum flow downstream of Euston Weir is defined as 2500 ML/day plus Murray are influenced by regulatory sufficient flow for the six main Sunraysia weirs at Torrumbarry, Euston, pumped diversions. This corresponds to a minimum flow of between 3900 ML/day in Mildura and Wentworth, and the weir summer and 2550 ML/day in winter. Although pools of each structure (Figure 9.1). releases are made from Hume Dam to supply these flows, there are occasions, typically in 9.1 HYDROLOGIC MANAGEMENT February and March, when it is necessary to draw upon the Euston Weir pool. The There are four regulatory structures between maximum drawdown for this purpose, Torrumbarry and Wentworth. occurring during the 1982 drought, was 2.7 m Torrumbarry Weir. The weir has a maximum or 27 GL. More typical drawdowns are between depth of 6 m and a mean depth of 4.8 m. During 0.5 and 1.5 m (6 to 18 GL). The Euston Weir the irrigation season from mid August to mid May, pool is directly connected to Lake Benanee and Torrumbarry Weir is operated to hold the weir its associated wetlands. Drawing down the weir pool level high enough for water to be diverted pool affects recreational users and some pump through the National Channel into the offtakes on the river and the lakes. Torrumbarry irrigation system. Because the Mildura and Wentworth Weirs. The purpose of National Channel is run at capacity for most of the these two weirs is to maintain a constant water irrigation season, there is little scope for varying level for navigation and as such they have little the level of the weir pool during this period. impact on flows. These weirs are removed during However, in June and July the weir pool level has floods and are reinstalled on the flood recession. often been lowered to allow maintenance works Being downstream of the major irrigation to be undertaken. During floods the weir is fully diversions and a number of unregulated opened to allow floodwaters to pass. tributaries, the seasonality of flow in this reach Euston Weir. Euston Weir has a maximum is similar to natural seasonality. However, the depth of 4.9 m and an average depth of 2.7 m. volume of flow has been much reduced and the It was originally installed to maintain a constant frequency of flooding also reduced. These effects water level in the river for navigation. It has are demonstrated for the flow at Euston in subsequently been used to maintain flow in the Figures 9.2 and 9.3.
Figure 9.1 Map of Zone 4: River Murray between Torrumbarry Weir and Wentworth
Dareton WENTWORTH MILDURA Merbein
Red Cliffs
Euston ROBINVALE BALRANALD
Piangil Moulamein OUYEN Manangatang
Nyah NYAH West Woorinen
SWAN HILL
Speed Wakool Lake Ultima Sea Boga N Lake
Barham Koondrook
02030km10 Woomelang KERANG Cohuna Base map copyright AUSLIG Map compiled by GIS Unit, MDBC Quambatook Leitchville Gunbower BIRCH IP Pyramid
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Figure 9.2 Typical pre and post regulation flow in the River Murray at Euston
100000
90000 1932 - Pre-Regulation (some diversion) 80000 1995 - Post Regulation and development 70000
60000
50000
Euston Flow (ML/day) 40000
30000
20000
10000
0 1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec Date
In summary, in this river zone compared with present as high-level flood runners. Hence there are natural conditions: a range of floodplain features (anabranches, billabongs, cutoffs and backwaters) that may vary in • the total volume of flow has been halved; significance along the length of this river zone. • the periods of prolonged low flow are much The main channel of the River Murray generally more frequent; and increases in size with distance west, with cross-section areas increasing from 400 m2 at Swan Hill to 1200 m2 • the frequency, duration and size of floods have at Mildura. The boundary sediments of the Mallee all been reduced. Tract contain a higher silt/clay content, and therefore the shape of the channel in this reach is notably 9.2 ENVIRONMENTAL CONDITION different to that observed upstream. As a consequence, width/depth ratios along this section of river can reach This part of the River Murray displays a number of 18 whereas in the Barmah region, maximum significant features as it moves from the Riverine width/depth ratios of 12 were recorded. Since 1927 Tract to the Mallee Tract. The river channel increases there has been substantial bed aggradation, up to 3 m in complexity downstream and is characterised by at some locations, along this river zone. large, well-defined benches. Downstream of Swan Instream and floodplain condition. In the long Hill, the River Murray flows through the Mallee section of river between Torrumbarry and Euston Tract, where the many anabranch channels of the weirs, there is considerable variability in overall river Riverine Tract coalesce into a single channel. The morphology. The upper part is characterised by large main channel is confined to a broad trench cut into in-channel benches, positioned at approximately ancient marine sediments. Subtle variations in the half bank-full level, with anabranches and width of this broad trench are associated with distributaries important features connecting the river changes in the morphology of the main channel. In with the floodplain. The floodplain is somewhat the Mallee Tract, the river exhibits some contained by levee banks but is in reasonably good anabranching, with slight increases in the width of condition, with accumulations of woody debris and the trench. However, these anabranch channels leaf litter; but again, the issue of river connectivity is only flow at high discharges and are much smaller crucial. Floodplain vegetation consists of mature in size than those present in the Riverine Tract of river red gum woodland with a diverse understorey the Murray. In some sections, anabranches are of shrubs and grasses.
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Figure 9.3 Change in montly flow in the River Murray at Euston
20th Percentile 120000 100000 Current Conditions 80000 Natural Conditions 60000 40000
Flow ML/day 20000 0 Jul Oct Jan Feb Sep Dec Jun Apr Nov Mar Aug May Month
50th Percentile
12000 10 000 Current Conditions 8000 Natural Conditions 6000 4000
Flow ML/day 2000 0 Jul Oct Jan Feb Sep Dec Jun Apr Nov Mar Aug May Month
80th Percentile 12000 10 000 8000 Current Conditions 6000 Natural Conditions 4000
Flow ML/day 2000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
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Around Tooleybuc, bench complexity increases, benthic algae and bacterial biofilm on snags and other with benches at low, medium and high levels. As in submerged surfaces. Another significant factor may be the river section above Torrumbarry Weir, there the return of the Edward–Wakool Anabranch, which appears to be an accelerated loss of in-channel is presumed to be a major source of sediment, organic features. This is likely to be due to prolonged carbon and nutrients to the main river channel from constant flows, rapid falls in water level and the anabranch floodplain. It is therefore an important desnagging. Overbank flows to the floodplain appear stimulating factor for instream productivity, provided to be rare and this places a greater importance on the suspended load of sediment is not so excessive as the instream benches as a habitat and site of organic to cause siltation on submerged surfaces and restrict matter accumulation. The riparian vegetation was as light penetration. structurally diverse as on the floodplain, and some Below Torrumbarry Weir, invertebrate diversity did not improve immediately. This was presumed to isolated stands of emergent macrophytes (e.g. be due to the build up of sediments on surfaces, the Phragmites) were observed. lack of benthic algae, and the overall poor instream At the Wakool Junction, the channel environment habitat. Fish populations were similarly poor and showed a complex assemblage of physical habitats. probably dominated by carp. Further downstream at The development of a fret line along the bank, Tooleybuc, the species richness and abundance of indicating constant elevated flows, is of some concern, invertebrates had increased markedly with mayflies, and some riverbank erosion was evident, with recent caddisflies and a few shrimps being caught. Instream slumping and loss of trees. The floodplain in the habitat was improved by an increase in the mass of immediate vicinity of the Wakool Junction appeared coarse organic material, woody debris (snags) and to be very healthy, with many flood runners and deep instream vegetation. Though increasing, snag holes containing large accumulations of leaf litter and abundance was still low, but concave and convex organic debris. Consequently, maintenance of a benches at least offered possible flood refuges for suitable flooding regime is extremely important at this native fish. site. The floodplain river red gum woodlands showed Below the Wakool Junction, instream habitat a mixed age structure, from young mature woodland availability increased significantly with a good to patches of older trees. The understorey was multi- distribution of large snags, many with hollows layered young trees, lignum, river coobah, and a suitable for fish breeding. As already noted, the range of grass species, all of which were contributing increased accumulation of organic matter on the to the rich organic matter accumulation. floodplain was important. Also important was the Below Euston Weir, the river channel is contained number of flood runners connecting the river to the in a wide trough which controls the meander pattern floodplain, as these provide a channel for lateral fish of the river. Within the channel, the river undergoes movements, assuming a suitable frequency of an internal meander alternating from one side of the inundation. The flood runners were estimated to fill at discharges in excess of 40,000 ML/day and channel to the other, with eddies and backwaters overbank flooding was predicted at 60,000 ML/day. being cut-off on the inside loop of the meander. Invertebrate species richness and abundance was high These so-called ‘dead-zones’ can be important sites of at the Wakool Junction site with shrimps, prawns and growth and development for river phytoplankton. blackfly larvae dominant on logs, and Caenid mayflies The channel is dominated by large benches and present. Native fish abundance is likely to increase silcrete outcrops, while the floodplain is confined, to because of the improved instream habitat, although some extent, by levees, both anthropogenic and high numbers of carp are still present. natural. Billabongs are numerous and semi- The habitat diversity in the section from the permanent but floodplain inundation seems Murrumbidgee River confluence to almost uncommon. The age structure of river red gum on Wentworth, including the important silcrete the floodplain was somewhat skewed, with mainly outcrops, is also expected to support a reasonable young to mature regrowth but few old trees. native fish population. However, as for nearly all Understorey vegetation was simple, presumably a river zones, the physical barriers provided by the consequence, at least in part, of heavy grazing. weirs will dramatically affect fish recruitment even if Biota. The diversity and abundance of most channel habitat and food sources are good. aquatic biota (invertebrates and fish) is comparatively Water quality. From the Murrumbidgee River poor above and immediately below Torrumbarry Weir, confluence almost to Wentworth, the turbidity of but improves significantly further downstream. The the river decreases due to a combination of saline main causes for the improvement appear to be an groundwater intrusions and decreasing river current increase in habitat abundance and diversity, especially velocity (a consequence of the widening river the number and size range of snags, and improving channel noted earlier). For example, at typical water clarity with consequently richer development of summer flows of 5000 ML/day, the velocity
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downstream of Euston is approximately 0.05 m/sec • constant river heights causing erosion of banks compared with an eight-fold higher velocity of 0.40 and instream benches, and increased m/sec below Torrumbarry Weir. The slope of the sedimentation downstream; riverbank is also greatly decreased and this, together • increasing risk of toxic cyanobacterial blooms; with the improved water clarity, greatly increases the size and range of instream habitat for benthic • significant wetlands permanently flooded by algae and associated biofilm bacteria and fungi. This weir pools; improved algal and biofilm development supported • weirs as physical barriers to fish passage; a rich and diverse population of benthic invertebrates including prawns, shrimps, midges, • over-grazing on the floodplain in some sections; and beetles, caddisflies and waterboatmen. However, in • increased river turbidity during the summer. spite of the improved water clarity, no submerged macrophytes were observed. 9.4 PRIORITY ISSUES The decreased current velocity during the summer also raises the potential for cyanobacterial growth in The priority issues for the Torrumbarry to weir pools from Torrumbarry downstream under Wentworth Zone are: summer low flows. Indeed, the four weir pools in this part of river (including Wentworth) are on the • reduction in the frequency of inundation of flood threshold of cyanobacterial bloom problems under runners and the floodplain causing degradation typical summer flow scenarios, with significant of these habitats, and a decrease in the input of problems to be expected in dry, reduced flow summers. organic carbon from the floodplain to the river; • full weir pools during the summer and autumn 9.3 THREATENING PROCESSES which cause: The major threatening processes for this river zone • permanent inundation of some significant are largely habitat related, though constant flows wetlands; and and bank and bench erosion are still significant • increased risk of cyanobacterial blooms at problems. They include: low flows; • reduction in frequency of inundation of instream • reduction in the number of snags leading to a benches, flood runners and the floodplain; major reduction in native fish and their • reduction in the number of snags, and the invertebrate and algal food sources, (which is possibly exacerbated by elevated turbidity); and re-alignment of many of those still present, compromising their biological function; • barriers to fish passage.
TABLE 9.1 Percentage of years in which the maximum monthly flow (expressed as a mean daily flow for that month) for the year exceeded 20,000 ML/day, for natural and current (1994) conditions – Torrumbarry to Euston (based on MDBC Monthly Simulation Model; supplied by MDBC)
Station Natural Current (1994) Torrumbarry 94 49 Swan Hill 94 45 Wakool Junction 95 61 Euston 97 65
9.5 MANAGEMENT RECOMMENDATIONS currently this rule is generally met for the inundation of instream benches, which require annual flows of > 9.5.1 Reduced Frequency of Inundation of 20,000 ML/day (Table 9.1). However, the rule was Flood Runners and the Floodplain – Medium To not met for inundation of flood runners (> 40,000 High Priority ML/day) or the floodplain (> 60,000 ML/day) (see Tables 9.2 and 9.3). Note that these target discharges In examining the severity of the reduction in flooding are only visual estimates made on-site by the Panel of in-channel benches, flood runners and the using the stage-height relationship for the site. Before floodplain itself, the Panel applied the ‘minimum any specific changes are implemented, the exact 50%’ rule outlined in section 4.1.3. Modelling inundation heights (and therefore target discharges) analyses undertaken by the MDBC show that should be determined by survey.
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TABLE 9.2 Natural and current (1994) frequency of 40,000 ML/day floods with 2 month duration between August and December – Torrumbarry to Wentworth (supplied by MDBC)
Station Natural Current (1994) Wakool Junction 50 16 Euston 71 30 Wentworth 84 31
Although the minimum 50% rule was generally floods > 20,000 ML/day for individual months, only met for the 20,000 ML/day flood, the Panel is not annual maxima. For individual months, the 50% suggesting that the flooding regime is entirely rule is broken in every month of the year, except for satisfactory from an ecological perspective. It merely August and September at Euston. indicates that the flooding frequency is not below a The estimated frequencies of natural and current level deemed to be totally unacceptable. Further, the flows > 40,000 ML/day (to fill flood runners for a figures in Table 9.1 do not describe the frequency of duration of at least two months) in the natural flood TABLE 9.3 Natural and current (1994) frequency of 60,000 ML/day floods with at least 1 month duration between August and December – Torrumbarry to Wentworth (supplied by MDBC)
Station Natural Current (1994) Wakool Junction 34 11 Euston 54 15 Wentworth 61 21
period from August to December are presented in wetlands (as discussed in section 4.1.3) are: Table 9.2. It can be seen that the Panel’s minimum • conservation of existing flood events; 50% rule is not met. At Wakool Junction the rule is satisfied only for two-month duration floods of less • enhanced watering of selected wetlands; and than 20,000 ML/day. At Euston the rule is satisfied • conservation of a reduced floodplain. for two-month floods of less than 26,000 ML/day while at Wentworth the criterion is only met for a) conservation of existing flood events flows less than 30,000 ML/day. This general option is not applicable in this zone of the The reduction in frequency floods of River Murray. The flow restrictions imposed by Barmah 60,000 ML/day, required to inundate the floodplain Choke and the absence of any large storages in sections for at least one month, is shown in Table 9.3. This immediately upstream, mean that flood events are not table shows that the Panel’s 50% criterion for mitigated through upstream pre-release strategies. The flooding of the floodplain is not met. The Panel reduction in flooding frequency is due to an overall decline in flood events being passed downstream from believes that these dramatically decreased flood the headwater storages or from tributaries. frequencies are causing substantial degradation of the floodplain vegetation and function. In general, the b) enhanced watering of selected wetlands major options available to deal with the decrease in Increasing the current flooding regime would require flood frequency of the flood runners and floodplain a general decrease in demand (i.e. allocation) for TABLE 9.4 Natural and current (1994) frequency of selected floods in the Torrumbarry to Wentworth river zone under 20% reduced demand scenario (supplied by MDBC)
Station Natural 1994 demand reduced by 20% Wakool Junction (40,000 ML/day) 50 24 Swan Hill (20,000 ML/day) 94 46 Euston (40,000 ML/day) 71 35 Wentworth (40,000 ML/day) 84 41
water. Modelling by the MDBC predicts that a options available for increasing the frequency of reduction in demand (Victorian and NSW flooding of some key wetlands in this river zone, e.g. diversions) of approximately 20% would be required Hattah Lakes. Assessment of flooding requirements to meet the Panel’s 50% rule (Table 9.4) Obviously for individual wetlands was beyond the charter of there would be a significant cost associated with such the Scientific Panel. a demand reduction. There are possibly other
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RECOMMENDATION Z4.1 should be assessed to determine the changes to snagging and resnagging over time. A review should be undertaken of the River Murray In key areas for native fish conservation and in this river zone to identify opportunities to restoration, an appropriate density and distribution increase the watering of targeted wetlands and the of snags be reintroduced. Key areas, methods and areas that would most benefit from this. implementation issues should be determined by the MDBC Fish Management Committee with funding c) conservation of a reduced floodplain – High Priority to be provided through Murray-Darling 2001. Revegetation and protection of riparian zones Where it is not possible to increase the frequency of along the River Murray be made a priority for floodplain inundation, the Panel considers that it funding under Murray-Darling 2001. will be necessary to explicitly conserve the reduced floodplain area to ensure that it remains a viable 9.5.3 Negative Impacts of Weir Pools – Medium component of river functioning. This could involve to High Priority land management activities, such as controls on a) preventing permanent or over-inundation of grazing and wood collection to conserve organic significant floodplain wetlands carbon and habitat diversity on the floodplain, and Lake Benanee was one example identified by the planning controls to prevent blockages to water flow Panel where direct hydrologic connection with a weir and fish passage through flood runners. In general, pool had dramatically altered the nature of a the larger the reduction in functional floodplain, the floodplain wetland. The Panel is aware that there are greater the imperative to maintain the remainder many other examples on the floodplains adjacent to free of all pressures which might impinge on its the Euston, Mildura and Wentworth Weir pools. function as part of the floodplain river ecosystem. Over watering of wetlands is undesirable.
RECOMMENDATION Z4.2 RECOMMENDATION Z4.4
Identification of floodplains or floodplain parts with In the immediate future, weir pools be drawn high functional value for special protection. down to the lowest possible level for two months Development of an ecological guide and in late winter–early spring to provide the ecological priorities amongst land management opportunity for drying of adjacent floodplain policy instruments likely to be used for all remaining wetlands and main channel. floodplain. Any future development on any part of the b) decreasing the risk of cyanobacterial blooms at floodplain be such that it does not further alienate low flows any of the floodplain, that land uses are compatible Recent research by Jones (1997) (with collaborative with ecological functioning, and that flooding and input by Peter Baker and others from the Australian flowpaths are not further impacted. Water Quality Centre, South Australia) has shown that there is a significant inverse linear relationship 9.5.2 Reduced abundance and distribution of between river flow rate and cyanobacterial snags – High Priority abundance for Torrumbarry, Euston and Lock 1 weir pools (Figure 9.4). The loss of snags from the river has probably Interestingly, the data shows that the behaviour stimulated a major reduction of instream habitat for of the three weir pools with respect to vertebrates (particularly fish), invertebrates and cyanobacterial development and flow rate is similar. benthic algae. In order to maintain the remaining This general relationship enables predictions to be snags and those created by future tree falls, and to made for target baseflows to prevent or minimise increase the density of snags in key areas of this the risk of toxic cyanobacterial blooms in the River river zone, the Panel formulated the following Murray weir pools. The relationship predicts that recommendation. severe problems can be expected at extended periods (> 1–2 weeks) of flow of less than RECOMMENDATION Z4.3 3000–4000 ML/day (relevant during the normal cyanobacterial growth period in southern The introduction of a policy to protect existing snag Australia from November to April, with December populations in terms of their number, size and to February the peak growth period). Table 9.5 position. This should cover alterations to and below shows the frequency percentiles for flows removal of snags and wood from the river, less than 4000 ML/day for the four weir pools in anabranches, channels and floodplain. This policy this river zone.
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TABLE 9.5 Percentage of years in which the mean modelled flow is less than 4000 ML/day at Torrumbarry, Euston, Mildura and Wentworth Weirs for the months from November to April under current (1994) conditions (supplied by MDBC) (Percentiles rounded to nearest 5% and major growth period shaded)
Weir Nov Dec Jan Feb Mar Apr Torrumbarry 25 40 55 5 50 15 Euston 0 0 < 5 5 40 < 5 Mildura152540355525 Wentworth < 5 < 5 5 < 5 45 20
It can be seen that under the current (1994) flow RECOMMENDATION Z4.5 regime, the statistical risk of major (Alert Level 3) cyanobacterial blooms is low (approximately 1 year For prevention of cyanobacterial problems in the in 20) for Euston and Wentworth, but considerably Alert Level 3 range (> 15,000 cells/mL for depth higher for Torrumbarry and Mildura. It is important integrated samples), river flows should not drop to note, however, that some cyanobacterial below 4000 ML/day for periods exceeding one week problems can be experienced at Alert Level 2 during the period November to April. concentrations (2000–15,000 cells/mL). For For prevention of cyanobacterial problems in the example, these problems include the need to Alert Level 2 range (2000–15,000 cells/mL for depth implement activated carbon treatment of drinking integrated samples), river flows should not drop water for the removal of the malodorous below 8000 ML/day for periods exceeding one week cyanobacterial metabolite geosmin. Alert Level 2 during the period November to April. cyanobacterial concentrations can be expected in the flow range of 4000–8000 ML/day. These flow targets should apply equally to Torrumbarry, Euston, Mildura and Wentworth weirs, although Wentworth Weir is complicated by in-flows from the Darling River. It may be possible for cyanobacterial blooms to establish in the Darling
Figure 9.4 Relationship between cyanobacterial abundance and discharge in the lower River Murray
5
4
3
2
log total cyanobacteria (cells/mL) 1
y = -1.8014x + 9.6889 2 R = 0.3971 0 3 3.5 4 4.5 5 14d mean discharge (ML/d)
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River reaches of this weir pool, even though the control of cyanobacteria in regulated rivers is the use discharge from the weir itself is high due to River of flow pulsing (Webster et al. 1997). It is possible that Murray flows. The converse situation also applies. a single flow pulse of 10,000–15,000 ML/day every 14 Another potential issue that should be addressed days may continually reset a developing cyanobacterial before a management decision to increase baseflows population and prevent bloom formation. However, for cyanobacteria is implemented, is that elevated this is an untested hypothesis for the River Murray. baseflows in summer may be ‘unnatural’ for some Furthermore, MDBC analyses show that there is very rivers of the Murray-Darling Basin, and therefore little scope to provide such brief high flows by drawing contrary to the intention and principles of the down, for example, Euston Weir (because its usable environmental flows assessment process. A storage volume is only about 20,000 ML). This comparison of ‘natural’ and current 1994 flows for strategy, therefore, appears unfeasible for the Mallee the 20, 50, and 80 percentiles in the four weir pools Tract area of the River Murray. referred to in Table 9.5 can be obtained by comparing this table to Table 9.6. 9.5.4 Barriers to Fish Passage – High Priority It is apparent from Table 9.6 that flows greater than 4000 ML/day are quite ‘natural’ for this zone The importance of unobstructed fish passage along of the River Murray, particularly during the peak the river throughout the year has been highlighted growth period from December to February. in this river zone, as it has for the river as a whole. Therefore implementation of the cyanobacterial prevention flow targets outlined in RECOMMENDATION Z4.6 Recommendation Z4.5 should not be problematic from a river ecology point of view. However, it is The function and utility of each weir and/or noteworthy that river flows do drop below the 4000 regulatory structure be reviewed, using a ML/day target approximately one year in twenty for cost–benefit approach and that plans be made for the four weirs during the peak growth period. These the provision of fish passage at each barrier. comparatively rare natural low flow events Provision of fish passage at Lake Victoria should also (presumably during major droughts) may be be examined. important to the long-term function of the river, and may need to be factored into the cyanobacterial In addition to the regulating structures on the river, control planning. there is a need to provide fish passage onto and The flow target of 4000 ML/day at Euston was across the floodplain. tested using the MDBC monthly simulation model to assess its probable economic impact on river RECOMMENDATION Z4.7 operations. It has an economic cost (through lost irrigation profit) of $1.9 million per year, although The introduction of planning controls on floodplains there are some savings as the increased baseflow to prevent and/or remove blockages of watercourses provides salinity control benefits to South Australia. and anabranches that change the movement of One other potential management strategy for the water across and through floodplain systems. TABLE 9.6 Modelled natural flows (ML/month) for the 20, 50 and 80 percentile frequencies at Torrumbarry, Euston, Mildura and Wentworth weirs between November and April (supplied by MDBC) (peak cyanobacteria growth period shaded)
Weir Percentile Nov Dec Jan Feb Mar Apr Torrumbarry 20 16377 9601 6070 3935 2639 2745 50 25180 13894 8729 6360 4323 4835 80 39059 23576 13750 9811 7139 8022 Euston 20 30227 14530 9102 6163 3894 4694 50 45927 24234 13696 9808 7081 7854 80 73070 47997 24726 16681 11868 13857 Mildura 20 31900 17300 10100 6790 4050 4330 50 54100 33200 15800 11000 7400 7900 80 85300 60500 35800 18900 12700 13400 Wentworth 20 34339 18681 11449 8069 5954 6871 50 57948 35691 21111 15796 12334 12461 80 111567 77428 43644 30136 26189 26653
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10– ZONE 5– WENTWORTH TO WELLINGTON
10.1 HYDROLOGIC MANAGEMENT lake to reduce river salinity. At times when the river flow is greater than 10,000 ML/day and the river is less saline than the lake, water is flushed through The major structures in this river zone are the lake to reduce its salinity. In the past, the lake has been used to reduce the Lake Victoria and twelve locks and weirs peak of floods greater than 100,000 ML/day and to boost smaller floods to water the floodplain. In the (Figure 10.1). first case, water is released on the rising limb of the flood to create airspace, which is filled at rates of up Lake Victoria. Lake Victoria is situated in New South to 8000 ML/day at the peak of the flood. In the Wales just upstream of the South Australian border. second case, releases of up to 7000 ML/day are It is used to re-regulate surplus flows in the River made as the flood peaks. This action creates airspace Murray and to supply South Australia with its in the lake and sharper flood recessions can be entitlement flow. created if this airspace is refilled as the flood recedes. Lake Victoria construction was completed in Locks and Weirs between Euston and 1929 and when full holds 680 GL, with a surface Blanchetown. There are 12 weirs and locks on the area of 11,200 ha. It has a maximum depth of 7.4 m River Murray between Euston and Blanchetown (see and an average depth of 6.1 m. Water is fed into Table 10.1). The primary purpose of these structures is Lake Victoria via Frenchmans Creek, which leaves to aid navigation and facilitate diversion of water by the River Murray just upstream of Lock 9. Inflows, holding the pool upstream at a constant level. which are controlled by an inlet regulator, are Eleven of the weirs (the Boulé Weirs) consist generally limited to 8000 ML/day although higher of a series of concrete buttresses carrying large rates can be obtained by surcharging Lock 9 by up concrete stop logs across most of the width of the to 0.3 m. Outflows are released into Rufus River river, and a series of hinged steel trestles across which returns water to the River Murray just the remainder. These trestles carry timber panels downstream of Lock 7. to hold back the water, and when these panels As mentioned, water is released from Lake are removed, as during periods of high river flow, Victoria to meet South Australia’s entitlement. This the trestles can be lowered flat on the bed of the entitlement totals 1850 GL per year and varies from river to provide a clear waterway for passage of 7000 ML/day in summer to 3000 ML/day in winter. vessels. The weir at Mildura (a Detheridge Weir) At times when storage in Menindee Lakes exceeds consists of steel trestles provided with wheels 1300 GL, an additional 3000 ML/day is supplied to carried on a reinforced concrete foundation, on South Australia as dilution flow. South Australia which they can be drawn out of the river during receives its entitlement flow in 55% of months. floods. When in position in the river, timber drop In the remaining 45% of months, the flow is bars are laid against the steel trestles to raise the unregulated and greater than the entitlement. water level to the required height. The purpose At times when the flow to South Australia is less of these two weir types is to maintain a constant than 10,000 ML/day and the salinity in the lake is water level for navigation. As such they have less than in the river, water is flushed through the little impact on flows (but effect water levels Figure 10.1 Map of Zone 5: River Murray between Wentworth and Wellington
Dareton MORGAN WENTWORTH Merbein RENMARK Eudunda Walkerie Red Cliffs Barmera Berri
Loxton
N Sedan Base map copyright AUSLIG 02030km10 Map compiled by GIS Unit, MDBC
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TABLE 10.1 Information on the locks and weirs on the River Murray between Euston and Blanchetown (supplied by MDBC)
No Name River Distance Capacity Type of Year Completed (kms) (GL) Weir 1 Blanchetown 274 63.7 Boulé 1922 2 Waikerie 362 44.3 Boulé 1928 3 Overland Corner 431 74.4 Boulé 1925 4 Bookpurnong 516 45.3 Boulé 1929 5 Renmark 562 62.8 Boulé 1927 6 Murtho 620 43.4 Boulé 1930 7 Rufus River 697 12.9 Boulé 1934 8 Wangumma 726 21.4 Boulé 1935 9 Kulnine 765 42.5 Boulé 1926 10 Wentworth 825 44.3 Boulé 1929 11 Mildura 878 36.7 Detheridge 1927 15 Euston 1110 38.6 Boulé 1937
significantly). They are removed during floods freely in this broad trough hence there are many and are reinstalled on the flood recession. anabranch channels, cutoffs, billabongs and Being downstream of the major irrigation deflation basins. The second sub-section, near diversions and a number of unregulated Overland Corner in the River Murray, enters a tributaries, the seasonality of flow in this river 30 m deep limestone gorge section where the zone is similar to natural seasonality. However, river and floodplain are constrained to 2–3 km. the volume of flow has been much reduced, as The river's course is structurally controlled by has the frequency of flooding. These effects are local faulting in the basement rocks. The channel demonstrated for the flow at the South is characterised by long straight reaches and short Australian border in Figure 10.2. angular reaches where the river abuts the In summary, changes in this river zone limestone cliffs. The morphological diversity of compared with natural conditions include: wetlands is much less than in the valley section • the mean volume of flow at the border is (Walker and Thoms 1993). The channel morphology of the lower River 44% of the natural flow; Murray is complex, with many in-channel • periods of prolonged low flow are much benches, typical of semi-arid rivers. The channel more frequent, with entitlement flow being gradient is low and the overall position of the received in 55% of months; and channel stable over the last 100 years. There • the frequency, duration and size of floods have been significant, but variable, cross- have all been reduced. sectional changes. These are associated with the construction of locks and weirs, and flow regulation. In reaches below weirs, width/depth 10.2 ENVIRONMENTAL CONDITION ratios have increased due to erosion. In some reaches, the in-channel benches have been The lower River Murray, the river below the completely eroded. The benches are still intact in Darling Junction, is contained within the Lower the middle sections of each weir pool and provide Murray Tract. This river zone is strongly shelving banks that encourage the development of influenced by flows from the Darling River, littoral communities and storage of organic although this has been disproportionately material. In the actual weir pool many of these increased by regulation in the River Murray features are now completely inundated. (Walker and Thoms 1993). There are two main The entire lower River Murray is grossly sub-sections in the lower River Murray. The first modified by the presence of locks and weirs. For is the valley section, which extends from the the most part, under normal regulated flows, the Darling Junction to Overland Corner. Here the River Murray is a series of pools and only retains river and floodplain are contained within a broad the character of a flowing river in high flow events. valley up to 10 km wide. The river meanders Sediment cores from the weir pools show an
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Figure 10.2 Change in monthly flow in the River Murray at South Australian border
20th Percentile 120000 100000 Current Conditions Natural Conditions 80000 60000 40000
Flow ML/day 20000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Aug Mar May Month
50th Percentile 120000 100000 Current Conditions Natural Conditions 80000 60000 40000
Flow ML/day 20000 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Aug Mar May Month
80th Percentile 120000 100000 Current Conditions Natural Conditions 80000 60000 40000
Flow ML/day 20000 0 Jul Oct Jan Feb Sep Dec Jun Apr Nov Mar Aug May Month
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abrupt transition to regulated conditions. Before number of areas, particularly those at the upstream regulation the riverbed was dominated by coarse end of weir pools and those furthest away from the sands whereas now it comprises fine silts and clays. river, are affected by a reduction in the frequency Corresponding to this change in the sediment and of flooding. Some of these areas further away from flow regime are changes in the diatom and benthic the river are also being affected by rising saline communities. The condition of the various habitats groundwater. This is causing tree death. are as follows. Salinisation is a major issue due to a combination Instream environment. The instream of flow reduction and natural saline areas such as environment has changed dramatically since flow Chowilla, and evaporation basins on the floodplain, regulation. Previously the in-channel environment e.g. Kararapko. Further, most of the floodplain is was dominated by large numbers of sandy bars, affected by grazing. benches and deep pools. The availability of these Wetlands just upstream of a lock are permanently physical compartments as habitat has been reduced inundated due to the presence of the weir pool. This substantially by either erosion or permanent is disrupting normal wetting/drying cycles and flooding, due to weirs. The diversity of in-channel causing wetland degradation. A considerable amount habitat has been further reduced as a consequence of woody debris has been removed from the of enhanced sedimentation. In excess of 800,000 floodplain, thus reducing the amount of organic tonnes of sediment were added to two weir pools, material available to the river. In addition, there are a as a result of bank erosion, in one year (Thoms and number of blockages on the floodplains which Walker 1989). Given the relatively high trap prevent the movement of water and fish. efficiencies of the weirs, most of this sediment now Biota. Fish populations are significantly affected blankets the riverbed and in some locations the by the presence of barriers. Fish passage will only riverbed has aggraded over 2 m. occur in this river zone during high flows when the There appears to be a reasonable amount of weirs are removed. Although a fish ladder is wood present at most sites visited although there has present at Lock 6, it causes high water velocities been significant snag removal throughout the lower which limit its use, and none of the other River Murray (see section 4.3.2). A number of other structures have adequate provisions for fish river operations have had an influence on the passage. The operation of lock gates should be fully instream environment. Dredging and river training evaluated. The disruption to the wetting/drying operations are common in those sections of the river cycles of wetlands and backwaters make these used by commercial pleasure boats. The majority of areas unattractive to native fish during flooding the dredged material is dumped on the adjacent and favours exotic species. Floods of about 40,000 floodplain modifying local floodplain habitats. ML/day are considered to be required in spring to Riverbanks. The availability of riverbank habitat ensure good Murray cod breeding and recruitment. has been virtually eliminated in the lower River There is a significant lack of snags as instream Murray. High rates of bank erosion, up to 2 m/year habitats, restricting native fish populations in this immediately downstream of locks and weirs are a river zone. Also, there is a potential issue with feature of the lower river. These are associated with water from Lake Victoria, which may be affecting prolonged wetting of the banks accompanied by fish with some form of disease. Current work rapid rates of fall after weir re-instatement following indicates there may be a correlation between floods. Bank erosion in the upper sections of the weir outbreaks of fish disease and releases from Lake pools has removed most of the bank complexity. In Victoria (Bryan Pierce pers. comm.). the lower sections, bank habitats are now Macroinvertebrate communities are now typical permanently flooded, reducing their functionality as of billabong communities, indicating the change in temporal refuges and processing areas. flow regime. In South Australia, the abundance of Riparian zone. Generally the riparian zone was macroinvertebrates had been reduced due to the in reasonable condition with river red gum and high turbidity sourced from the Darling River black box communities, although most areas have during summer. However, when the River Murray been heavily grazed. In some cases, these dominates the water supply, the macroinvertebrate communities have been affected by a reduction in numbers show evidence of some recovery. flooding frequency. However, this is partly Water quality. The change in the texture of compensated for by increased fresh groundwater sediment being transported in the lower River tables, caused by the presence of weir pools. It Murray has had an impact on turbidities. The should be noted that there is also some evidence of water is now highly turbid, particularly in summer. salinity from saline groundwater causing tree death This will have the effect of limiting instream in areas. benthic algae productivity. It may increase the risk Floodplain. The floodplain in the area upstream of cyanobacteria blooms in most of the weir pools of the gorge section is in fair to poor condition. A in summer, except in the lowest part of the river
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zone, where wind can be an ameliorating factor. 10.5.1 Unseasonal Wetting and Drying of However, this depends on the relative magnitude Fringing Riverine Wetlands – High Priority of turbidities in any summer. There are potentially good development sites for blue-green algal blooms As discussed in section 4.5.2, the presence of weir in the lower weir pools, depending on flows and pools will affect wetlands which are closely wind exposure. associated with the river in different ways. Those connected to the river at a point just upstream of 10.3 THREATENING PROCESSES the weir (referred to here as fringing riverine wetlands) will be inundated more frequently than The major threatening processes for this reach include: natural. Those occurring further upstream on the edge of the weir pool will suffer a reduction in • unseasonal wetting and drying of wetlands flooding frequency due to the effect of regulation. closely associated with the river, i.e. wetlands at For some of these wetlands and floodplain areas the upstream end of the weir pool have a which are close to the river, it is possible to reduction in flooding frequency whilst those just enhance flooding frequency via the manipulation upstream of the weir are permanently flooded; of weir levels. However, in considering these • reduction in the frequency of flooding of most wetlands and floodplain areas, the Panel is more areas of the floodplain; concerned about those wetlands suffering frequent or continuous inundation than those with a • barriers to fish passage; reduced flooding regime. • reduction in the number of snags; This zone of the river is dominated by weir pools. Associated with each weir pool is a • bank erosion downstream of weirs due to rapid groundwater mound of fresh water. This benefits rates of fall after reinstalling weirs; riparian trees and may compensate, locally, for • risk of algal blooms; reduced flooding. In these cases, targeted watering is not necessary and could have undesirable effects on • increased turbidity in summer months affecting wetland and floodplain communities by raising instream productivity with consequential groundwater tables or even waterlogging trees. impacts on the food chain; Wetlands and floodplain areas of the lower River Murray likely to be targeted for enhanced watering • saline groundwater levels affecting floodplain generally looked to be in good condition. Therefore wetlands in some areas; and the Panel considers that caution should be observed • possible impact on fish health caused by water before attempting to enhance the watering regimes released from Lake Victoria. of these wetlands and floodplain areas. More information on the influence of groundwater on 10.4 PRIORITY ISSUES these areas and the implications for groundwater tables of increasing flooding is necessary. The priority issues are: As mentioned above, the Panel is more concerned about the fringing riverine wetlands • unseasonal wetting and drying of wetlands; permanently flooded due to the presence of weirs. • reduction in the frequency of flooding of most These wetlands and the high level benches within areas of the floodplain (affecting floodplain the river need to be drained, for some period, on a health and fish breeding); regular basis to restore their functionality. Manipulation of weir levels to achieve this would • barriers to fish passage; also benefit bank and riparian vegetation. Although • bank erosion downstream of weirs due to rapid the Panel does not favour the use of regulators for rate of fall after reinstatement of weirs; this purpose in all cases, it is clear that in some cases their use may be necessary. The Panel • reduction in the number of snags; strongly recommends care when using regulators. • risk of algal blooms; and RECOMMENDATION Z5.1 • increased turbidity in summer months affecting instream productivity with consequential A set of ecological, engineering and hydrological impacts on food chain. guidelines for the use of regulators to exclude high summer flows be developed; the ecological criteria 10.5 MANAGEMENT RECOMMENDATIONS for developing these should be based on the impact of altered linkages (two-way) between floodplain Many of these priority issues have been discussed and river; and local and regional benefits or in Chapter 4. disbenefits.
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Options to deal with this issue were discussed in • enhanced wetland watering; and section 4.5.2. • conservation of a reduced floodplain. RECOMMENDATION Z5.2 a) conservation of natural flood events Fringing riverine wetlands within the influence of weir pools should not be further watered. RECOMMENDATION Z5.3 In the immediate future, weir levels be drawn down to the lowest possible level for two months To conserve flood events, abandon the practice of in late winter–early spring to provide the pre-releases from Lake Victoria. opportunity for drying of adjacent floodplain wetlands and main channel. b) enhanced wetland watering A weir be removed 6–8 weeks prior to a flood It may be possible in the upper reaches of this river with reinstatement after the flood as an experiment zone to enhance the flooding regime of some to demonstrate the benefits of reintroducing wetlands by the use of water from Lake Victoria. wetting/drying cycles. On some occasions, it will be possible to use 10,000 ML/day from Lake Victoria to augment flooding 10.5.2 Reduction in the Frequency of either by enhancing the flood peak or extending Flooding of Areas other than Fringing the duration. Further work will be required to Riverine Wetlands of the Floodplain – develop an operating schedule that would make High Priority the best use of this water. Areas of floodplain which are not as closely associated with the river are not be influenced by RECOMMENDATION Z5.4 the fresh groundwater mound associated with the weir pools. These areas are affected by a reduction Additional water in Lake Victoria be used to in the frequency of flooding. These areas include: augment floods that will water the floodplain • a number of benches upstream of the gorge that rather than fringing river wetlands. would be flooded by flows of 30,000 ML/day; c) conservation of a reduced floodplain • the floodplain upstream of the gorge that would In areas where it is not possible to increase the be flooded at flows of 100,000 ML/day; and frequency of flooding, it is necessary to accept that the floodplain in these areas is now contracted. The Panel • the floodplain downstream of the gorge that considers that it will be necessary to explicitly would be flooded at flows of 60,000 ML/day. conserve the reduced floodplain area to ensure that it remains a viable component of the river. This could The frequency of these flows has been significantly reduced, see Table 10.2. involve land management activities, e.g. controls on From Table 10.2 it can be seen that flooding of the grazing and wood collection to conserve the organic upstream benches almost satisfies the Scientific debris on the floodplain, and planning controls to Panel’s criterion that the frequency of flooding be prevent blockages to water flow and fish passage. at least 50% of the natural frequency. However, inundation of the floodplain along this entire river RECOMMENDATION Z5.5 zone occurs at less than 30% of its natural frequency. These floodplain areas will not be Identification of floodplains or floodplain parts with affected by manipulations of weir levels. As high functional value for special protection. described in section 4.1.3, options to improve the Development of an ecological guide and flooding frequency and conserve the floodplain ecological priorities amongst land management areas include: policy instruments likely to be used for all remaining floodplain. • conservation of natural flood events; Any future development on any part of the
TABLE 10.2 Percentage of years in which the maximum mean monthly flow at the South Australian border exceeds 30,000 ML/day, 60,000 ML/day and 100,000 ML/day for at least one month (supplied by MDBC)
Flow Criteria Natural Conditions Current (1994) Conditions (ML/day) (% of years) (% of years) < 30,000 94 46 < 60,000 52 14 < 100,000 26 5
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floodplain be such that it does not further alienate anabranches, channels and floodplain. This policy any of the floodplain, that land uses are compatible should be assessed to determine the changes to with ecological functioning, and that flooding and snagging and resnagging over time. flowpaths are not further impacted. In key areas for native fish conservation and restoration, an appropriate density and distribution 10.5.3 Barriers to Fish Passage – High Priority of snags be reintroduced. Key areas, methods and implementation issues should be determined by the This issue was discussed in section 4.2.2. MDBC Fish Management Committee with funding to be provided through Murray-Darling 2001. RECOMMENDATION Z5.6 Revegetation and protection of riparian zones along the River Murray be made a priority for The function and utility of each weir and/or funding under Murray-Darling 2001. regulatory structure be reviewed, using a cost–benefit approach and that plans be made for 10.5.6 Risk of Algal Blooms – Medium Priority the provision of fish passage at each barrier. Provision of fish passage at Lake Victoria should Algal bloom risk was discussed in sections 4.5.1 also be examined. and 9.5.3. In addition to the regulating structures on the river, RECOMMENDATION Z5.1O there is a need to provide fish passage onto and across the floodplain. For prevention of cyanobacterial problems in the Alert Level 3 range (> 15,000 cells/mL for depth RECOMMENDATION Z5.7 integrated samples), river flows should not drop The introduction of planning controls on floodplains below 4000 ML/day for periods exceeding two to prevent and/or remove blockages of watercourses weeks during the period November to April for the and anabranches that change the movement of weir pools in this entire zone of the River Murray water across and through floodplain systems. (not just in South Australia). For prevention of cyanobacterial problems in 10.5.4 Bank Erosion Downstream of Weirs the Alert Level 2 range (2000–15,000 cells/mL for due to Rapid Rate of Fall after Reinstalling depth integrated samples), river flows should not Weir – High Priority drop below 8000 ML/day for periods exceeding two weeks during the period November to April for As discussed in section 4.5.3, a number of reaches the weir pools in this entire zone of the River in this river zone are showing evidence of severe Murray (not just in South Australia). bank erosion due to block failure in areas downstream of the weirs. This is probably due to 10.5.7 Increased Turbidity Sourced from the rapid rates of fall after weir reinstatement. Darling River in Summer Months – High Priority RECOMMENDATION Z5.8 This issue, which was discussed in some detail in If the level upstream of a weir falls below pool level section 4.4.2, is considered to be a high priority in during weir reinstatement, the pool should not be this river zone. However, it will become a higher refilled immediately but should be filled gradually priority still if Lake Victoria is not able to be used to over the next week to avoid rapid falls in water supply the South Australian entitlement and much level downstream. of the entitlement has to be supplied by the Darling River directly from the Menindee Lakes. The Panel 10.5.5 Increase in Abundance and considers that there is a need to introduce some Distribution of Snags – High Priority variation in weir pool heights over summer to effectively increase the euphotic zone. This will also In order to maintain remaining and future snags, have the benefit of introducing some variation for and to increase the density of snags in key areas: the maintenance of bank vegetation.
RECOMMENDATION Z5.9 RECOMMENDATION Z5.11
The introduction of a policy to protect existing snag In years when Darling River water is the populations in terms of their number, size and predominant supply to South Australia, weir pool position. This should cover alterations to and heights be manipulated over a range of 30 cm (i.e. removal of snags and wood from the river, the target river height ± 15 cm) in a month.
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11– ZONE 6– LOWER DARLING RIVER AND THE GREAT ANABRANCH
This river zone comprises two distinct flows to South Australia. Under the Murray Darling Basin Agreement, river sections, the lower Darling River New South Wales has exclusive rights to the Menindee Lakes once the storage has fallen proper and the Great Anabranch below 480 GL. At other times the MDBC has the (Figure 11.1). right to release water from Menindee Lakes to supply the River Murray. MDBC releases are The Darling River spills into the Great made to help supply South Australia’s Anabranch at their junction about 55 km south entitlement flow, including a special dilution flow of Menindee, as well as through one, two or a of 3000 ML/day, whenever the Menindee Lakes complex of channels or flowpaths, depending on hold more than 1300 GL. MDBC releases are also the size of the flood. River and Anabranch then used to achieve monthly target storage levels in travel south for nearly 200 km without joining; Lake Victoria. These targets aim to have Lake the Darling River reaches the River Murray at Victoria full over summer, to reduce evaporation Wentworth and the Great Anabranch enters 15 losses in Menindee Lakes, and to draw down km further west. The combined sections make a Lake Victoria in autumn so that it can catch river zone that is geomorphologically quite surplus River Murray flows in winter and spring. distinct and diverse, with an immense MDBC releases to achieve these targets can be as floodplain and anabranch system, a spectacular high as 9000 ML/day, which is the effective series of ephemeral lakes as well as the main channel capacity in the lower Darling River. river channel. However as the storage in Menindee drops, the Figure 11.1 Map of Zone 6: Darling River 11.1 HYDROLOGIC MANAGEMENT between Minindee Lakes and Wentworth, including the Great Anabranch. Darling River This is the main tributary to the River Murray and the only tributary contributing substantial flows downstream of the Murrumbidgee confluence. Water in the Darling River comes from the uplands of northern New South Wales and southern Queensland, and from the inland plains of southern Queensland. Most of the major tributaries are upstream of Walgett, making a long travel through a flat landscape, and this influences the flow regime and the shape of the hydrograph. The source areas have a geological history and character not found in the Murray catchment, with basalt outcrops and sandy soils, and a flow regime driven by summer rainstorms and episodic and variable rain events. These make the Darling River distinct and separate from the River Murray in terms of flow regime and water quality. N The lower Darling River is impounded at Darling River Menindee. The Menindee Lakes are an extensive on-river water storage scheme, 02030km10 completed in 1960, with a flood-mitigation role. Base map copyright AUSLIG Map compiled by GIS Unit, MDBC The pool behind the weir, Lake Wetherell, and the entrained lakes, of which the largest are Dareton WENTWORTH Cawndilla, Menindee and Pamamaroo, can be MILDURA surcharged to 2000 GL. When fully surcharged, Merbein the lakes have a surface area of 490 km2, a Red Cliffs maximum depth of 12.8 m and an average depth of 4.1 m. The Menindee Lakes supply part (approximately 39%) of annual entitlement
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release rates from the Lake outlet regulators also domestic and irrigation use. None of these have reduce. This limits the maximum release rate and provision for fish passage. also constrains the operation of the individual Operations of the Menindee Lakes Scheme lakes in the Menindee Lakes scheme. are not part of this study but they impact At times when there is no requirement to directly on the lower Darling River. The Panel is release water to supply the River Murray, flows in aware that the New South Wales Department of the lower Darling River are set to supply losses Land and Water Conservation is completing a and demand in the river and to provide dilution three-year study of environmental flows for this flows. Prior to 1989, riparian flow at Weir 32, just section of the Darling River. downstream of the lakes, was set at between 100 The effect of the Menindee Lakes Scheme and and 200 ML/day. However, since then riparian its operations on the flow regime of the lower releases of up to 500 ML/day have been made. Darling River has been significant, as seen in The current rules vary the riparian release at Weir Figures 11.2 and 11.3. The seasonal pattern has 32 from 200 ML/day in winter to 350 ML/day in been shifted, instead of spring or autumn high flow summer, although the Commission has, in the periods, the higher flows now occur in summer. past, approved releases of up to 5000 ML/day for Winter flows have lost their variability and are now a week to break up algal blooms. relatively constant, with flows in the 200–500 The Menindee Lakes are also operated to ML/day range occurring 65% of the time. The reduce flooding. Pre-releases of up to incidence of bankfull flows is reduced, with flows 23,000 ML/day (NSW-DWR 1994) have been greater than 10,000 ML/day now occurring only made to create airspace to catch subsequent floods. 10% of the time compared with 25% naturally. There are three low level weirs on this Development in the Darling River catchment, 200 km section of the Darling River. Weir 32, combined with the water harvesting function of completed in 1958, is the largest and supplies the lakes and their high levels of evaporation, has town water for Menindee as well as supplying reduced flow volumes by almost 50%. the Broken Hill pumping station. Pooncarie Weir, built in 1968, is the water supply for Great Anabranch Pooncarie, while Burtundy Weir was built The Great Anabranch is normally dry, being an privately in 1942 and enlarged in 1956 for ephemeral system and carrying flood flows from
Figure 11.2 Typical pre and post regulation flows in the Darling River at Menindee.
30000 1947 - Pre-Regulation Conditions at Menindee 25000 1992 - Post Regulation Conditions at Weir 32
20000
15000
10000
Menindee/Weir 32 Flow (ML/day) Menindee/Weir 5000
0 1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec Date
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Figure 11.3 Change in monthly flow in the Darling River at Menindee.
20th Percentile 18000 Current Conditions 13000 Natural Conditions
8000
Flow ML/day 3000
-2000 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
50th Percentile 18000 Current Conditions 13000 Natural Conditions 8000
Flow ML/day 3000
-2000 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
80th Percentile 20000
15000
10000 Current Conditions 5000
Flow ML/day Natural Conditions 0 Jul Oct Jan Feb Sep Jun Dec Apr Nov Mar Aug May Month
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the Darling River (Irish 1993). In its uppermost can have in a fluvial system in a flat landscape. reaches near the Darling River, it flowed as much Water from this structure backed up and affected as two years out of three but flows large enough to Nearie Lake, requiring the construction of a reach Nearie Lake, one of the furthest down the regulator to exclude water and avoid water- line of lakes on the Anabranch, probably occurred logging. in only 45% of years. Under present conditions, the Anabranch begins to flood when flows in the 11.2 ENVIRONMENTAL CONDITION Darling reach 10,000 ML/day. Flows travel down Tandou Creek when Darling River flows reach The lower Darling River and the Great 20,000 ML/day. Extended flows and a large flood Anabranch lie in the Mallee Tract which is the are required to make the Anabranch flow down to same physiographic province as the the River Murray, such as in 1956, 1974 and 1976. Torrumbarry to Wentworth Zone (Bowler et al. Gauging records at Bulpunga show short pulsed 1977). The recent formation of worked flows up to 400 ML/day with rapid rates of rise and sedimentary materials is characterised by fall during the 1980s. riverine, lacustrine and aeolian features. The Because the lakes are in series down the river flows in a bed of alluvium, heavy grey clay Anabranch, they fill in rough sequence from the with occasional lenses of sand or gravel. There north, with the southerly lakes such as Nearie Lake are no tributaries and no bedrock outcrops, only filling last and only if the flood is big enough. One occasional lateritic sills cross the river channel, estimate is 500 GL (Briggs and Townsend 1993). such as at Burtundy. The landscape is very flat, Lakes fill via a feeder channel from the Anabranch with river fall of only 4–6 cm/km (Hoetzel and Creek and only one lake is on-channel. The Croome 1994). northern lakes are larger; with Mindona, Travellers, The channel of the Darling River formed Popio and Popiltah having surface areas of only 11,000 years ago and so it is much younger 6700–10,000 ha and a full volume of 280–580 GL. than the Great Anabranch, its parent river. The The southern lakes from Nialia to Warrawenia, are river floodplain is relatively narrow and the generally smaller and do not exceed 140 GL river channel has low sinuosity with small (Jenkins and Briggs 1995). Lake Tandou and its meanders and in some sections, such as small satellite lakes are, in geomorphic terms, part downstream of Pooncarie, is relatively straight. of the Menindee Lakes scheme. In contrast, the Great Anabranch, being the The complexity of the flowpaths and the ancestral river, has a floodplain that is at times difficulty of making accurate readings, together very wide, a river channel that is sinuous with with a shortage of information has prompted a large meanders and several large ephemeral specific investigation into the frequency of flooding lakes (Bowler et al. 1977), similar to those in down the Anabranch and how much this flooding other parts of the Murray-Darling Basin such as has been affected by water resource development the defunct Willandra Lakes system. The Great (Irish 1993). There are two main sources of Anabranch is the longest part of the anabranch change: the activities of early settlers and river system on the Darling River (Bowler et al. regulation. Early settlers lowered the main off-take 1977), other upstream parts being the over 130 years ago. Thus, by lowering the Talyawalka and Acres billabong. commence to flow threshold, they probably The large ephemeral lakes give a distinctive increased the frequency and duration of flooding. character to the Great Anabranch. They are River regulation and the development of the shallow, rounded, flat-bottomed lakes fringed by Menindee Lakes have had the opposite effect, lunettes on their eastern shore. Elsewhere in the decreasing flood frequency. In the lower reaches of basin such lunettes have been recognised as rich the Anabranch these two changes have cancelled archaeological sites of cultural significance. The each other resulting in flooding frequency in the lakes are resource-rich habitats in an otherwise arid end lakes, such as Nearie Lake, similar to the pre- and resource-poor landscape. Their dependent European frequencies (Irish 1993). biota and flora alternate between aquatic and The Anabranch supplies an annual allocation of terrestrial species. In this way the full diversity of 50,000 ML for stock and domestic use, usually in arid zone floodplains and wetlands is expressed. late winter–early spring over a period of about 3 The lakes are also used for opportunistic months (King and Green 1993). This comes cropping following natural flooding. Recognising directly from Lake Cawndilla, rather than down these two needs and developing a means of the Darling River, and the water is stored behind accommodating them has been the focus of small structures. The largest of these structures is research on these and other Darling anabranch Dam 183. Dam 183 is a strong reminder of the lakes (Jenkins and Briggs 1995). Floods, therefore, large ecological consequences that a small structure reach these lake systems in a fairly natural
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sequence, the only alteration to their hydrology been cut over as fuel for the paddle steamers. being when a recently-receded lake bed is sown to Floodplain. The floodplain of the Darling River crops and a follow up flood is excluded to protect a is not large and its condition is uncertain. The crop. Whilst the general hydrological pattern down original vegetation appears to still be dominant the Anabranch resembles the natural flow regime, over much of the area but a long history of grazing the principal threats (actual and future) are and rabbits will affect regeneration of trees and associated with structures, such as culverts, weirs, shrubs, and of understorey grasses and species retaining walls and bridges. These and the composition generally. There would be significant regularity of the stock and domestic supply create out-of-channel flows at 20–25,000 ML/day patches of changed environment, through localised through flood runners. This would have occurred ponding effects, and an altered water regime. naturally 35% of years but now only occurs 15% of years. Extensive flooding would occur at flows of Lower Darling River 80,000 ML/day. The major effect of flooding in this The Lower Darling is affected by the three weir area is to sweep organic matter into the river. The pools which were built prior to the Menindee reduction in frequency of flooding is affecting Lakes. The Darling River in this river zone consists floodplain vegetation health and input of carbon mainly of an incised channel with one small into the river. section of floodplain. Flood runners are present in Biota. There are good fish assemblages in this some areas. A description of major habitat types section of the Darling River. However, fish and their condition follows. movement, recruitment and recolonisation will be Instream environment. The river cross-section is affected by a number of barriers between the River quite complex with a number of instream benches. Murray and Menindee Lakes, by the constant flow The value of these benches is being lost. The levels which will inhibit some fish movement, and benches themselves are being eroded by constant by the reduced availability of flood runners and high flows and their habitat values are being floodplain due to reduced flooding frequency. The reduced by persistent unseasonal high flows caused weir at Menindee Lakes has allowed fish passage by the use of the Darling River to supply flows to only twice since construction, in the large floods of South Australia. Comparatively rapid rates of rise the mid 1970s and in 1990s. Large number of fish and fall within the river also occur. The constant were reported to have moved upstream during flow level and unseasonal flows have resulted in an these times. In one of the most famous fish-tagging apparent lack of macrophytes. The changed flow exercises in Australia, Golden Perch, a migratory regime has increased the risk of algal blooms. fish that had been tagged in South Australia were Riverbanks. Due to constant flows, the bank recovered in the Barwon and Balonne rivers, a face is steep with little microcomplexity and is not distance of over 1000 km which was achieved a significant habitat. Within the banks there are a because flood waters overtopped the weirs number of high level benches which would flood (Reynolds 1983). at approximately 15,000 ML/day. This level of The significance of impeded fish passage is flooding has been reduced from 60% of years to the very obvious accumulation of fish 30% of years. This reduction in the frequency of downstream of weirs in small floods, unable to flooding is affecting vegetation health and input of continue their migration upwards. Whilst it is organic material into the river. recognised that these barriers are breached Riparian zone. Almost three quarters of the during high flows, which may correlate with 250,000 ha in this combined river and Anabranch peak migration times for the adults of some zone is black box woodland, typically with a short species, they will still pose a barrier (either sparse shrubby understorey, and one fifth is lake partial or full) for the majority of times. These bed (King and Green 1993). Lignum, canegrass, issues are likely to further affect fish recruitment nitre goosefoot and sedgelands are not common, in South Australia. The status of fish nor are stands of river red gum, other than as communities in the Darling River is also likely channel fringing vegetation. The general dryness of to be affected by sustained flows, seasonal shifts the regional climate restricts river red gums to a and reduced access to breeding sites. narrow riverine fringe; the variable flow regime of Macroinvertebrate communities are typical of the past was evidently not enough to compensate lowland rivers. for this. Perennial vegetation surrounding the lakes Water quality. The Darling River typically is typically either low shrubby chenopods such as carries a high load of suspended sediment, largely burrs, salt bush, nitre goosefoot and ruby salt bush fine montmorillonite clays. Consequently water is or black box woodland sometimes, but not turbid, high in total phosphorus but low in commonly, with spiny lignum and sparse grasses. nitrogen. Algal cell counts can be high, especially Riparian vegetation along the Darling would have during low or no flows, such as on the
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Anabranch, where the critical value of 15,000 inspected were evidently subject to heavy grazing. cells/mL was exceeded in 12 of the 46 samples Fish populations were dominated by Carp. The collected over the summer–autumn of 1991 Anabranch contained very little specific habitat for (Briggs and Townsend 1993). native fish. Native species would utilise the Anabranch channel when it was freely available The Great Anabranch and the barriers present had been breached to Lakes. These are important habitats for aquatic provide fish passage. In contrast, fish in the lower biota when flooded and for terrestrial fauna at or flooded Great Anabranch are diverse and other times. Twelve of the seventeen lakes on the abundant (King and Green 1993) and include in Great Anabranch are opportunistically cropped addition to the more usual riverine species, (Jenkins and Briggs 1995), and one is in a Nature Midgeley’s carp gudgeon and reports of catfish. Reserve. Closer to Menindee, Lake Tandou and two ancillary lakes are now dedicated to cropping, 11.3 THREATENING PROCESSES representing a complete change and absolute habitat loss. The habitat value of all of these lakes is The major threatening processes for this river not specifically known but based on the section are: importance known elsewhere about the value of • barriers to fish movement affecting fish ephemeral habitats, is expected to be high. These communities; lakes were not specifically inspected due to time • reduced frequency of flooding of high level constraints. As these are the largest and most benches, flood runners and floodplain; numerous in the basin, this makes this river zone • constant flow levels causing erosion and unique. The lakes are highly significant in terms of reduced habitat availability; their contribution to terrestrial and aquatic • unseasonal flows including rapid rates of rise biodiversity and hence their ecological value, which and fall affecting biological communities; lies in their ephemerality, should be respected and • risk of algal blooms; and preserved as much as possible. • permanent inundation of parts of the Pools. The pools behind structures on the Anabranch channel completely modifying the Anabranch, have virtually no bed diversity and, Anabranch environment. because water is ponded, have high levels of sedimentation. The stable, near-permanent wet 11.4 PRIORITY ISSUES conditions provided by these pools are similar to billabongs elsewhere and are suitable for plants. In this river section, the aforementioned issues Hence macrophyte species were found, including should be viewed as of equal priority. the invasive species cumbungi Typha which is currently controlled by cattle foraging and 11.5 MANAGEMENT RECOMMENDATIONS herbicide applications. Reliable water probably also encourages the growth of deep-rooted perennials 11.5.1 Barriers to Fish Passage – High Priority such as riparian trees and shrubs. The pools have Within this 200 km river zone there are four high levels of instream productivity but also have a barriers to fish passage, three weirs and one major very high risk of algal blooms. Macroinvertebrate barrier (Menindee Lakes Scheme), making the and fish communities are those characteristic river a fragmented habitat and isolating fish in the of billabongs. lower Darling from the upstream catchment. As Floodplain. There is a natural flooding gradient these are low level weirs, they will be drowned from north to south of the Anabranch with out at relatively low flows but the value of this for flooding of the southern end probably occurring fish passage is uncertain due to velocity and less frequently than in the north. Whereas the turbulence effects around the submersed weirs. southern end may continue to have a natural flood Without the weirs, there would be a continuous frequency, the northern floodplain may be stretch of river as habitat connecting the River suffering from reduced flooding and be slowly Murray to the Menindee Lakes. This will become drying out. It is not known what effect the important should management and operations of development of the Menindee Lakes scheme has Menindee Lakes be changed to encourage a had on flow paths at the top of this reach. A grassy native fishery. understorey was absent close to anabranch pools There are two ways to achieve a continuous and channels in areas near permanent water or stretch of river habitat: drinking holes but the floodplain may be in better • remove the weirs; or condition. There is little organic matter present and virtually no regeneration. All parts of the area • build fish ladders on weirs.
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The first option would have the advantage of also RECOMMENDATION Z6.2 reinstating natural habitat features by eliminating weir pools, and reestablishing some aspects of a To conserve flood events, abandon the practice of natural flow regime. However, it would require pre-releases from Menindee Lakes. the provision of an alternative water supply for those users affected. The second option would b) enhanced wetland watering provide some form of fish passage. As these are It may be possible to enhance the flooding regime low level weirs, any fish passage device would be of selected wetlands by using water from the relatively small in size and rock fishways may be a Menindee Lakes. Opportunities should be taken cheap option, although their applicability and where possible to augment flooding by enhancing effectiveness would need to be assessed. A proper the flood peak to ensure that more areas of assessment of the most appropriate type of fish floodplain are inundated. Further work will be passage should be undertaken. required to develop an operating schedule.
RECOMMENDATION Z6.1 RECOMMENDATION Z6.3
Year round fish passage suitable for all native Further investigation into use of water from fish species be provided as a matter of urgency in Menindee Lakes to enhance floodplain watering the lower Darling River, possibly by the removal along the Darling River and Anabranch system. of weirs between the Menindee Lakes and the River Murray. 11.5.3 Constant Flows – High Priority
Although not within the area to be considered by As discussed already (see section 4.1.1), the this Panel, providing fish passage into Menindee approach of the Panel to the issue of constant flows Lakes should be investigated. is to introduce some variability into the flow regime. This would require implementing an operational 11.5.2 Reduced Frequency of Flooding – requirement for variability as done on the River High Priority Murray, partly achieved by the removal of weirs as previously recommended (recommendation Z6.1). The high level benches with their litter in the However, there will still be a need to deal with river channel are micro-sites for zooplankton constant release from Menindee Lakes. productivity. However, they require flooding to In this river zone, the Panel considers that become active and to contribute organic material there is some scope to introduce a transparent to the river. Similarly, flooding is needed to dam rule where the release hydrograph mimics provide fish passage to potential breeding sites on the inflow hydrograph in shape, even though its the floodplain and even in flood runners, as well height will be affected by the requirements to as to flush carbon into the river. Reduced flooding store or release water. The introduction of such a frequency limits this input. rule would provide a degree of variability that As previously described (see section 4.1.3), mimics natural conditions. This is consistent with options to improve the flooding frequency for this the no pre-release strategy outlined in river zone include: recommendation Z6.2.
• conservation of existing flood events; and RECOMMENDATION Z6.4 • enhanced watering for specific wetlands. During the period June to September, when a) conservation of existing flood events storages are filling, the MDBC explore the option The Panel considers that in this river zone, the of passing a percentage (e.g. 10%) of the inflows, major significance of the floodplain is the input of thereby ensuring the hydrographic characteristics organic matter into the river. Therefore, it is more (other than volume) of water flowing into important to conserve the flood peak, allowing storages are not altered and that releases reflect water to move into more areas on the floodplain the arrival and timing of these in-flowing waters. than to lengthen the duration of the flood event.
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Should this not be feasible, the Panel therefore subject to considerable uncertainty. recommends releases at a constant discharge Further monitoring and physical modelling of should be avoided. Instead releases should mimic weir pools should be undertaken to establish a natural rainfall event in the catchment by using targets of higher certainty. Historic MDBC data for a step function to vary flows as shown in Burtundy River also shows that elevated Figure 4.4. The flow would rise over two days and cyanobacterial abundance can occur with minor then recede gradually. A suitable time scale for flow pulses. This is presumably due to ‘seeding’ this variation would be 2 weeks giving two peaks from the Menindee Lakes. per month. The amplitude of the variation around the desired level should be ± 20% (in terms of 11.5.6 Permanent Inundation of Anabranch river height, not flow) truncated where necessary Channel – Medium Priority by minimum and channel capacity flows. The issue of the permanent inundation of the 11.5.4 Unseasonal Flows including Rapid Anabranch channel is a philosophical one Rates of Rise and Fall – High Priority analogous to the weir pools in the lower River Murray, except that weir pools on the Anabranch The actual flow regime for this river zone in are well established. The presence of weir pools summer is a mixture of low minimum flows with has changed the localised environment so much high flows released whenever the Menindee from the natural regime that a decision has to be Lakes are used to supply the South Australian made to either return it to a natural state or to entitlement. The result is unnaturally rapid leave it as it is. The Panel considers that the fluctuations in water levels and prolonged periods release of 50 GL of water in the Great Anabranch of high summer flows. to provide water users with only 5 GL is an inefficient use of water from an economic and RECOMMENDATION Z6.5 environmental point of view. This is an adequate reason to consider returning the Anabranch to its To deal with unnaturally rapid fluctuations in natural state in the long term. water levels and prolonged periods of high summer flows, the Panel recommends the RECOMMENDATION Z6.7 transparent dam rule, as recommended above in recommendation Z6.4. The provision of alternative means of supplying water to users along the Anabranch in order to This will not change the high summer flows but reinstate a natural flow regime. will address the issue of comparatively rapid rates of rise and fall, and will introduce some variability In the short term, the current ecological to the system. condition of the Anabranch could be improved by reversing the alien water regime. Accordingly, 11.5.5 Risk of Algal Blooms – High Priority the Panel suggests that pools within the Anabranch be managed to dry out completely There is potentially a serious risk of algal blooms for at least one month per year as this will in the weir pools over summer. discourage much of the weed growth and so favour growth of native species. RECOMMENDATION Z6.6
A minimum baseflow be provided during January, February and March of 500–1000 ML/day for periods of greater than one week.
This would act as a minimum threshold to the transparent dam rule recommended above (Recommendation Z6.4). This minimum baseflow estimate is based on very limited data and is
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GLOSSARY
abiotic non-living components of an ecosystem such as basic elements and compounds of the environment allochthonous material originating from a locality external to where it is deposited alluvial fan fan-shaped alluvial deposit formed by a stream where its velocity is abruptly decreased amplitude the distance or range from one extremity of an oscillation to the middle point anabranch a minor stream that leaves and rejoins the main river anoxic deficient of oxygen anthropogenic borne of the actions of humankind assemblage the collection of plants and/or animals that are found in a community autochthonous of sedimentary materials originating and deposited at about the same location backwater a body of stagnant water connected to a river; water held back or forced back, as by a dam, flood, tide or current bankfull the carrying capacity of a stream just prior to spilling onto the adjacent area bar a ridge of sand or gravel near or slightly above the surface and extending across or running parallel to a stream or shoreline baseflow sustained fair weather runoff bedload the solids that are carried downstream by a river, excluding those carried in suspension or solution; they range from the heavier particles, which are moved by saltation, to the largest, which are moved by normal traction; bed load has two sources, hillside material that is washed into a stream and the erosion of river banks benches in-channel morphological features benthic living on or in the bottom of a body of water billabong an old river meander that has been cut off and become isolated from the main channel. biodiversity the variety of life on earth at the genetic, species and ecosystem levels. biofilm a film of bacterial slime or matter accumulating on solid grains of a porous medium. biota all the species of plants and animals occurring within a given area (or time period). biotic refers to living components of an ecosystem, e.g. plants and animals. block a solid mass of rock or stone, with one or more plane or approximately plane faces. canopy the leafy branches of forest trees which form a dense cover for underlying vegetation; the vertical projection downward of the aerial portion of vegetation, usually expressed as a percentage of the grounds so occupied; the aerial portion of the overstorey vegetation. cap limit placed on taking water from streams in the Murray-Darling Basin. catchment the region which drains all the rainfall, other than that removed by evaporation, into a stream, which then carries the water to the sea or lake. channel the part of the river where the water usually flows; it includes the bed and the lower part of the banks. chemocline the depth where dissolved oxygen concentration goes to zero. chlorophytes complex multi-cellular algae.
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cohesive attraction between similar polar molecules, such as hydrogen bonding with water molecules. culvert a lined channel or pipe(s) for carrying a waterway beneath a road or railway. cyanobacteria photosynthetic eubacteria that have chlorophyll a and produce oxygen as a byproduct of photosynthesis. desiccation a drying out adaptation for existence during dry periods whereby a plant or animal deprives itself of moisture. detrital/detritus fragmented particulate organic matter derived from the decomposition of debris. diatoms any of numerous microscopic, unicellular, marine or freshwater algae having siliceous cell walls. diffusion net passive movement of molecules from a region where they are in high concentration to one where they are in low concentration, due to random thermal motion of molecules; passive movement of molecules along their electrochemical gradient. displacement the offset of rocks due to movement along a fault. diversion any natural or artificial method or means by which part or the whole of a river flow is taken from its natural course. diversity the distribution and abundance of different plant and animal communities within an area. drown-out flows flows that submerse the weir. dynamic changing through time. ecology the study of the interrelation between living organisms and their environment. ecosystem a community of organisms, interacting with one another, plus the environment in which they live and with which they also interact. ecotones the transition zone between two plant communities, such as that between rainforest and sclerophyll forest. eddies a current at variance with the main current in a stream of liquid or gas, especially one having a rotary, whirling motion. entrainment action whereby water carries particles along by its flow. environmental flow the flow of water required to improve environmental values of a stream and associated riparian and floodplain areas including wetland biological values. ephemeral (stream) a stream which only flows after a rain event. epilimnion the surface layer of a lake formed as a result of stratification. During summer this layer, if formed, is warmer than the bottom layer (hypolimnion). However, in winter, the situation may well be reversed if the surface temperature is lower than the temperature in the bottom layer. epiphytic displaying characteristics typical of a plant which grows upon another but from which it does not get food, water or minerals. estuarine formed in an estuary; found in estuaries. euphotic zone the zone or level in water where enough light penetrates to allow active photosynthesis. exotic an organism or species which is not native to the region in which it is found. fault a fracture in the earth’s crust which has moved so the rock strata on the two sides do not match.
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filaments a long slender cell or series of attached cells. flagellate having flagella, long lashlike appendages that serve as an organ of locomotion in certain reproductive bodies, bacteria, protozoans, etc. flashy flow flow which is sudden, high and abrupt. flood mitigation the reduction or attempted reduction in damages resulting from floods. floodplain the relatively smooth valley floors adjacent to and formed by alluviating rivers that are subject to overflow. flood runner a shallow path followed by flood waters sometimes rejoining the main trunk of the river some hundreds of kilometres downstream. flow, natural flow of a stream under natural, as opposed to regulated, conditions. flow, regulated flow of a stream that has been subjected to regulation by reservoirs, diversions or other works of humankind. fluvio-lacustrine of, pertaining to, or produced by a river or lake. generic pertaining to a genus; referring to all the members of a genus or class. geomorphology the study of structure, origin and development of the earth’s land forms. groundwater an underground zone of saturation where the soil and rock pore spaces are completely filled with water. habitat the native environment or kind of place where a given animal or plant naturally lives or grows. headwaters inflow water which flows in at the origin of a stream. hollow a depression or cavity; a valley. hydraulic conductivity the ease with which water is conducted through an acquifer; a measure of permeability to water. hydrology the science dealing with surface and groundwaters of the earth; their occurrence; circulation and distribution; their chemical and physical properties and their reaction with the environment. hypolimnion the lower layer of water within a lake. Lack of mixing of the layers often results in deoxygenation of the hypolimnion with resultant release of nutrients and other chemicals from the sediments. igneous pertaining to or of the nature of fire; (rock) – formed from magma which has cooled and solidified either at the earth’s surface (volcanic) or deep within the earth’s crust (plutonic). infiltration the slow movement of water through or into the pores of a soil or underground sediments. intermittent (stream) a stream which flows irregularly. inundation to overspread with a flood; overflow. lacustrine of or pertaining to a lake; living or occurring on or in lakes, as various animals and plants; formed at the bottom or along the shore of lakes, as geological strata. lagoon a shallow stretch of water that is partly or completely separated from the sea by a narrow strip of land. lateral linkages ecological connections between the river and its floodplain. lentic standing or static water. levee an artificial bank or length of raised ground, constructed along a stream to confine flood water to the main waterway and so protect land further away.
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lignum a tall, almost leafless shrub, common on low lying ground in the interior of Australia. linkage the act of linking; the state or manner of being linked. longitudinal linkages ecological connections between different parts of the river itself. lotic flowing water. macroinvertebrate an invertebrate (animal without a backbone) that is larger than 1 mm. macrophyte a water plant that is not an alga and which may be either floated or rooted. meander a curve in the course of a river which continually swings from side to side in wide loops, as it progresses across relatively flat country. meromictic (water) water which is permanently stratified. metabolic the act of the sum of processes or chemical changes in an organism or a single cell by which food is built up (anabolism) into living protoplasm and by which protoplasm is broken down (catabolism) into simpler compounds with the exchange of energy. metalimnion the temperature transition zone between the surface and bottom layers of water. metamorphic pertaining to or characterised by change of form, structural change, or metamorphosis. microbial pertaining to a micro-organism, usually of vegetable nature; a germ; a bacterium, especially one causing disease. middens mounds consisting of shells of edible molluscs and other refuse, marking the site of prehistoric human habitation. milfoil a herbaceous plant with finely divided leaves and small white to red flowers. monomictic (water) water which is stratified but mixing completely once a year. myrtaceous belonging to the Myrtaceae, or myrtle family of plants, which includes the myrtle, the guava, the eucalyptus, etc. neuro-toxin a chemical that causes nervous systems to malfunction, produced as offensive or defensive weapons in both plants and animals. niche the position or function of an organism in a community of plants and/or animals. nitrification conversion by soil bacteria of organic compounds of nitrogen to nitrites or nitrates by oxidation. oxic having oxygen. peak flow the maximum volume of flow attained during any given flow event. perennial (stream) a stream which flows for most of the year. permeable the ability to transmit water, measured at a rate by which water can move through soil in a given interval of time. phytoplankton plant plankton. planktonic of, or pertaining to, the small animals and plant organisms that float or drift in the water, especially at or near the surface (free-floating). point bar a deposit of sand and gravel that develops on the inside of a meander bend. pool a body of water or portion of stream that is deep and quiet relative to the main current. precautionary principle the precautionary principle states that ‘where there are threats of serious or irreversible environmental damage, lack of full scientific certainty should not be used as a reason for postponing measures to prevent environmental degradation’.
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pristine a state of ecological stability or condition existing in the absence of direct disturbance by modern man. productivity the measure of efficiency of production which implies comparison of input with output (in terms of energy). propagules any part of an organism, produced sexually or asexually, that is capable of giving rise to a new individual. pulse (flood or flow pulse) change in river stage. rain rejection flows small floods that result because of reduced demand of re-regulating due to rainfall in irrigation areas. reach a relatively homogenous section of river with a repetitious sequence of physical processes and habitat types. Individual elements (habitat types) of a reach (e.g. pools, riffles) should not be called reaches. recharge replenishment of a depleted aquifer by normal infiltration or by artificial means such as injection of surface waters. regime the prevailing system of stability within the river channel. resilience the capacity of an (e.g.) ecosystem, community or species to return to its original state following repeated alterations to that state as a result of disturbance. re-suspension the act by which particles of a solid are returned to a state where they are mixed with a fluid but are undissolved after being altered from this state. riffle shallow rapids in an open stream, where the water surface is broken into waves by obstructions wholly or partially submerged. riparian referring to or relating to areas adjacent to water or influenced by water associated with streams or rivers on geological surfaces occupying the lowest position on a watershed. riparian release the minimum release of water down a stream. riverine of or pertaining to a river. salinity the amount of sodium chloride or dissolved salts in a unit of water. This can be measured in parts per hundred (percent), parts per thousand, milligrams per litre or in units of electrical conductivity (microsiemens per centimetre at 25 degrees Celsius or ‘EC units’). scroll bars a bar displaying a spiral, coiled or rolled form. seasonality pertaining to or dependent on the seasons of the year or some particular season. sedentary abiding in one place; not migratory. sedimentary of, pertaining to, or of the nature of sediments; formed by deposition of sediment, as rock. sedimentation the deposition or accumulation of sediment. serial discontinuity a lack of continuity, uninterrupted connection or cohesion within a series. sill a tabular body of intrusive igneous rock, normally between beds of sedimentary rocks or layers of volcanic ejecta. siltation the process of deposition of silt, especially in reservoirs. sinuous having many curves, bends or turns; winding. slope an inclined or slanting surface. slumping the mass movement of incoherent sediment down a slope; sliding takes place on a definite plane which may be a structural plane (e.g. beds, joints), giving rise to slumps such as are found in clays.
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snag a dead tree or part thereof that has fallen into a stream and forms an obstruction to the flow of that stream. species richness the number of species within a community. stratification the separation of water into two layers, characterised by such differences as density, water temperature, water clarity and dissolved oxygen levels. substrate something which underlies or serves as a basis or foundation. suspended load sedimentary matter transported in suspension by a flowing stream. swale a marshy depression or a depression in groundmoraine. tectonic pertaining to the structure of the earth’s crust; referring to the forces or conditions within the earth that cause movements of the crust such as earthquakes, folds, faults and the like. temporal of or pertaining to time. thermocline the layer of water in lakes and seas that rapidly changes temperature due to seasonal temperature variations; the transitional area between the hypolimnion and epilimnion layers in a stratified waterbody. thermocline seiching the occasional rhythmical movement from side to side of the thermocline, with fluctuation of the water level. thermocline tilting the tilting, slanting, sloping or incline of the thermocline. tract a stretch or extent of land, water, etc; region. tributary a stream contributing its flow to a larger stream or other body of water. turbidity visible pollution due to suspended material in water causing a reduction in the transmission of light. understorey plants growing beneath the canopy of other plants, usually referring to grasses, forbs and low shrubs under a tree or shrub canopy. wetland an area of marsh, fen, peatland or water, either natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including an area of marine water the depth of which at low tide does not exceed six metres. zone any continuous tract or area, which differs in some respect, or is distinguished for some purpose, from adjoining tracts or areas, or within which certain distinguishing circumstances exist or are established. zooplankton animal plankton.
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REFERENCES
ARMCANZ (Agriculture and Resource Management Council of Australia and New Zealand) (1995) Water Allocations and Entitlements: A National Framework for the Implementation of Property Rights in Water. Standing Committee on Agriculture and Resource Management, Canberra. ARMCANZ and ANZECC (Australian and New Zealand Environment and Conservation Council) (1996) National Principles for the Provision of Water for Ecosystems. Sustainable Land and Water Resources Management Committee, Sub-committee on Water Resources, Sydney. Allen, J.D. (1995) Stream Ecology. Structure and function of running waters. Chapman and Hall, London. Bennison, G.L., Hillman, T.J. and Suter, P.J. (1989) Macroinvertebrates of the River Murray (Survey and Monitoring: 1980- 1985). Water Quality Report no. 3. Murray-Darling Basin Commission, Canberra. Blanch, S.J. and Walker, K.F. (1997) Littoral plant life history strategies and water regime gradients in the River Murray, South Australia. Verh. Internat. Verein. Limnol. Blanch, S.J., Burns, A., Vilizzi, L. and Walker, K.F. (1996a) Ecological effects of shallow winter-spring flooding in the lower Murray, 1995. Report to the Natural Resources Management Strategy, November 1996. Blanch, S. Walker, K.F. and Ganf, G.G. (1996b) Vallisneria: cruising on empty in the twilight zone. Australian Society of Limnology 35th congress, Berri. 29 September - 2 October 1996. Blyth, J.D., Doeg, T.J. and St. Clair, R.M. (1984) Response of the macroinvertebrate fauna of the Mitta Mitta River, Victoria, to the construction and operation of Dartmouth Dam. 1. Construction and initial filling period. Occasional Papers from the Museum of Victoria 1(2): 83-100. Boon, P., Frankenberg, J., Hillman, T., Oliver, R. and Shiel, R. (1990) Billabongs. In Mackay, N. and Eastburn, D. (eds), The Murray. Murray Darling Basin Commission, Canberra. Bormans, M. and Webster, I.T. (1993) Modelling algal distributions in the River Murray. Centre for Environmental Mechanics, Technical Report 57, report to Murray-Darling Basin Commission. Bormans, M., Maier, H., Burch, M. and Baker, P. (1997) Temperature Stratification in the lower River Murray, Australia. Implication for cyanobacterial development. Australian Journal of Marine and Freshwater Research 48: 647-654. Bowler, J. M. (1978) Quaternary climate and tectonics in the evolution of the riverine plain, Southeastern Australia. In: Landform evolution in Australasia (Ed. by J. L. Davies and M. A. J. Williams), pp. 70-112. ANU Press, Canberra. Bowler, J.M., Stockton, E. and Walker, M.J. (1977) Quaternary stratigraphy of the Darling River near Tilpa, New South Wales. Proceedings of the Royal Society of Victoria 90: 79-88. Boulton, A.J., Sheldon, F., Thoms, M.C. and Stanley, E.H. (1998) Problems and Constraints in Managing Rivers With Variable Flow Regimes, In Boon, P.J., Davies, B.R. and Petts, G.E. (eds) Global Perspectives on River Conservation: Science, Policy and Practice. Bren, L.J. (1992). Tree invasion of an intermittent wetland in relation to changes in the flooding frequency of the River Murray, Australia. Australian Journal of Ecology 17: 395-408 Briggs, S.V. and Townsend, G. (1993) Restoration Management of Nearie Lake. Final report, National Resources Management Strategy Project 209. Brookes, A. N. (1988) Channelised rivers. Wiley, Chichester. Brymner, M.H. (1985) Effects of drought on water quality in Lake Hume, a reservoir on the River Murray. Technical Report no. 3, Albury-Wodonga Development Corporation. Burgess, G. K. and Thoms, M. C. (1997) Environmental flow management in Queensland river systems. In: Hydrology and Water Resources Symposium, Vol. 24, pp. 274-278, Auckland. Cadwallader, P.L. and Backhouse, G.N. (1983) A Guide to the Freshwater Fish of Victoria. Fisheries and Wildlife Department, Ministry for Conservation, Melbourne. Cadwallader, P.L. and Lawrence, B. (1990) Fish. In: Mackay, N.J. and Eastburn, D. (eds), The Murray. Murray-Darling Basin Commission, Canberra. Crabb, P. (1997) Murray-Darling Basin Resources. Murray-Darling Basin Commission, Canberra. Cranston, P.S. and Hillman, T.J. (1992) Rapid assessment of biodiversity using 'biological diversity technicians'. Australian Biologist 5: 144-155. Croome, R. and Welch, D. (1988) Lake Dartmouth artificial destratification trial, 8 December 1987 - 3 January 1988. Report no. 95, Water, Materials and Environmental Science Branch, Rural Water Commission, Victoria.
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Davies, B.R., Thoms, M.C., Walker, K.F., O'Keeffe, J.H., and Gore, J.A. (1994) Dryland rivers: their ecology, conservation and management. In Calow, P. and Petts, G.E. (eds), The Rivers Handbook. Vol 2, Blackwell Scientific Oxford, pp. 484-512. DLWC (Department of Land and Water Conservation) (1996). Eastern Millewa Forest Investigation of Regulator Sites. Report TS 96.076. Department of Land and Water Conservation, NSW. Doeg, T.J. (1984) Response of the macroinvertebrate fauna of the Mitta Mitta River, Victoria, to the construction and operation of Dartmouth Dam. 2. Irrigation release. Occasional. Papers from the Museum of Victoria 1(2): 101-127. Ebsary, R. (1990) Effect of Dartmouth Reservoir on stream temperatures in the Mitta Mitta River. Technical Report 90/10, Murray Darling Basin Commission, Canberra. Erskine, W., Lake, P.S., Brown, P., Pendlebury, P, Pulsford, I., Banks, J., Nixon, J. and Robertson, C. (1996) Expert Panel Environmental Flow Assessment of the Snowy River Below Jindabyne Dam. Unpublished report to Snowy Genoa Catchment Management Committee, February 1996. Frankenberg, J. and Tilleard, J.T. (1991) Protecting riverbanks from erosion. Australian Planner 29: 107-110. Geddes, M. (1990) Crayfish. In: Mackay, N.J. and Eastburn, D. (eds), The Murray. Murray-Darling Basin Commission, Canberra. Gehrke, P.C. (1991) Avoidance of inundated floodplain habitat by larvae of golden perch (Macquaria ambigua Richardson): influence of water quality or food distribution? Australian Journal of Marine and Freshwater Research 42: 707-719. Hart, B., Oliver, R., Grace, M., Beckett, R., Rees, C. and Baldwin, D. (1996). Darling River ( Nutrient cycling and algal bloom triggers. In Banens, R.J. and Lehane, R. (eds) 1995 Riverine Environment Research Forum. Murray-Darling Basin Commission, Canberra. Hillman, T.J. (1995) Billabongs, floodplains, and the health of rivers. Water 22: 16-19. Hillman, T.J. (1996) Floodplains and billabongs. In Science and the Murray-Darling. Proceedings CRCFE Seminar, Mildura, October 1996. Cooperative Research Centre for Freshwater Ecology, Canberra. Hillman T.J. and Bren L. (1996) Practicalities of managing environmental flows for different ecological timescales. Wetlands for the Future, INTECOL’S V International Wetlands Conference, Perth, September 27. Hoetzel, G. and Croome, R. (1994) Long-term phytoplankton monitoring of the Darling River at Burtundy, New South Wales; incidence and significance of cyanobacterial blooms. Australian Journal of Marine & Freshwater Research 45: 747-59. Irish, J. (1993) Nearie Lake Nature Reserve: Historical frequency of inflows. TS 92.017, New South Wales Department of Water Resources. Jenkins, K. M. and Briggs, S. V. (1995) Ecological management of lakebed cropping on the lakes of the Great Anabranch of the Darling River. Final report, Australian Nature Conservation Agency. Jolly, I. (1996) The effects of river management on the hydrology and ecology of arid and semi-arid floodplains. In: Anderson, M.G., Walling, D.E. and Banes, P. (eds), Floodplain processes. John Wiley and Sons, Ltd. Jones, G.J. (1994) Weir pool conditions stimulating cyanobacterial blooms in the Murrumbidgee River. In: Roberts, J. and Oliver, R. (eds) The Murrumbidgee, Past and Present, CSIRO Division of Water Resources, Griffith. Jones, G.J. (1997) Setting target river flows for the prevention of cyanobacterial blooms in weir pools. Final Grant Report, LWRRDC, Canberra. Junk, W.J., Bayley, P.B., and Sparks, R.E. (1989) The flood pulse concept in river-floodplain systems. Canadian Special Publication in Fisheries and Aquatic Sciences. 106: 110-127. King, A. M. and Green, D. L. (1993) Wetlands of the Lower Darling River and Great Darling Anabranch. TS 93.032, Progress Report to Murray-Darling Basin Commission for the Barwon-Darling Wetland Survey, NSW Department of Water Resources. King, J.M. and Louw, D. (1998) Instream flow assessments for regulated rivers in South Africa using the Building Block Methodology. Aquatic Ecosystem Health and Restoration. (in press) Koehn, J. D. (1996) Habitats and movements of freshwater fish in the Murray-Darling Basin. In Banens, R.J. and Lehane, R. (eds) 1995 Riverine Environment Research Forum. Murray-Darling Basin Commission, Canberra. Koehn, J.D. and O'Connor, W.G. (1990) Biological Information for Management of Native Freshwater Fish in Victoria. Department of Conservation and Environment, Melbourne. Koehn, J.D., Doeg, T.J., Harrington, D.J. and Milledge, G.A. (1995) The effects of the Dartmouth Dam on the aquatic fauna of the Mitta Mitta River. Arthur Rylah Institute for Environmental Research Report to the Murray-Darling Basin Commission. Lawrence, B.W. (1991) Fish Management Plan. Murray-Darling Basin Commission, Canberra.
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Mackay, N., Hillman, T. and Rolls, J. (1988) Water Quality of the River Murray. Review of Monitoring 1978 to 1986. Murray-Darling Basin Commission, Canberra. Mallen-Cooper, M., Stuart, I.G., Hides-Pearson, F. and Harris, J. (1996) Fish migration in the River Murray and assessment of the Torrumbarry fishway. In Banens, R.J. and Lehane, R. (eds) 1995 Riverine Environment Research Forum. Murray-Darling Basin Commission, Canberra. McKinnon, L. & Shepheard, N. (1995) Factors contributing to a fish kill in Broken Creek. Victorian Naturalist 112: 93-99. MDBMC (1995) An Audit of Water Use in the Murray-Darling Basin. Murray-Darling Basin Ministerial Council, Canberra. MPPL (1990). Riparian vegetation of the River Murray. Prepared by Margules and Partners Pty Ltd, P and J Smith and Department of Conservation Forests and Lands. Report to the Murray-Darling Basin Commission, Canberra. Murray, P. and Philcox, M. (1995) An assessment of irrigation runoff quality draining from flood irrigated dairy pastures of the lower Murray. In: Nutrient management in irrigated agriculture: Research and Implementation Conference Proceedings, 19-20 June 1995, Echuca, pp 25-30. Negri, A.P. and Jones, G.J. (1995) Bioaccumulation of paralytic shellfish poisoning (PSP) toxins from the cyanobacterium Anabaena circinalis by the freshwater mussel Alathyaria condola. Toxicon. 33 (5): 667-678. NSW-DWR (1994) Menindee Lakes and the Lower Darling River: Issues in management. Summary Report, NSW Department of Water Resources. Olley, J. (1996) Sources of suspended sediment and phosphorus in the Murrumbidgee River. Report to NSW EPA and MDBC, CSIRO Division of Water Resources. Parkinson, A. (1996) Macrohabitat Use by Birds on the Ovens River Floodplain. Hons thesis, Monash University, Melbourne. Petts, G.E. (1992) Impounded Rivers. Perspectives for ecological management. John Wiley and Sons, New York. Pressey, R. L. (1986) Wetlands of the River Murray. Environmental Report 86/1, River Murray Commission, Canberra. Reynolds, L.F. (1983) Migration patterns of five fish species in the Murray-Darling River system. Australian Journal of Marine and Freshwater Research 34: 857-871. Richter, B.D., Baumgartner, J.V., Powell, J. and Braun, D.P. (1996) A method of assessing hydrologic alteration within ecosystems. Conservation Biology 10: 1163-1174. Roberts, J. (1994) Riverbanks, plants and water management. In: Roberts, J. and Oliver, R. (eds), The Murrumbidgee, past and present. CSIRO Division Water Resources, Griffith. Roberts, J., Chick, A.J., Oswald, L. and Thompson, P. (1995) Effect of carp, Cyprinus carpio L., an exotic benthivorous fish, on aquatic plants and water quality in experimental ponds. Marine Freshwater Research 46: 1171-1180. Roberts, J. and Sainty, G. (1996) Listening to the Lachlan. Sainty and Associates, Potts Point, Sydney. Roberts, J. and Sainty, G. (1997) Oral History as a tool in historical ecology: the Lachlan river as a case history. CSIRO Land and Water, Canberra. Robertson, A.I., Boon, P.I., Bunn, S.E., Ganf, G.G., Herzceg, A.L., Hillman T.J. and Walker, K.F. (1996) A Scoping Study into the Role, Importance, Sources, Transformation and Cycling of Carbon in the Riverine Environment. A Consultancy for the NRMS of the MDBC (Project R6067). Rutherfurd, I. (1991) The geomorphology of the River Murray. Monash University, Clayton, Victoria. Sainty, G. and Jacobs, S. (1990) Water plants. In: MacKay, N. and Eastburn, D. (eds), The Murray. Murray-Darling Basin Commission, Canberra. Schumm, S. A. (1988) Geomorphic hazards - Problems and prediction. Zeitschrift f¸r Geomorphologie, Supplementary Bd. 647, 17-24. Schumm, S. A. (1991) To Interpret the Earth: Ten Ways to be Wrong. Cambridge University Press, Cambridge, 133 pp. Smith, P. and Smith, J. (1990) Floodplain vegetation. In: Mackay, N. and Eastburn, D.(eds), The Murray. Murray- Darling Basin Commission, Canberra. Smith, P. and Smith, J. (1991) The significance of riparian vegetation to the Murray System and the requirements for its survival. In: Dendy, T. and Coombe, M. (eds), Conservation in Management of the River Murray system. Proceedings of the Third Fenner Conference on the Environment, South Australian Department of Environment and Planning, Adelaide.
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Specht, R.L. (1990) Forested Wetlands in Australia. In Lugo, A.E., Brinson, M. and Brown, S. (eds), Forested Wetlands. Ecosystems of the World 15. Elsevier, Amsterdam. Sullivan, C., Saunders, J. and Welch, D. (1988) Phytoplankton of the River Murray, 1980-1985. Murray-Darling Basin Commission, Canberra. Suter, P.J. (1996) Source water and its impacts on the macroinvertebrates of the River Murray in South Australia. Australian Society for Limnology 35th Congress, Berri, S.A. (Abstract only). Suter, P.J., Goonan, P.M., Boer, J.A. and Thompson, T.B. (1993) A biological and physico-chemical monitoring study of wetlands from the River Murray floodplain in South Australia. Report no. 7/93, Final Report on Project S002, Murray-Darling Basin Commission Natural Resources Management Strategy, Australian Centre for Water Quality Research, Adelaide. Thoms, M. C. (1995) The impact of catchment development on a semiarid wetland complex: the Barmah Forest, Australia. IAHS, 230, 121-130. Thoms, M. C. and Sheldon, F. (1996) River channel complexity and ecosystem processes: the Barwon Darling River, Australia. In: Frontiers in Ecology: Building the Links (Ed. by N. Klomp and I. Lunt), pp. 193-206. Elsevier, Oxford. Thoms, M. C. and Walker, K. F. (1989) Some preliminary observations of the environmental impact of river regulation in the River Murray, South Australia. South Australian Geographical Journal, 89, 1-14. Thoms, M. C. and Walker, K. F. (1992a) Channel changes related to low-level weirs on the River Murray, South Australia. In: Lowland Floodplain Rivers (Ed. by P. A. Carling and G. E. Petts), pp. 235-249. Wiley, Chichester. Thoms, M. C. and Walker, K. F. (1992b) Morphological changes along the River Murray, South Australia. In: Lowland floodplain rivers: Geomorphological perspectives. (Ed. by P. A. Carling and G. E. Petts), pp. 235-249. Wiley, Chichester. Thoms, M. C. and Walker, K. F. (1992c) Sediment transport in a regulated semi-arid river: The River Murray, Australia. Thoms, M.C., Sheldon,. F., Roberts, J., Harris, J. and Hillman, T.J. (1996) Scientific Panel Assessment of Environmental Flows for the Barwon-Darling River. New South Wales Department of Land and Water Conservation. Thorburn, P.J., Mensforth, L.J. and Walker, G.R. (1994) Reliance of creek-side river red gums on creek water. Australian Journal Marine Freshwater Research 45: 1439-43. Treadwell, S. and Koehn, J. (1997) The ecological significance of large woody debris (snags) in Australian riverine environments. Report to the Land and Water Resources Research and Development Corporation, Canberra. Wager, R. and Jackson, P. (1995) The Action Plan for Freshwater Fishes. Australian Nature Conservation Agency, Canberra. Walker, K.F. (1985) A review of the ecological effects of river regulation in Australia. Hydrobiologia 125: 111-129. Walker, K.F. (1990) Freshwater Mussels. In: Mackay, N.J. and Eastburn, D. (eds), The Murray. Murray-Darling Basin Commission, Canberra, pp. 309-316. Walker, K.F. (1996) The river snail Notoparla hanleyi: an endangered pest. Xanthopus 14: 5-7. Walker, K.F. and Hillman, T.J. (1977) Limnological Survey of the River Murray in Relation to Albury-Wodonga, 1973-1976. Report on behalf of Gutteridge, Haskins and Davey to Albury-Wodonga Development Corporation, Albury, NSW. Walker, K.F. and Thoms, M.C. (1993) Environmental effects of flow regulation on the River Murray, Australia. Regulated Rivers: Research and Management 8: 103-119. Walker, K., Boulton, A.J., Thoms, M.C. and Sheldon, F. (1994) Effects of water level changes induced by weirs on the distribution of littoral plants along the River Murray, South Australia. Australian Journal Marine Freshwater Research 45: 53-70. Walker, K.F., Sheldon, F. and Puckridge, J.T. (1995) A perspective on dryland river ecosystems. Regulated Rivers: Research and Management 11: 85-104. Ward, J.V. and Stanford, J.A., eds (1979). The Ecology of Regulated Streams. Plenum Press, New York. Ward, K.A. (1991) Investigation of the flood requirements of Moira grass plains in Barmah forest, Victoria. Department of Conservation and Environment, Melbourne. Webster, I.T., Jones, G.J., Oliver, R.L., Bormans, M. and Sherman, B.S. (1997) Control Strategies for cyanobacterial blooms in weir pools. CEM Technical Report no. 119, CSIRO, Australia. Welch, D. (1984) An Assessment of Water Quality and Monitoring at Dartmouth Reservoir. Rural Water Commission of Victoria, Melbourne. Woodyer, K.D. (1968) The Bankfull Frequency Of Rivers. Journal Of Hydrology 6: 114-142.
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APPENDIX 1 – STEERING COMMITTEE
Campbell Fitzpatrick (Chair) Dept Natural Resources and Environment (Vic)
Jane Doolan (Project Manager) Dept Natural Resources and Environment (Vic)
Gillian Dunkerly Environment Protection Authority (NSW)
Dick Francis Murray-Darling Basin Commission
David Forsythe Dept Environment, Sport and Territories (Com)
Ken Harris Dept Land and Water Conservation (NSW)
Anne Jensen Dept Natural Resources and Environment (SA)
Penny Knight Dept Land and Water Conservation (NSW)
John O’Donnell Environment Protection Authority (NSW)
Representative Dept Land and Water (QLD)
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APPENDIX 2 – PROJECT BRIEF
REVIEW OF 2. PROJECT OBJECTIVE
ENVIRONMENTAL FLOWS As stated previously, the purpose of the project FOR THE RIVER MURRAY is to identify short term actions which would AND LOWER DARLING improve the environmental flow conditions of the River Murray. Given this, the major objective of the project is: - 11 March 1996 to identify changes in river operations for the River 1. BACKGROUND Murray and Lower Darling that should result in general improvements in the environmental On 30 June 1995, the Murray Darling Basin condition of these river reaches whilst considering Ministerial Council made the decision to cap all water diversions within the Basin. The the current needs of existing water users. permanent cap on diversions is to be implemented by June 1997. In the meantime 3. SCOPE the States are to identify and resolve issues The project is limited to an investigation of the Mitta associated with this decision and to develop Mitta River below Dartmouth Dam, the mainstream arrangements to implement this Cap. A key of the River Murray and the Lower Darling River issue to be resolved before arrangements for the below the Menindee Lakes. Work on the Upper Cap can be finalised is the environmental flow Darling River will be undertaken by DLWR (NSW) regimes to be provided in both the River and DPI (Qld) and it will be the responsibility of Murray and the lower Darling River. these groups to ensure that this project and their Given the timeframe for this exercise, the best ongoing activities are integrated. way to review the current flow regimes is by the For the purposes of this study, the area has been use of expert panels. The advice of the expert divided into the following 13 reaches. panel will be considered by the Murray-Darling Basin Commission and feed into negotiations by Reach 1 Mitta Mitta River, Dartmouth Dam to each State on the actions necessary to implement Hume Reservoir the Cap within their territories Reach 2 Hume Dam to Yarrawonga Weir It is anticipated that the process will identify Reach 3 Yarrawonga Weir to Tocumwal short term actions to improve the environmental flow Reach 4a Tocumwal to Barmah regime of the River Murray which may be Reach 4b Tocumwal to Deniliquin implemented by the MDBC. It should be noted that Reach 5 Edward and Wakool Rivers this will not herald a return to natural or Reach 6 Barmah to end of Euston Weir pool pristine conditions for the Murray. It will simply Reach 7 Euston Weir to Wentworth (Lock 10) identify a range of actions that could be Reach 8 Lock 10 to lock 5 undertaken and that would provide some Reach 9 Lock 5 to lock 3 improvement in environmental values. These Reach 10 Lock 3 to lock 1 actions will be taken into account by each State Reach 11 Lock 1 to but not including Lake in their actions to cap diversions. Alexandrina The information and advice provided by the Reach 12 Darling River – Menindee Lakes to expert panel will also provide valuable Wentworth information to assist the New South Wales Reach 13 Darling Anabranch Environment Protection Authority if it decides to proceed with the development of river flow The panel is not to consider the environmental objectives for the River Murray. It should be issues within the pools of the major storages (e.g. noted that the MDBC needs to determine how Dartmouth, Hume dam, Lake Victoria and the the New South Wales River Flow Objective Menindee Lakes). process is to be applied to the Murray River given that the River’s flow regime is determined by 4. MAJOR TASKS FOR EXPERT PANEL statutory agreement between four governments. This project brief describes the process and In order to identify a range of short term actions outcomes for the review of current flow regimes that will improve environmental flows in the within the River Murray and Darling Rivers by specified river reaches, there are 5 broad tasks that expert panels. the expert panel will be requested to undertake for
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each reach. These include: importance in Task 1, the difference between the 1. identify the major characteristics of a flow regime current regime and the likely natural regime should which would maintain or restore biodiversity be identified. The positive or negative benefits of and ecological processes these changes should be outlined. Then, the 2. assess the impact of the current operating regime hydrological parameters considered to be of most on biodiversity and ecological processes significance in determining ecological condition 3. identify changes in current management which should be identified. would improve ecological values 4. set priorities on the possible management actions 4.3 Task 3 according to their predicted environmental benefits and a broad assessment of their ease of identify changes in current management which would implementation improve ecological values 5. predict likely environmental benefits under a range of water management and allocation Using the conclusions from Task 2, identify a range scenarios of possible changes to the water management In addition, the Panel will be requested to make any operations of the reach, predicting the likely recommendations on the integrated management environmental benefits of each. These management on the river as a whole. options should cover at a minimum: • minor changes to weir operations with no 4.1 Task 1 further allocations of water; • changes to weir operations coupled with possible identify the major characteristics of a flow regime further allocations of water and proposed which would maintain or restore biodiversity and schedules for their use; ecological processes • major changes to water allocations to meet the desired flow regime identified in Task 1; and This should involve the following steps: • any interim strategies. • establish the current status of the reach including It should include consideration of low flow, regulated an assessment of: and flood flow requirements and the need to meet • major biological habitat wetland watering requirements. It should be noted types/communities/species - instream and that the Panel is only being asked to identify possible riparian; operating regimes for each reach and not policy • condition of major habitat types; • major geomorphological influences; decisions that may have to undertaken to implement • stability of substrate; and these. In predicting likely environmental benefits, the • associated wetlands. Expert Panel should focus on the instream and This will be done using an agreed information riparian environment but also comment on the collection format. This initial assessment should be degree to which wetland watering requirements are based on literature reviews and site inspections met by the proposed operating regime. where necessary. • establish the current operating regime 4.4. Task 4 • assess how far removed the current status is from the likely natural state set priorities on the possible management • determine the major hydrological requirements actions according to their predicted (using agreed data formats) to maintain/restore environmental benefits and a broad the likely natural state of riverine ecosystems. assessment of the ease of implementation This should include an assessment of low flow, regulated flow and flood flow requirements and Rank the possible changes in operating regime take into account the requirements of associated identified in Task 3 according to their predicted wetlands. It should also take into account the ecological benefits. Then for each option, river requirements of any individual species operations experts should provide a very coarse assessment of ease of implementation. This should 4.2 Task 2 include consideration of: • impact on other users; and assess the impact of the current operating regime on • financial costs of any works required or changes biodiversity and ecological processes to operating regime. For each hydrological parameter identified to be of If possible, the management actions should be
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grouped into packages of actions that: core members who would stay with the Panel for the • can be immediately implemented with little entire task to ensure consistency of approach and to impact on other users with some idea as to the ensure an integrated overview of river management. ecological benefits likely to be derived; These would be supplemented by bringing people in to • will have a medium impact on users and major comment on their areas of expertise in the river environmental benefits; and reaches where they have knowledge. • high cost options and likely environmental benefits. 5.2 Expert Panel Process The output of this section will be used in negotiations on water allocation and changes to operating regimes It is proposed that the study would comprise of water management structures so that as much three stages. detail as possible should be given to provide information and direction into these discussions. 1. Initial Workshop It is proposed to hold an initial workshop. The aim 4.5 Task 5 of this would be to get a common understanding amongst the Panel of the project, to present the predict likely environmental benefits under Panel with relevant existing information and to a range of water management and allocation select field sites, where appropriate. scenarios Existing information given to the Panel will include for each reach: In addition to the above tasks, the Expert Panel will • pre and post regulation hydrology; be asked to comment on the likely ecological • current system operations; benefits of several water management scenarios( e.g. • cross-sectional data; see attached) and consider them within the ranking • instream and wetland environmental values; identified in Task 4. • aerial photographs; and • existing and draft recommendations for wetland 4.6 Task 6 watering. This may include a fly down the river using CD-ROM make any recommendations on the integrated if possible. The Panel would then choose field sites, management on the river as a whole where appropriate, for each river reach. They would also comment on the data collation sheets for the After consideration of the 13 reaches, the Panel exercise and the proposed methodology and tasks. should examine any commonalities in their results and make any relevant recommendations for the 2. Field Inspections integrated management of the river reaches The Panel would then undertake field inspections. It included in the study. would be desirable to undertake all field inspections in the one trip. It is estimated that this will take a 5. OPERATION OF THE EXPERT PANEL full week and should occur preferentially in late May or early June. If, however, it is not possible to 5.1 Composition of the Expert Panel get a group of experts for a week at this time, it is proposed to undertake two field trips. The first The Panel should include the following areas of incorporating Reaches 1 to 7 to be undertaken in expertise: late May/early June. The second, covering the lower • hydrology; Murray reaches could then be undertaken later, in • geomorphology; August or September. • wetland ecology; River operators responsible for operation of each • macroinvertebrate community ecology; of the river reaches will be present at the each of the • fish biology; field sites and will contribute to the Expert Panel. • macrophyte biology; • algal ecology; 3. Final Meeting • riparian zone ecology; and After the site inspections, there will be a period of • river operations. a month for iteration of the Panel's It is preferred that a single panel cover the entire 13 recommendations with a final meeting to discuss river reaches. However, if it is not possible to get and sign off the final report. experts with this breadth, it is proposed to have several
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5.3 Support to the Expert Panel 7. PROJECT MANAGEMENT The Panel will be provided with a collation of existing information at the Initial Workshop. Each The project will be overseen by a Steering State will collate information on their reaches of Committee comprising the following people interest. In addition, the MDBC will provide briefings on river operations, river cross-sections and Campbell Fitzpatrick (Chair) – Water Audit other relevant information. Committee The Panel will be provided with several support staff to assist with information collection and Jane Doolan – CNR Vic (Project Manager) collation, field site surveys and writing up reports. Penny Knights – DLWR NSW This support staff will include a project manager whose role will be to organise the entire project, Ken Harris – DLWR NSW keep the panel focused on the task and collate the Gillian Dunkerly – EPA NSW final report. In addition, the Panel will include a hydrologic John O'Donnell – EPA NSW (Albury) modeller. Anne Jensen – DERN SA
5.4 Final Report Dick Francis – MDBC David Forsythe – DEST The draft report of the Expert Panel will be collated from the field notes and data sheets collected on each Representative – DLW (Qld) reach during the field inspections. As much work as possible will be done on site. This will be collated by the project manager and circulated to the Panel for The Steering Committee will report the further work and any further modelling. The Panel results of the project to the Water Audit will have a month after the field inspections to redraft the report before the Final meeting. Committee. Project management will be provided by CNR (Vic). 6. KEY OUTPUTS
The output will be a report outlining for each reach: • current ecological values; • environmental requirements of natural habitats/communities; • the major factors of the current water regime contributing to current condition; • a range of operating regimes which would lead to improved environmental outcomes; this should be identified according to their environmental benefits and ease of implementation; and • prediction of the ecological benefits of several water management scenarios. In addition, the report will also cover any recommen- dations for the integrated management of the river.
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APPENDIX 3 – OPERATIONAL MODELLING SECTION 1
MODELLING RESULTS FOR (b) Winter release 200-600 ML/day with no THE EXPERT PANEL ON pre-releases and vary in steps of 50 ML/d. The question of pre-release rules for Dartmouth will ENVIRONMENTAL FLOWS be tested in detail in the review of the operation of IN THE RIVER MURRAY Hume and Dartmouth. It is recommended that AND LOWER DARLING modelling to assess the pre-release rules be left to that study. However discussion by the expert panel Following the first field trip of the Expert Panel a of desired environmental outcomes for the each number of studies were requested by the Panel to would be valuable to the operations review. enable it to determine the impact on existing water The original operating rules for Dartmouth did users of proposals to change the operation of the not envisage pre-release until the storage got to system. This paper discusses the results of some of within 0.6 to 1.2 metres from full supply at which those studies. time the power station operators could operate the storage as they wished (provided they allowed the REACH - DARTMOUTH TO HUME storage to be filled in spring). It is expected that those operators would have attempted to prevent Issue - Constant Flows water from spilling so as to maximise electricity generation. The power station operators have less 1 (a) Pulsed releases - 2 pulses/month with control over releases under the current airspace 20% variation in river height in summer and harmony operation. Under that operation, the target 50% variation in winter. airspace in Dartmouth is a function of the total Because this proposal does not alter the volume of airspace in Hume and Dartmouth. The harmony water released but only changes the pattern of its operation provides a degree of flood protection release, it should have no impact on existing water although it is not strictly a flood pre-release. users outside the Mitta Mitta Valley. Table 1 shows The Commission has recently adopted the the variation in flows that would be required to riparian flow policy similar to the 200-600 ML/day achieve the desired level of variations for different proposal by the expert panel. target flows in the Mitta Mitta River (based on the Riparian releases at Colemans are: rating table at Colemans which is just downstream 200 ML/d when Dartmouth is less than 60% full of the Reregulating Pond). 300 ML/d when Dartmouth is between 60 and 70% The large variation in flow for the winter pulses 400 ML/d when Dartmouth is between 70 and 80% suggests that the 50% target may be too large. Note 500 ML/d when Dartmouth is greater than 80% full. also that since the maximum normal release at This policy is now part of the benchmark Colemans is 9500 ML/d, level fluctuations would conditions for the MDBC modelling studies. have to be reduced for the higher target flows. Returning to the old policy of 200 ML/day at all
TABLE 1 Maximum and minimum flows at Colemans Station to achieve Expert Panel pulses (20% level variation in summer and 50% in winter)
Target Flow Mean Gauge Peak Summer Min Summer Peak Winter Min Winter (ML/d) Ht (m) Flow (ML/d) Flow (ML/d) Flow (ML/d) Flow (ML/d) 100 0.73 140 60 193 7 200 0.89 260 140 350 50 400 1.13 530 270 720 80 600 1.24 830 370 1070 130 1000 1.42 1380 620 1900 100 2000 1.72 2700 1300 3500 500 3000 1.93 4000 2000 5400 600 4000 2.10 5300 2700 7000 1000 5000 2.26 6500 3500 8400 1600 6000 2.40 7800 4200 10000 2000 7000 2.52 9000 5000 11600 2400 8000 2.64 10200 5800 13000 3000 9000 2.77 11300 6700 15000 3000 10000 2.80 12500 7500 15900 4100
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times (run 2480000 on Table 2) would increase twenty metres before shuttering would affect outlet average diversions by 0.9 GL/year but would reduce temperatures. Shuttering would cease to have any the expected value of hydro-electricity generation. affect once the storage had been drawn down by more than forty metres. The storage would be within (c) Not to run at channel capacity for longer this range of levels in only 20% of months. There than five days at a time. would be difficulty lowering shutters in the outlet This option will reduce the total volume of water tower in its current arrangement as the work would that can be released in a month which effectively need to take place under thirty metres of water and reduces the outlet capacity of the storage. This in would probably require divers. Despite these turn increases restrictions to irrigation supplies. consideration, shuttering using the existing tower There are two mechanisms by which this can occur: could result in higher temperatures in some months. • Inability to get water to Lake Hume fast enough to enable water to be released from Hume at a MDBC Technical report 90/10 concludes that rate that satisfies demand, and during riparian releases from Dartmouth, the • Releases from Dartmouth would have to start operation of the reregulating pond is sufficient to earlier thereby increasing the risk that water heat up the released water so that, by the time it released from Dartmouth would subsequently passes Colemans its temperature is almost the same spill from Hume thus reducing the water as the water flowing into the storage. This is shown available for use. in Figure 19 of that report. During irrigation releases Although the exact mechanism for not running at from the storage however, such as occurred in 1981, capacity was not developed, the MDBC model was 1982/83 and 1988 the release from the Dam does used to test the impact of a 10% and 20% reduction not have time to warm up and the flows in the in the Dartmouth outlet capacity. The results of Mitta Mitta River are significantly cooler than usual. these tests are presented in Table 3. They show that Figures 2 and 3 compare recorded temperatures at reducing the outlet capacity by 20% would cost Tallanddon on the Mitta Mitta River with those irrigators about $500,000/year but would benefit recorded at Jingellic on the River Murray upstream hydro-electricity by $240,000/year and reduce of Hume Dam. These figures bear out that the salinity costs by $170,000/year. A 10% reduction in temperatures are only affected at high release rates. capacity would have about half these impacts. The In this regard the adoption of the airspace harmony change in the distribution of flow for these two rules which results in medium size releases for options is shown if Figure 1. longer periods may have resulted in more prolonged (d) Maintain current rate of rise and fall for occurrences of lower temperature water. hydro demand. This does not change existing practice so it does not 3. Epilimnion deepening by aerators. require modelling. The plumbing for aerators was installed when the dam was constructed and three attempts were made Issue - Low water temperature to mix the water using them. The latest of these was in January 1988. The report into the latest test 2. Stop logs on existing offtake. concluded that the destratification facility, in its Details of the high level outlet works are that: present form, is unlikely to mix the reservoir during • the top of the outlet works are 31 metres above summer and hence increase the temperature of the the sill of the outlet pipe, water released. MDBC Technical Report 90/10 used • the height of the outlet pipe is 8.5 metres, models to test other configurations of aerators. It • the top of the works is 31 metres below full concluded that, when the storage was full, an supply level, aerator pumping four times the volume of air of the • there is provision for shutters to be fed into slots tested system could increase the autumn outlet on the outside of the tower to select the depth at temperatures by 6°C. At lower lake levels, higher which water is drawn into the tower, increases could be obtained. However the operation • no shutters are installed at present, and of such a system would be expensive. • there is provision for the tower to be extended to the surface. MCBC Technical report 90/10 contains information 4. New Variable Level Offtake. about recorded temperatures in the lake. This reveals The base of the High Level Offtake Tower was that the depth to the thermocline typically increases constructed such that it could be extended to the from 8 metres in October to 15 metres in March surface should that be required. The base of the tower before increasing to 40 metres in July and August. is also constructed to handle shutters that could be Given that an unshuttered depth of tower of at least shifted to select the offtake level. The Murray-Darling ten metres is likely to be required to draw water off, Basin Commission is yet to consider a detailed the storage would have to be drawn down by about proposal for installation of a variable level off-take.
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TABLE 2 Impact of changes to riparian releases from Dartmouth. (Note: All benefits and costs are in December 1996 dollars) Index: 2088000 Benchmark Run – includes Dartmouth Riparian Releases which increase with storage 2480000 Dartmouth Riparian Releases Set at 200 ML/day
Benchmark Difference (this run minus the Benchmark) Run number 2088000 2480000 Average Net Diversion (GL) New South Wales 1878.74 0.91 Victoria 1624.18 0.02 Anabranch Use 54.00 0.00 Tandou 55.44 0.04 Tandou Irrigation Use (GL) 33.80 0.03 Average Shortfall (GL) New South Wales 296.84 -0.52 Victoria 97.96 0.02 South Australia 9.61 0.01 Lower Darling Diversions 0.060 0.000 Broken Hill Pumps 0.050 0.000 Peak Shortfall (GL) New South Wales 2459 0 Victoria 862 -4 South Australia 416 1 % of Years With Shortfalls New South Wales 69.31 -1.98 Victoria 32.67 0.00 South Australia 14.85 0.00 % of Years With NSW Alloc < 40% 5.94 0.00 % of Years With VIC Alloc < 100% 1.98 0.00 % of Years With SA Alloc < 90% 1.98 0.00 Minimum NSW Diversion (GL) 198 0 Salinities (EC) Torrumbarry 108.07 0.09 Swan Hill 267.65 0.94 Euston 267.49 -0.04 Red Cliffs 316.70 0.00 Merbein 353.32 0.08 Lock 9 361.40 -0.05 Renmark 399.19 -0.13 Berri 431.43 -0.12 Morgan 549.23 -0.02 Murray Bridge 592.22 0.04 Average Salinity Costs ($1000/year) New South Wales 95.0 0.4 Victoria 234.4 -0.4 South Australia 65966.1 7.4 Total (a) 66295.5 7.4 Average Shortfall Costs ($1000/year) New South Wales 23154 -72 Victoria 7641 2 Total (b) 30795 -70 Value of Hydro-elec ($1000/year) (c) 8742.1 -74.4 Total Costs Less Hydro Value (a+b-c) 88318 -74.4 % Months Darling is Off Allocation 23.51 0.00 Hume Recreation Value Added ($1000/year) 2160.4 -0.9
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Figure 1 Dartmouth Flow distribution with reduced outlet capacity.
14000
12000 Base Conditions 10000 Reduce Dartmouth Release Capacity by 10% Reduce Dartmouth Release Capacity by 20 %
8000
6000
4000
2000 Monthly Discharge Expressed as ML/day
0 0 102030405060708090100 Percent of Months Exceeded
Figure 2 Comparison of Tallandoon and Jingellic Temperatures
35 Tallandoon Temperature Jingellic Temperature Dartmouth Dam Release (GL/day) 30
25
20
15
10
5
0 Jul-78 Jul-79 Jul-80 Jul-81 Jul-82 Jul-83 Jul-84 Jul-85 Jul-86 Jul-87 Jul-88 Jul-89 Jul-90 Jul-91 Jul-92 Jul-93 Jul-94 Jul-95
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Figure 3 Comparison of water temperature at Tallandoon and Jingellic.
Murray Darling Basin Commission 2 Mar 97 28 * * * * * * * 26 * * * * * * * * * * * * * * * * * * * * * * * 24 * * * * * * * ** * * * * * * * * * * * * * * * * * * *** * * * * * * * * * * * * 22 * * * * * *** * * * * * * * * * * * * * * * * * * * * * * * * * * ** * * * * * * * * * * * * * * * * 20 * * * * * * *** * * * * * * ** * * * * * * *** * * ** * * * * * * * * ** * * * * * * * * * * * * * * ** 18 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *** *** * * * * * * * * * * * * * *** * * * * * 16 * * * * * * ** *** * * * * * * * * * * * ** ** * * * * * * ** * ** * * 14 *** * * * * *** * *** * ***** * * *** * * * * * * * * ** ** * * 12 * ** * *** * * * * * * * * * * * ** * * ** * * ** * * ***** * ** 10 * ** **** ** * * ** ** * * * * *** * * ** *** ***** * * 8 * ** * * * * * *** * * * ** * * *** * * 6 * * * * * * * * ** * * 4
River Murray - Jingellic, Temperature (Deg C) River Murray - Jingellic, Temperature 2
0 0 2468 10 12 14 16 18 20 22 24 26 28 Mitta Mitta River - Tallandoon, Temperature (Deg C) Best Fit - Y=A+BX Where A = 0.8773 B = 1.0383 R2 = 0.6986
Figure 4 Impact of changed minimum flow at Euston to Flow Distribution
Murray Darling Basin Commission 2 Mar 97 400 Date range = 1891.05 to 1992.04 350 All Months
300
250
200
150
100
50 Minimum flow at Euston = 4000 ML/day Benchmark Conditions (GL/Month) 0 0 10 20 30 40 50 60 70 80 90 100 % of time exceeded
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TABLE 3 Impact of reduced outlet capacity at Dartmouth. (Note: All benefits and costs are in December 1996 dollars) Index: 2088000 Benchmark Run 2476000 Reduce Dartmouth release capacity by 10% 2479000 Reduce Dartmouth release capacity by 20%
Benchmark Difference (this run minus the Benchmark) Run number 2088000 2476000 2479000 Average Net Diversion (GL) New South Wales 1878.74 -3.13 -5.84 Victoria 1624.18 -0.23 -0.69 Anabranch Use 54.00 -0.01 -0.03 Anabranch Off Allocation 26.41 0 0.02 Tandou 55.44 0.03 0.09 Tandou Irrigation Use (GL) 33.80 0.02 0.02 Average Shortfall (GL) New South Wales 296.84 3.12 5.83 Victoria 97.96 0.23 0.42 South Australia 9.61 0.04 0.09 Lower Darling Diversions 0.060 0 0 Broken Hill Pumps 0.050 0 0 Peak Shortfall (GL) New South Wales 2459 4 12 Victoria 862 3 9 South Australia 416 2 3 % of Years With Shortfalls New South Wales 69.31 0 -0.99 Victoria 32.67 -0.99 -0.99 South Australia 14.85 0 -0.99 % of Years With NSW Alloc < 40% 5.94 0 0 % of Years With VIC Alloc < 100% 1.98 0 0 % of Years With SA Alloc < 90% 1.98 0 0 Minimum NSW Diversion (GL) 198 -1 0 Salinities (EC) Torrumbarry 108.07 -0.09 -0.30 Swan Hill 267.65 -0.58 -2.54 Euston 267.49 0.18 -0.01 Red Cliffs 316.70 0.20 0.05 Merbein 353.32 1.11 0.58 Lock 9 361.40 0.75 0.63 Renmark 399.19 0.99 0.59 Berri 431.43 0.89 0.47 Morgan 549.23 0.77 0.17 Murray Bridge 592.22 -0.10 -0.68 Average Salinity Costs ($1000/year) New South Wales 95.0 -1.8 -4.7 Victoria 234.4 -7.3 -11.5 South Australia 65966.1 17.0 -151.3 Total (a) 66295.5 7.9 -167.5 Average Shortfall Costs ($1000/year) New South Wales 23154 243 455 Victoria 7641 18 23 Total (b) 30795 261 488 Value of Hydro-elec ($1000/year) (c) 8742.1 121.7 239.6 Total Costs Less Hydro Value (a+b-c) 88318 148 80 % Months Darling is Off Allocation 23.51 0 0.09 Hume Recreation Value Added ($1000/year) 2160.4 30.2 69.1
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REACH - HUME TO YARRAWONGA namely that the 50% level fluctuation may be slightly excessive at times and not achievable at Issue - Constant Flows others because it would cause flooding in the Barmah/Millewa Forest. However it is very unlikely 5. Pulsed released - 2 pulse/month with 20% that the proposed changes would cause a loss in variation in river height in summer and 50% water resources in this part of the River. variation in winter. Vary in steps of 50 ML. This option may affect resources as it will at times REACH - CHOKE AREA involve reducing airspace in Yarrawonga which might lead to increased spill and increased rain Issue - Unseasonal Flooding rejection flows. The MDBC currently does not have a model which can test this option although this 8. Create more airspace in the system by capability may be developed for the reducing the permissible target flows at Hume–Dartmouth operations review. The change in Tocumwal flow required to achieve the expert panel’s water At time of writing, the MDBC model was being level fluctuations are presented in Table 4. The modified to test these rules. Storage in Lake Mulwala to smooth out these pulses is shown in Table 5 while the rate of fall in water 9. Reduce permissible Tocumwal target flow level needed to reach the Expert Panel targets are by 1000 ML/day presented in Table 6. These tables show that a 50% fluctuation at low 10. Reduce permissible Tocumwal target flow flows might leave the river at times with by 2000 ML/day unacceptably low flows. At higher flows the desired level fluctuations will sometimes not be able to be 11. Reduce permissible Tocumwal target flow met because they would: by 5000 ML/day • exceed channel capacity, • exceed the maximum permissible rate of fall, or 12. 10000 ML/day for 2-3 days every 10-12 days • cause water to spill from Yarrawonga. to run over weir pool. These limitations do not rule out the possibility of There is very limited scope to deliver these flows to pulsed releases but mean that the target fluctuations the Euston Weir in time for them to be effective. may need to be reduced in some instances. The only feasible option would be by drawing on the reregulating storages on the bottom of the 6. Review impact of pre-release rules on a Murrumbidgee. The MDBC has only a limited call daily basis on these resources and it would still take over a The MDBC does not currently have models suitable week for water released from them to reach Euston. to test these options although these models will be developed for the Hume and Dartmouth operations Issue - Algal bloom in weir pool review. It is recommended that the Expert Panel leave the assessment of these options to that review. 15. (a) 10000 ML/day for 2-3 days every 10-12 However, the review team would appreciate any days to turn over weir pool. comments that the expert panel might like to make There is very limited scope to deliver these flows to on the environmental desirability of changes to the the Lock 11 and Lock 10 Weir Pools in time for existing pre-release policy. them to be effective. Drawing down the Euston Weir Pool (effective storage about 20 GL) could REACH - YARRAWONGA TO TOCUMWAL provide for a short duration flush but would be followed by reduced flows as the pool refilled. Issue - Constant Flows (b) Discharge 4000 ML/day from Euston 7. Pulsed releases - 2 pulses/month with 20% between December and May This option was tested using the MDBC Monthly variation in river height in summer and 50% Simulation Model and the results are summarised in variation in winter. Vary in steps of 50 ML. Table 7. It has a significant economic impact on A table of the changes in flow at Yarrawonga that irrigators of $1.9 million per year although it correspond to the target fluctuations recommended provides considerable salinity benefits in South by the Expert Panel has not been prepared but Australia. Its impact on the flow regime at Euston would probably show similar results to Table 4 between December and May is shown in Figure 4.
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TABLE 4 Maximum and minimum flows at Doctors Point to achieve Expert Panel pulses (20% level variation in summer and 50% in winter) Note Channel Capacity = 25000 ML/d
Target Flow Mean Gauge Peak Summer Min Summer Peak Winter Min Winter (ML/d) Ht (m) Flow (ML/d) Flow (ML/d) Flow (ML/d) Flow (ML/d) 1200 1.50 1800 600 2250 150 5000 2.10 7100 2900 9000 1000 10000 2.72 12700 7300 16600 3400 15000 3.18 18500 11500 24000 6000 20000 3.60 24300 15700 30800 9200 25000 3.90 31000 19000 38300 11700
TABLE 5 Required storage in Lake Mulwala to smooth out pulsed releases from Hume (Note: The effective storage in Lake Mulwala is between 20 and 25 GL)
Target Flow (ML/d) 20% Level Variation (metres) 50% Level Variation (metres) 1200 0.040 0.100 5000 0.056 0.140 10000 0.072 0.181 15000 0.085 0.212 20000 0.096 0.240 25000 0.104 0.260
TABLE 6 Daily fall in level required at Doctors Point to achieve the Expert Panel Level Fluctuations. (Note: Water level falls in this reach restricted to 0.150 m/day)
Target Flow (ML/d) 20% Level Variation (metres) 50% Level Variation (metres) 1200 0.040 0.100 5000 0.056 0.140 10000 0.072 0.181 15000 0.085 0.212 20000 0.096 0.240 25000 0.104 0.260
It will increase the frequency of days with 6000 REACH - TOCUMWAL TO DENILIQUIN ML/d (180 GL/month) at Euston because in some occasions water released from Hume to supply this Issue - Unseasonal flooding in Wetlands extra dilution flow will prove unnecessary. 17. Least water through Edward River during REACH - BARMAH TO TORRUMBARRY Summer/Autumn. Options:
Issue - Algal bloom in weir pool (a) Least water through Edward Escape, and/or 16. 10000 ML/day for 2-3 days every 10-12 days A recent study by the Department of Land and to turn over weir pool. Water Conservation investigating management of There is very limited scope to deliver these flows to Torrumbarry Weir in time for them to be effective. rain rejections concluded that when in full The only practical way of making a release would be operational use there is currently available capacity from the Goulburn from Goulburn Weir which will of only 300 ML/day to pass rejected water orders have limited effective storage during the irrigation through escapes. In the past the MDBC has made as season. Also under the Goulburn Bulk Entitlement much use as possible of this route but it is not the Murray entitlement to call on Goulburn supplies sufficient to prevent unseasonal flooding. is limited to 30 GL per year.
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TABLE 7 Impact of increased Euston minimum flows from December to May (Note: All benefits and costs are in December 1996 dollars) Index: 2088000 Benchmark Run 2483000 Increase Euston Target Flow to 4000 ML/d from December to May
Benchmark Difference (this run minus the Benchmark) Run number 2088000 2483000 Average Net Diversion (GL) New South Wales 1878.74 -21.71 Victoria 1624.18 -1.64 Anabranch Use 54.00 -0.05 Anabranch Off Allocation 26.41 0.08 Tandou 55.44 0.08 Tandou Irrigation Use (GL) 33.80 0 Average Shortfall (GL) New South Wales 296.84 21.71 Victoria 97.96 2.54 South Australia 9.61 0.19 Lower Darling Diversions 0.060 0 Broken Hill Pumps 0.050 0 Peak Shortfall (GL) New South Wales 2459 -5 Victoria 862 -21 South Australia 416 18 % of Years With Shortfalls New South Wales 69.31 7.92 Victoria 32.67 0.99 South Australia 14.85 -0.99 % of Years With NSW Alloc < 40% 5.94 0 % of Years With VIC Alloc < 100% 1.98 0 % of Years With SA Alloc < 90% 1.98 0 Minimum NSW Diversion (GL) 198 -1 Salinities (EC) Torrumbarry 108.07 -0.28 Swan Hill 267.65 5.47 Euston 267.49 -2.31 Red Cliffs 316.70 -3.94 Merbein 353.32 -4.81 Lock 9 361.40 -3.50 Renmark 399.19 -2.61 Berri 431.43 -2.84 Morgan 549.23 -3.37 Murray Bridge 592.22 -2.03 Average Salinity Costs ($1000/year) New South Wales 95.0 -2.2 Victoria 234.4 -1.2 South Australia 65966.1 -197.1 Total (a) 66295.5 -200.5 Average Shortfall Costs ($1000/year) New South Wales 23154 1693 Victoria 7641 198 Total (b) 30795 1892 Value of Hydro-elec ($1000/year) (c) 8742.1 -79.9 Total Costs Less Hydro Value (a+b-c) 88318 1771 % Months Darling is Off Allocation 23.51 0.17 Hume Recreation Value Added ($1000/year) 2160.4 -28.7
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(c) Open Mary Ada, and/or 'Unseasonal Surplus Flow Management - Barmah Water diverted through the Mary Ada joins the Millewa Forest June 1996'. That report recommends Edward River upstream of the site of the a range of options for management which the Expert flooded wetlands so this option appears to be of Panel could consider if they wished. limited benefit. The expert panel may need to consider whether they are more interested in managing rain rejection (d) Use Tuppal Creek, and/or events or whether they are more concerned with the The offtake to Tuppal Creek only starts to flow at river continual high flows in the Edward River. The levels that are considerably higher than normal rain options that they have recommended may have rejection levels. The DLWC report has investigated some benefit in reducing the high regulated flow other options to divert rain rejections into Bullatale down the Edward River during summer and autumn. Creek and believe that these could be used to divert small events albeit at considerable cost ($6m).
(e) Use Gulpa Creek This is of limited value during rain rejection events because most of the rain rejection will have left the A. F. Close river prior to arrival at this site. 31 March 1997 The options for handling rain rejections are discussed at some length in the DLWC draft report entitled
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SECTION 2
MODELLING SCENARIOS FOR Using the rating tables for station "Mitta Mitta River THE MURRAY EXPERT PANEL at Colemans", gauge heights were determined for varying flows. For each indicated flow, the gauge Scenario 1: Dartmouth to Hume height at 10% higher and lower (summer), and 25% higher and lower (winter) were calculated and Point 1: Two pulses/month with 20% variation in river corresponding flows recorded. Table 1 summarises height (not volume) in summer and 50% in winter the results.
TABLE 1 Mean gauge height, and peak and minimum flow in summer and winter at Colemans Station
Flow Mean Gauge Peak Summer Min Summer Peak Winter Min Winter (ML/d) Ht (m) Flow (ML/d) Flow (ML/d) Flow (ML/d) Flow (ML/d) 100 0.725 140 60 193 7 200 0.89 260 140 350 50 400 1.125 530 270 720* 80* 600 1.24 830 370 1070* 130* 1000 1.415 1380 620 1900* 100* 2000 1.715 2700 1300 3500* 500* 3000 1.93 4000 2000 5400* 600* 4000 2.1 5300 2700 7000* 1000* 5000 2.26 6500 3500 8400* 1600* 6000 2.395 7800 4200 10000* 2000* 7000 2.52 9000 5000 11600* 2400* 8000 2.64 10200 5800 13000* 3000* 9000 2.765 11300 6700 15000 3000 10000 2.8 12500 7500 15900 4100 Note: * denotes flow factored down to equal the flow in the first column.
Scenario 5: Hume to Yarrawonga The model was re-run with reduced demands (Victorian and New South Wales diversions) to Point 1: Two pulses/month with 20% variation in assess the impacts on the stations Swan Hill, Wakool river height (not volume) in summer and 50% in Junction, Euston and Wentworth that did not satisfy winter the Expert Panel's targets. The frequency of Using the rating tables for station "Doctors Point", distribution of flow in each year was plotted as per gauge heights were determined for varying flows. the original analyses (including the rule line), with For each indicated flow, the gauge height at 10% the addition of reduced demand (refer to Figures 8- higher and lower (summer), and 25% higher and 11). Statistical summary sheets were also produced lower (winter) were calculated and corresponding for reduced demand. flows recorded. Table 2 summarises the results. The results indicate that a reduced demand of at least 20% is needed to satisfy the Expert Panel's Model Re-run: Reduced Demand by 20% targets, as summarised in Table 4.
TABLE 2 Mean gauge height, and peak and minimum flow in summer and winter at Doctors Point Station
Flow Mean Gauge Peak Summer Min Summer Peak Winter Min Winter (ML/d) Ht (m) Flow (ML/d) Flow (ML/d) Flow (ML/d) Flow (ML/d) 1200 1.5 1800 600 2250 150 5000 2.1 7100 2900 9000 1000 10000 2.715 12700 7300 16600 3400 15000 3.18 18500 11500 24000 6000 20000 3.6 24300 15700 30800 9200 25000 3.9 31000 19000 38300 11700
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Figure 1 River Murray at Torrumbarry – Frequency of distribution (ML/day)
70000 Natural Conditions Current Conditions 60000 Half of Natural Frequency
50000 Expressed as ML/day – 40000
30000
20000
10000
0 Maximum Monthly Flow in Year Maximum Monthly Flow in Year 0 102030405060708090100 Probabilityof being exceeded (% of Years)
Figure 2 River Murray at d/s Swan Hill – Frequency of distribution (ML/day)
40000 Natural Conditions 35000 Current Conditions Half of Natural Frequency
30000
Expressed as ML/day 25000 –
20000
15000
10000
5000 Maximum Monthly Flow in Year Maximum Monthly Flow in Year 0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
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Figure 3 River Murray at Wakool Junction
250000
200000
Natural Conditions Current Conditions Expressed as ML/day
– 150000 Half of Natural Frequency
100000
50000 Maximum Monthly Flow in Year Maximum Monthly Flow in Year 0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
Figure 4 River Murray d/s Euston Mean Monthly Flow (ML/day)
300000
Natural Conditions 250000 Current Conditions Half of Natural Frequency
200000 Expressed as ML/day –
150000
100000
50000 Maximum Monthly Flow in Year Maximum Monthly Flow in Year 0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
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Figure 5 River Murray at Wakool Junction
200000
– 180000
160000 Natural Conditions 140000 Current Conditions 120000 Half of Natural Frequency
100000
80000 Expressed in ML/day 60000
40000
20000 Maximum Continuous Two Monthly Flow in Year Monthly Flow in Year Maximum Continuous Two
0 0 10 20 30 40 50 60 70 80 90 100 % of Time Exceeded
Figure 6 River Murray d/s Euston
250000 –
200000 Natural Conditions Current Conditions Half of Natural Frequency 150000
100000 Expressed as ML/day
5000 Maximum Continuos Two Monthly Flow in Year Monthly Flow in Year Maximum Continuos Two
0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
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Figure 7 River Murray d/s of Wentworth
350000 –
300000
Natural Conditions 250000 Current Conditions Half of Natural Frequency 200000
150000 Expressed as ML/day
100000
50000 Maximum Continuos Two Monthly Flow in Year Monthly Flow in Year Maximum Continuos Two
0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
Figure 8 River Murray d/s Swan Hill – Frequency of distribution (ML/day)
40000
Natural Conditions 35000 Reduce Cap by 20% – Current Conditions 30000 Half of Natural Frequency
25000
20000
15000 Expressed as ML/day
10000 Maximum Monthly Flow in Year Maximum Monthly Flow in Year
5000
0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
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Figure 9 River Murray at Wakool Junction – Frequency of distribution (ML/day)
200000 – 180000 Natural Conditions Reduce Capacity by 20% 160000 Current Conditions Half of Natural Frequency 140000
120000
100000
80000
Expressed as ML/day 60000
40000
20000 Maximum Continuous Two Monthly Flow in Year Monthly Flow in Year Maximum Continuous Two 0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
Figure 10 River Murray d/s Euston – Frequency of distribution (ML/day)
250000 –
200000 Natural Conditions Current Conditions Half of Natural Frequency 150000 Reduce Cap by 20%
100000 Expressed as ML/day
50000 Maximum Continuous Two Monthly Flow in Year Maximum Continuous Two
0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
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Figure 11 River Murray d/s Wentworth – Frequency of distribution (ML/day)
350000 – 300000
Natural Conditions 250000 Current Conditions Half of Natural Frequency Reduce Cap by 20% 200000
150000 Expressed as ML/day 100000
50000 Maximum Continuous Two Monthly Flow in Year Maximum Continuous Two
0 0 10 20 30 40 50 60 70 80 90 100 Probability of Being Exceeded (% of Years)
TABLE 4 Frequency of years that target flow (indicated in brackets) is exceeded.
Station Natural Conditions Reduced Demand by 20% Swan Hill (20000 ML/d) 94 46 Wakool Junction (40000 ML/d) 50 24 Euston (400000 ML/d) 71 35 Wentworth (400000 ML/d) 84 41
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SECTION 3
TABLE 1 Impact of reducing the channel capacity downstream of Yarrawonga (Note: All benefits and costs are in December 1997 dollars) Index: 2486000 Benchmark – Yarrawonga Channel capacity 10, 600 ML/d Jan-May 2487000 Yarrawonga Channel capacity 9600 ML/d Jan-May 2488000 Yarrawonga Channel capacity 8600 ML/d Jan-May 2489000 Yarrawonga Channel capacity 5600 ML/d
Benchmark Difference (this run minus the Benchmark) Run number 2486000 2487000 2488000 2489000 Date of Run Average Net Diversion (GL) New South Wales 1880.73 3.16 6.50 -0.87 Victoria 1620.07 -16.62 -57.14 -203.72 Anabranch Use 53.98 -0.02 -0.01 0.11 Anabranch Off Allocation 26.42 -0.06 0.01 0.39 Tandou 55.50 0.05 0.08 0.42 Tandou Irrigation Use (GL) 33.83 0.04 -0.03 0.13 Average Shortfall (GL) New South Wales 294.85 -3.16 -6.51 0.87 Victoria 101.65 15.08 53.11 194.82 South Australia 9.41 -0.41 -1.15 -4.39 Lower Darling Diversions 0.060 0 0 0.010 Broken Hill Pumps 0.050 0 0 0 Peak Shortfall (GL) New South Wales 2459 22 59 -262 Victoria 853 -41 -47 -65 South Australia 423 11 4 -67 % of Years With Shortfalls New South Wales 67.33 -6.93 11.88 31.68 Victoria 38.61 39.61 56.44 61.28 South Australia 14.85 0 -0.99 -5.94 % of Years With NSW Alloc < 40% 5.94 0 0 -0.99 % of Years With VIC Alloc < 100% 1.98 -0.99 0 -1.98 % of Years With SA Alloc < 90% 1.98 -0.99 -0.99 -0.99 Minimum NSW Diversion (GL) 200 103 83 62 Salinities (EC) Torrumbarry 108.24 0.03 0.36 0.95 Swan Hill 268.05 -1.06 -1.29 -2.83 Euston 267.81 -1.87 -3.82 -4.58 Red Cliffs 317.05 -1.98 -3.75 -5.31 Merbein 353.52 -1.96 -3.81 -6.59 Lock 9 361.65 -0.69 -2.75 -4.67 Renmark 399.81 -1.53 -3.52 -6.82 Berri 431.97 -1.73 -4.29 -8.33 Morgan 549.58 -2.51 -6.33 -13.26 Murray Bridge 592.44 -2.42 -5.64 -14.42 Average Salinity Costs ($1000/year) New South Wales 94.4 -12.1 -26.8 -37.5 Victoria 232.8 -35.3 -78.2 -99.2 South Australia 65833.4 -284.1 -574.8 -1303.6 Total (a) 66160.6 -331.9 -679.8 -1440.1 Average Shortfall Costs ($1000/year) New South Wales 22949 -246 -507 68 Victoria 7912 1174 4134 15163 Total (b) 30860 928 3627 15331 Value of Hydro-elec ($1000/year) (c) 8736.2 83.0 240.8 770.1 Total Costs Less Hydro Value (a+b-c) 88285 513 2706 13021 Flow to South Australia (GL/year) 23.51 0.00 0.17 0.33 Hume Recreation Value Added ($1000/year) 2157.4 20.1 65.5 249.1 Gross margin for irrigated agriculture (Million/year) in 1996 dollars NSW (Million/Year) 0 0 0 0 Victoria (Million/Year) 0 0 0 0 165 4465 Murray Dar book 27/8/01 1:48 PM Page 166
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SECTION 4
FEASIBILITY OF minimum flow of 200 ML/day. The study indicates FLOW PULSING that the desired ± 20% level fluctuation could be achieved for target releases in the range of 281 ML/day to 6563 ML/day. Above and below those The Scientific Panel has proposed a system of pulsed target flows the maximum achievable fluctuation releases downstream of both Hume and Dartmouth would have to be less than ± 20%. Reservoirs in which the water level is fluctuated The last two columns of Table 1 list the rates of ± 20% over a two-week cycle. The panel has rise and fall that would be needed for a saw-tooth proposed that this fluctuation should have a release pattern in which the flow rose steadily for saw-toothed pattern rising over a two-day period two days and then fell steadily for the next twelve. and falling over the next twelve days. A small study These were calculated to test whether the allowable was conducted to test the range of target releases rates of rise and fall would be exceeded. The lowest over which this type of operation was feasible. of the constraints at Colemans are 0.48 m/day for rises and 0.24 m/day for falls. The proposed Releases from Dartmouth – Levels at operation does not exceed these constraints. Colemans Gauge 401211 Releases from Hume – Levels at Doctors Point The results of this study for releases from Dartmouth Gauge 409017 are shown in Table 1. The channel capacity for the Mitta Mitta River is 10,000 ML/day at Tallandoon. The results for releases from Hume are shown in In practice, the operators restrict releases from Table 2. In preparing this table it was assumed that Dartmouth to 9,500 ML/day to allow for variation releases could not exceed the channel capacity of in the local inflows. In preparing this table it was 25000 ML/day and that they could not be less than assumed that releases could not exceed 9,500 the minimum flow of 1200 ML/day. The study ML/day and that they could not be less than the indicates that the desired ± 20% level fluctuation
Table 1. Range of Target Releases from Dartmouth for which a ± 20% Level Fluctuation is achievable. (% change in levels based on the level above the cease to flow (= 0.46 m))
Target Flow Max Flow Min Flow Max Level Min Level Achievable Max Daily Max Daily (ML/d) (ML/d) (ML/d) (m) (m) Change in Rise in Fall in Level (±%) Level (m) Level (m) 200 200 200 0.87 0.87 0% 0 0 281 362 200 1.07 0.87 20% 0.102 0.017 400 546 260 1.20 0.95 20% 0.123 0.021 600 853 343 1.34 1.05 20% 0.146 0.024 1000 1479 528 1.55 1.19 20% 0.181 0.030 2000 2938 1047 1.89 1.41 20% 0.238 0.040 3000 4472 1579 2.14 1.58 20% 0.280 0.047 4000 5953 2068 2.34 1.71 20% 0.313 0.052 5000 7338 2627 2.52 1.83 20% 0.343 0.057 6000 8691 3270 2.69 1.95 20% 0.371 0.062 6563 9500 3625 2.78 2.01 20% 0.386 0.064 7000 9500 4500 2.78 2.14 16% 0.320 0.053 8000 9500 6500 2.78 2.41 9% 0.185 0.031 9000 9500 8500 2.78 2.67 2% 0.055 0.009 9500 9500 9500 2.78 2.78 0% 0 0
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could be achieved for target releases in the range of rising limb of the saw-tooth pattern to four days and 1865 ML/day to 19000 ML/day. Above and below reducing the falling limb to ten days. those target flows the maximum achievable The final column in Table 2 is the storage that fluctuation would have to be less than ± 20%. would be required in Lake Mulwala to smooth out the fluctuations below Yarrawonga. The effective The seventh and eighth columns of Table 2 list the storage in Lake Mulwala is only about 10,000 ML rates of rise and fall that would be needed for a and some of this must be set aside to re-regulate saw-tooth release pattern in which the flow rose surplus flows from the Ovens and Kiewa and rain steadily for two days and then fell steadily for the rejections from Mulwala Canal and Yarrawonga next twelve. These were calculated to test whether Channel. Given that up to 21,900 ML would be the allowable rates of rise and fall are exceeded. The required to smooth out the flows, it will obviously maximum allowable fall at Doctors Point is 0.15 be impossible to pulse the releases from Hume m/day while the target maximum rise is 0.30 unless the pulse can be transmitted downstream of m/day. The proposed operation would exceed the Yarrawonga. It may also be necessary in some target rate of rise for target flows between 8000 instances to pulse the irrigation supplies that are fed ML/d and 23000 ML/d. For these flows, the rate of from Yarrawonga Weir as well. rise could be reduced to the target by extending the
Table 2. Range of Target Flows below Hume for which a ± 20% level fluctuation is achievable (% change in levels based on the level above the cease to flow (= 1.04 m))
Target Flow Max Flow Min Flow Max Level Min Level Achievable Max Daily Max Daily Required (ML/d) (ML/d) (ML/d) (m) (m) Change in Rise in Fall in Storage in Level (±%) Level (m) Level (m) Lake Mulwala(ML) 1200 1200 1200 1.54 1.54 0% 0 0 1865 2530 1200 1.79 1.54 20% 0.125 0.021 232 5000 6790 3260 2.35 1.91 20% 0.218 0.036 617 10000 13400 6600 2.97 2.33 20% 0.322 0.054 1190 15000 20000 10000 3.48 2.67 20% 0.407 0.068 1750 19000 25000 13000 3.88 2.93 20% 0.473 0.079 2100 20000 25000 15000 3.88 3.10 16% 0.390 0.065 1750 25000 25000 25000 3.88 3.88 0% 0 0
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