Review of Environmental Impacts of Flow Regulation and Other Water Resource Developments in the River Murray and Lower Darling River System

Includes Glossary of Terms

Final Report to

Murray-Darling Basin Commission

Fluvial Systems Pty Ltd

July 2002 Review of Environmental Impacts of Flow Regulation and Other Water Resource Developments in the River Murray and Lower Darling River System

Includes Glossary of Terms

Final Report to

Murray-Darling Basin Commission by

Fluvial Systems Pty Ltd

Fluvial Systems Pty ltd PO Box 4117 Melbourne University Retail Parkville Vic 3010 ABN 71 085 579 095

Ph 03 93447780 Ph 0411855573 Email: [email protected]

Authors Christopher Gippel Dominic Blackham

July 2002

This document should be referenced as follows:

Gippel, C.J. and Blackham, D. 2002. Review of environmental impacts of flow regulation and other water resource developments in the River Murray and Lower Darling River system. Final Report by Fluvial Systems Pty Ltd, Stockton, to Murray-Darling Basin Commission, Canberra, ACT. Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Contents EXECUTIVE SUMMARY...... iv

1 INTRODUCTION ...... 1 1.1 ENVIRONMENTAL FLOWS AND WATER QUALITY OBJECTIVES FOR THE RIVER MURRAY PROJECT ...... 1 1.2 OBJECTIVES OF THIS REPORT ...... 1 1.3 STUDY AREA...... 4 1.4 FLOW REGULATION AND ENVIRONMENTAL FLOWS ...... 5 1.5 REPORT STRUCTURE ...... 7 1.6 KEY REFERENCES ...... 8 2 RIVER MURRAY FLOW REGULATION BACKGROUND...... 13 2.1 BRIEF HISTORY OF REGULATION ...... 13 2.2 HYDROLOGICAL IMPACTS ...... 14 3 GENERAL ECOLOGICAL AND GEOMORPHIC RESPONSES TO FLOW REGULATION ...... 19

4 DARTMOUTH DAM TO HUME DAM (MITTA MITTA RIVER)...... 27 4.1 SUMMARY OF IMPACTS AND CAUSES ...... 27 4.2 REGULATION INFLUENCES ...... 27 4.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES BETWEEN DARTMOUTH DAM AND HUME DAM...... 28 5 HUME DAM TO YARRAWONGA ...... 32 5.1 SUMMARY OF IMPACTS AND CAUSES ...... 32 5.2 REGULATION INFLUENCES ...... 32 5.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES BETWEEN HUME DAM AND YARRAWONGA...... 33 6 YARRAWONGA TO TORRUMBARRY WEIR (INCLUDES EDWARD- WAKOOL SYSTEM)...... 38 6.1 SUMMARY OF IMPACTS AND CAUSES ...... 38 6.2 REGULATION INFLUENCES ...... 38 6.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES FROM YARRAWONGA TO TORRUMBARRY WEIR ...... 42 7 TORRUMBARRY WEIR TO LOCK 11 MILDURA...... 47 7.1 SUMMARY OF IMPACTS AND CAUSES ...... 47 7.2 REGULATION INFLUENCES ...... 47 7.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES FROM TORRUMBARRY WEIR TO LOCK 11 MILDURA...... 50 8 LOCK 11 MILDURA TO LOCK 3 ...... 53 8.1 SUMMARY OF IMPACTS AND CAUSES ...... 53 8.2 REGULATION INFLUENCES ...... 54 8.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES FROM LOCK 11 MILDURA TO LOCK 3...... 56 9 LOCK 3 TO WELLINGTON ...... 61 9.1 SUMMARY OF IMPACTS AND CAUSES ...... 61 9.2 REGULATION INFLUENCES ...... 61 9.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES FROM LOCK 3 TO WELLINGTON ...... 61 10 WELLINGTON TO THE MOUTH ...... 63 10.1 SUMMARY OF IMPACTS AND CAUSES ...... 63

Fluvial Systems Pty Ltd ii

Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

10.2 REGULATION INFLUENCES ...... 63 10.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES DOWNSTREAM OF WELLINGTON ...... 64 11 LOWER DARLING RIVER AND GREAT ANABRANCH...... 68 11.1 SUMMARY OF IMPACTS AND CAUSES ...... 68 11.2 REGULATION INFLUENCES ...... 69 11.3 IDENTIFIED IMPACTS OF REGULATION ON ENVIRONMENTAL VALUES IN THE LOWER DARLING RIVER AND GREAT ANABRANCH ...... 71 12 SUMMARY OF ENVIRONMENTAL IMPACTS DUE TO REGULATION73 12.1 HYDROLOGY...... 73 12.2 GEOMORPHOLOGY AND WATER QUALITY ...... 73 12.3 ECOLOGY...... 75 13 IMPLICATIONS FOR ENVIRONMENTAL FLOW OPTIONS ...... 76 13.1 PROCESS FOR DEVELOPMENT OF ENVIRONMENTAL FLOW OPTIONS...... 76 13.2 RELEVANCE OF THIS REPORT TO DEVELOPMENT OF FLOW OPTIONS...... 76 14 REFERENCES ...... 78

APENDIX. GLOSSARY OF TERMS USED IN THE ENVIRONMENTAL FLOWS FIELD...... 85

Fluvial Systems Pty Ltd iii Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Executive Summary Introduction Over the past decade, there have been numerous discussions, investigations and decisions made regarding aspects of managing flows in the River Murray to provide environmental benefits. The Murray-Darling Basin Commission (The Commission) has implemented a number of initiatives addressing environmental flow needs of the River. In March 1999 the Environmental Flows and Water Quality Objectives for the River Murray Project (the Project) was established. An important first step in developing environmental flow options for the River Murray is to clearly establish the nature of the flow-related problems. A credible account of the scope and scale of the flow-related environmental problems provides the governments, communities and various interest groups with the basis for developing a common understanding of the issues. If solutions to environmental problems are to be sought through new ways of managing river flows, then the links between environmental condition and river flow should be known with a reasonable level of confidence. The Commission contracted Fluvial Systems Pty Ltd to conduct a review of the environmental impacts of flow regulation and other forms of water resource development along the main stem of the River Murray system. The objective of the review is to collate the key references that document the decline, or otherwise, of the in-channel, riparian and floodplain environments of the River Murray. One major problem faced by researchers of this topic has been the difficulty in isolating changes in the aquatic environment that are due to flow regulation from those that are due to changes in other factors such as catchment land use, fishing pressure, introduced species, riparian vegetation cover, large woody debris distribution, and natural variations in flow regime. Another major problem in determining the environmental impacts of regulation is that regulation of the River Murray occurred progressively over a long period of time, as water resources were developed to meet demands. While availability of data allows the hydrology of the system to be investigated in terms of the 7 phases of regulation, the ecological consequences cannot be investigated in such chronological detail. This is partly because collection of suitable data did not begin until the regulated flow regime was well established, but also because ecological responses are complex, often delayed, and can manifest in a location that is distant from the site of the hydrological disturbance. When the terms pre- and post-regulation are used in this report without reference to a specific regulating structure or practice, it can be assumed that the comparison is between natural (unregulated), and post- Dartmouth Dam conditions. The first main regulating structure in the catchment was Goulburn Weir on the Goulburn River, built in 1890, but construction of locks and dams on the River Murray did not begin until 1922. The flow in the main stem of the River Murray began to be affected by regulation at different times in different parts of the system, but it is reasonable to assume that the process of regulation (in the form of diversions) started to become a significant phenomenon at the turn of the 20th century. This document summarises the known changes in the natural environment of the River Murray that are thought to be largely due to flow regulation and other forms of water resource development. One important outcome of this review is identification of the major issues that can potentially be addressed by the Environmental Flows and Water Quality Objectives for the River Murray Project.

Fluvial Systems Pty Ltd iv Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

For the purposes of this review flow regulation is considered to include any activity that alters the hydrological behaviour of the system from its pre- regulation state. The term ‘environmental flows’ is used in the context of managing flows in regulated rivers (including those affected water abstraction and floodplain disconnection) to describe the component of in-stream flows that should be maintained in order to achieve environmental objectives. Environmental flows vary from river to river because the environmental objectives vary, and because the ecological requirements of the flora and fauna that inhabit rivers also vary. The Environmental Flows and Water Quality Objectives for the River Murray Project aims to identify alternative operating strategies that meet the needs of off-stream users, but which enhance or maintain the ecologically important values of the river. The impact of regulation on the River Murray and lower Darling River environment was assessed according to eight distinct river zones: 1. Dartmouth Dam to Hume Dam 2. Hume Dam to Yarrawonga 3. Yarrawonga to Torrumbarry Weir (including Barmah Choke and Edward River system) 4. Torrumbarry Weir to Lock 11 Mildura 5. Lock 11 Mildura to Lock 3 6. Lock 3 to Wellington 7. Wellington to the Mouth 8. Lower Darling River downstream of Menindee Lakes and the Great Anabranch A great deal of information is available on the environmental condition of the River Murray, and the impact of flow regulation. This information is available in a variety of forms: refereed scientific journal articles based on rigorous research in a specific location on the Murray; management and consulting reports and expert knowledge. This review is focussed on quantifiable changes to the environmental condition of the river, but there is a great deal of information that is not published in scientific journals that was considered worthy of inclusion. Thoms et al. (2000), Jensen et al. (2000) and Young (2001) were the key references used in the preparation of this report. These documents contain a mixture of existing published information, existing unpublished data and knowledge, and new hypotheses generated throughout the course of their production (consultation with and interaction between scientific experts was integral to the process of preparing these manuscripts). We have also searched the literature for additional material. Our approach was to favour information that specifically addressed the impacts of flow regulation and was well supported by data. Where we included less reliable or apparently speculative information, it was identified as such. Flow regulation background Catchment and river diversions have reduced the flow in the lower River Murray (below Euston) to half the natural levels, or less. Inflows from the Darling River do not improve the situation because the current (regulated) average annual discharge is only 57% of the natural level. Flow variability can be defined at various temporal scales, from annual flow variation to the rates of water level variation during flood events. Regulation has

Fluvial Systems Pty Ltd v Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission generally reduced flow variability. Variation of flow through the year is reduced through the release of relatively constant flow volumes during the periods of water harvesting (relatively constant low flows are released) and irrigation water supply (relatively constant channel capacity flows are released). Weirs are managed to maintain the water level at a fairly constant level for long periods, and this further reduces natural flow variability. Under pre-regulation conditions, peak flows occurred in spring and then rapidly receded to low levels in late summer and autumn. The construction of a large volume of storage on the system has altered this seasonal variation, by holding winter and spring inflows to the major storages in the upper reaches (Hume and Dartmouth Dams) and releasing water to meet peak irrigation demands in the summer and autumn. At Euston and further downstream, the combination of reduced summer flows due to irrigation diversions and unregulated inflows are sufficient to restore the natural pattern of higher flows in the late winter to early spring period. The frequencies of peak flows with recurrence intervals of 20 years or more did not change appreciably with regulation. The major hydrological impact of the construction of storages has been to reduce the frequency of occurrence of mid- range flows, or minor-medium floods. Regulation has also reduced the duration of mid-range floods. For much of its course, the River Murray flows through a semi-arid environment, so it is not surprising that prior to regulation, during times of extreme drought, it was reduced to a chain of saline ponds. Under regulated conditions, there is always some flow in the river. Near the Murray mouth, prior to regulation there was flow in the river for more than 95% of the time, keeping Lake Alexandrina quite fresh for extended periods. Under the regulated regime, flow through the Mouth effectively ceases (i.e. <10 GL/month) for around 20% of the time. The Mouth now experiences periods of up to four or five years with little outflow. Dartmouth Dam to Hume Dam Dartmouth Dam is a large storage that has altered the pattern of low flows, flood flows, as well as seasonal and daily flow variations. Accelerated channel erosion has been reported as a problem. Long periods of relatively constant regulated flow appears to be the main cause of this erosion, rather than drawdown, which has long been the conventional wisdom. The channel bed has less pronounced pool and riffle morphology than expected. This has been attributed to excessive sedimentation during the dam construction period, with the regulated flow conditions maintaining the poorly defined morphology, and degrading the habitat quality of the substrate. The cold water released at the base of the dam prevents spawning of native fish. The cold water favours alien species (trout) at the expense of native fish. Also, the macroinvertebrate communities have changed their composition in response to the regulated flows, degraded substrate, and the cold water releases. The environmental impacts of Dartmouth Dam can extend downstream as far as Lake Hume, depending on the nature of flow releases and flow conditions in the tributaries. Hume Dam to Yarrawonga The section of the River Murray, immediately downstream of Hume Dam, receives the full hydrological impact of flow regulation. For a distance of 180 km downstream to Yarrawonga Weir, the channel conveys all regulated flow destined for downstream uses. Hume Dam is operated to store winter-spring floods and release water for irrigation during summer-autumn. This effectively

Fluvial Systems Pty Ltd vi Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission reverses the natural seasonal flow pattern, and severely reduces the occurrence of small and medium-sized floods. Compared with rivers of a similar catchment area and discharge, the River Murray is a highly stable river by world standards. Downstream of Hume there has been localised bend migration of the order of 1,000 mm/yr, but there is no evidence to suggest that the rates of migration have altered in response to regulation. While rapid drawdown is a threat to bank stability in anabranches, the main cause of bank erosion in the main channel is the extended periods of regulated near-to-banktop flow. Bank erosion is prevalent throughout the reach, occurring at an average rate of 160 mm/yr. Downstream of Hume Dam the bed has deepened by as much as 24%, but downstream of Albury the river has shallowed. It is important to note that the observed morphological changes are small by world standards. The River Murray is relatively stable because of its low gradient and clay rich bank material. There has been a reduction in water temperatures and dissolved oxygen levels downstream of Hume Dam. Data suggest unusual seasonal trends in nitrate and phosphorous concentration. Turbidity and associated nutrient concentrations may have increased due to accelerated bank erosion, but data are lacking. Primary production (algal growth), native fish and macroinvertebrate populations have been adversely affected by the altered flow regime and associated changes in water quality. However, towards the lower end of this zone, fish populations are in good condition, probably due to the positive influence of Lake Mulwala. Yarrawonga to Torrumbarry Weir The influence of Hume Dam reduces with distance downstream, but it still exerts a profound influence through the Yarrawonga-Torrumbarry Weir zone. Additional regulation is exerted through diversion of water from Yarrawonga Weir, regulators on side-channels through the Barmah-Millewa Forest to help protect against unseasonal flooding, and offtakes on the Edward River and Gulpa Creek distributary system. Major tributaries of the River Murray upstream of Torrumbarry Weir include the Kiewa, Ovens, Goulburn and Campaspe rivers. The weir pool level at Torrumbarry determines the river level as far upstream as Echuca (74 km upstream) during the fully regulated mode of operation. Downstream of Yarrawonga, annual flow is 25% less than under natural conditions, but summer flow is 19% greater than natural. Flow variability has been decreased and water level is held at relatively constant near capacity discharge for much of the year. Seasonality has been altered, and the frequency and duration of winter/spring flooding has been reduced. Regulation has reduced the frequency of flooding of Barmah-Millewa Forest, but this ecologically important area also suffers from unseasonal flooding. The Edward River has also been affected by regulation, with flows through the Millewa Forest section at or near channel capacity for much of the year. In this section of the River Murray, persistently high summer flows characteristic of the regulated regime have led to widening of the upper part of the cross- section through the creation of in-channel over-cut benches, but this appears to have slowed or ceased over much of the zone since 1980. There has been no appreciable long-term change in bed elevation. Dramatic channel widening and bed degradation in response to regulation has been documented on the Edward River. It appears that the influence of cold water releases from Dartmouth and Hume dams is insignificant downstream of Yarrawonga. Regulation severely impacted waterbird populations. The river is dominated by alien fish species such as carp,

Fluvial Systems Pty Ltd vii Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission which appear to be using unseasonally flooded wetlands for breeding. Despite this, the River Murray between Lake Mulwala and Barmah Choke contains excellent native fish populations, including the only remaining truly natural population of the endangered Trout cod. Macroinvertebrate communities may be adversely affected by unseasonally high summer and autumn flows. The quality of habitats and the distribution of vegetation types in the Barmah-Millewa Forest have been significantly affected by the changed flooding regime. Torrumbarry Weir to Lock 11 Mildura The primary impact of regulation in this zone is the significant decrease in flow volume due to large upstream diversions, but weirs have the effect of maintaining relatively constant water levels in the pools upstream. Mean annual flow at Euston reduced by 49% from natural levels. Periods of prolonged low flow are more frequent. Frequency, duration and magnitude of all but the largest floods have been reduced. There appears to be an accelerated loss of in-channel benches due to prolonged constant regulated flows and rapid falls in water level, although de-snagging is also implicated. Between 1927 and 1981, the river aggraded up to 3 m in some locations along this zone, with the average being around 1.5-2.0 m. There is an unequivocal increase in turbidity between Torrumbarry and Euston Weir pool, followed by a decrease in turbidity at Euston and a further decrease to Wentworth. This is probably partly due to the regional geomorphic change (from Riverine Plain to Mallee tract), but reduced velocities in the weir pool, and saline inflows may also be involved. The Mallee tract showed consistent bed aggradation between surveys undertaken in 1927 and 1981. The four weir pools in this area can potentially be on the threshold of cyanobacterial bloom problems during dry periods, with Torrumbarry and Mildura Weir pools having the highest risk. Wetland and floodplain habitats have been adversely impacted by reduced frequency of inundation, while the artificial elevation of water levels upstream of weirs has led to the permanent inundation of some wetlands, reducing their productivity. The diversity and abundance of most aquatic biota (fish and macroinvertebrates) is comparatively poor immediately below Torrumbarry Weir, but improves significantly further downstream. The improvement was associated with increasing water clarity downstream which appears to promote richer development of benthic algae and bacterial biofilm on hard surfaces, which in turn improves the situation for macroinvertebrates and fish. The altered flooding pattern is thought to have reduced the effectiveness of inoculation of the river with micro-organisms and micro-invertebrates, and hence reduced the availability of larval fish food. Weirs create severe restrictions on faunal movement, which leads to population discontinuities. Flow regulation has removed cease-to-flow conditions from the flow pattern, but these were quite rare in the natural flow regime (once every 100 years) so the ecological impact of this change is probably minor. However, the replacement of naturally low summer flows with higher regulated flows, is of greater ecological significance. Lock 11 Mildura to Lock 3 Lake Victoria is used to re-regulate surplus flow in the River Murray, and ensure that South Australia is supplied with its entitlement flow. The role of the locks on the River Murray between Mildura and Lock 3 is to aid navigation and facilitate the diversion of water by maintaining a constant level in the weir pools.

Fluvial Systems Pty Ltd viii Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

This zone, being located downstream of the major irrigation diversions and being fed by a number of unregulated tributaries, has a flow seasonality that is similar to that of the pre-regulation regime. However, the volume of flow has been much reduced, as has the frequency, duration and magnitude of flooding. The variability of mid-range flows has been reduced, so that the present regime is dominated by prolonged low flows, with the occasional high flows. The planform of the river did not alter between 1906 and 1988, reflecting the low stream energy and cohesive bank material, but significant changes in the internal dimensions and slope of the channel have occurred following weir construction. Bank retreat of over two metres per year downstream of weirs is associated with extremely rapid flood recessions. Bank slopes have also increased. Width-depth ratios have increased by an average of 32% between Locks 3 and 4. Prior to regulation, the bed sediments were predominantly coarse sand, while now they are comprised mainly of fine silts and clays. The channel is developing a stepped gradient associated with the weirs. Due to the operation of Lake Victoria, the length of the period of highly turbid Darling River water impacting significantly on the turbidity of the River Murray has historically been extended from two months to approximately seven months. In more recent seasons (since 1989) Lake Victoria has been filled with lower turbidity water originating from the Murray-Murrumbidgee catchment (although this decision was driven by the lower salinity of Murray-Murrumbidgee water). This had the effect of increasing the growth of macrophytes and macroinvertebrates in wetlands and in the main river channel. While for an algal bloom to fully develop, low flow conditions must be combined with other favourable conditions of warm temperatures and stable weather, a flow at least as low as 6,000–7,000 ML/d was found to be critical to the process. This flow is similar to that currently used to supply the minimum entitlement flow to South Australia, suggesting that the current operating conditions may be conducive to development of algal blooms. Increased salinity and higher groundwater levels, combined with the regulated flow regime, has had an adverse effect on the growth and regeneration of floodplain trees, including black box and Lignum. The impact of water level management has changed the nature of the littoral zone by limiting exchanges between the river and its floodplain. The wider range of water level variation in the upper part of the weir pools (compared to the lower end) is reflected in the wider elevational range over which specific littoral flora are observed. Artificially raised water levels have led to a general shift in wetland type from temporary to permanent. Reducing floodplain inundation frequency through regulation probably severely reduces the reserve of invertebrates that can contribute to the floodplain foodweb following inundation. Stabilisation of the water level through flow regulation has promoted the growth of filamentous green algae. This shift in phytoplanton composition has had repercussions at higher levels in the food chain, with the loss of two species of gastropod. The riverine freshwater mussel, Murray crayfish, and the river snail have either declined or become regionally extinct. Although there are few historical data, a decline in the populations of many native fish, including Murray cod, trout cod, golden perch and river blackfish has been reported under regulated flow conditions. Regional extinctions are well advanced for five native species in the lower Murray, and another two are threatened. Native fish represent only about 5% of the total fish biomass. The main problem is reduced opportunities for recruitment because of the elimination of small floods, but sudden changes in water levels below weirs can strand fish eggs and cause fish to abandon nests.

Fluvial Systems Pty Ltd ix Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Lock 3 to Wellington Lock 3 to Wellington Zone contains the Murray Gorge. This zone has a similar degree of regulation as the Valley Zone upstream. The Darling River does not assist much in returning the River Murray to a less regulated regime, because it too is regulated. The cross-sectional area between Lock 2 and 3 has fluctuated through time. Weir pools have aggraded, and the bed has degraded downstream of the weirs. Littoral plant growth is limited in this Zone. In the absence of overbank flows (reduced in frequency through regulation), Gorge wetlands are more prone to sediment blockage than Valley wetlands, because they usually have only a single inlet channel. Wellington to the Mouth The Murray Barrages comprise five low head weirs and earthen causeways linking the islands that once formed a previous shoreline. The barrages block 7.6 km of previously navigable channels, and prevent ingress of water to Lake Alexandrina. Regulation has reduced mean volume of flow at the Barrages and Mouth by 62% of the natural volume, while median flow has been reduced by 80%. The frequency of cease-to-flow at the mouth has increased from 1 in 20 years to 1 in 2 years. Minor- to medium-sized floods (up to 1 in 7 year event) have been eliminated. Rates of flow recession following moderate/high events have increased. Currently there is very restricted flow through Mundoo and Boundary Creek Barrages, even when they are open. There is now an abrupt interface between the fluvial and tidal reaches, reducing the size of the estuarine component to 11% of its pre-Barrage scale. Increasing frequency and duration of periods of very low flow have contributed greatly to sedimentation at the Mouth and nearby channels. The sediment regime has also been impacted by the restriction of flow by the barrages causing a shift towards a more strongly depositional regime. The continuing growth of Bird Island has the potential to result in more frequent and more permanent blockage of the Mouth. Sedimentation upstream of Goolwa Barrage has been associated with a change from bioclastic sands to muds, attributed to the capture of coarser material in the upstream storages. The turbidity of the lakes has increased as a result of the relatively highly turbid Darling River water making a more significant contribution to the lower River Murray water budget. Although algal blooms did occur before regulation, there is general consensus that the incidence of cyanobacterial blooms in the Lower Lakes has increased with time. The Barrages have created an abrupt fresh-saline interface, whereas in its natural state there would have been a very large and transient interface, creating estuarine conditions. The management of the saline-freshwater interface over a very narrow range has effectively removed the habitats that represent the transition from saline to freshwater. As a consequence, flora adapted to this transition zone is poorly represented. The lack of many submerged plant species within the Lower Lakes appears to be correlated with the reduced light penetration associated with increased turbidity. The numbers of almost all species of waders and waterbirds using the wetlands of the Coorong and Lower Lakes have declined, particularly over the past 20 years. While this trend also applied on regional and national scales, the decline in total populations in the region between the Barrages and the Mouth are considered to be greater than the general levels of decline noted elsewhere. The confusion of ecological signals resulting from hydrological and geomorphic changes interferes with breeding/recruitment of fish and macroinvertebrates. The reduction in area of the highly productive estuarine habitat has affected the abundance of commercial and non-commercial fish

Fluvial Systems Pty Ltd x Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission species. Similarly, estuarine macroinvertebrates have also suffered reduced habitat area. Lower Darling River and Great Anabranch The Great Anabranch of the Darling River leaves the River 55 km south of Menindee, but further downstream it is connected to the River by other flow paths, depending on flow level. The Great Anabranch is separate from the Darling River for a distance of 200 km, joining the River Murray 15 km west of the Darling River confluence at Wentworth. The Great Anabranch is an ephemeral channel that carries flood flows from the Darling River (every 2-3 years), and regulated releases, but otherwise it does not naturally flow. Mean annual flow in the Darling River has been reduced by around 43% as a result of abstractions in the Barwon-Darling River system. The major impoundment on the lower Darling River, Menindee Lakes is an extensive online storage scheme, supplying approximately 40% of South Australia’s entitlement flow. Operation of the Menindee Lakes Scheme has shifted the seasonal flow pattern such that high flows now occur in summer. Winter flows are now less variable, and bankfull floods are less frequent. Early settlers lowered the main Anabranch offtake level over 130 years ago, thus lowering the threshold for commencement of flow. Operation of Menindee Lakes had the opposite effect on flow in the Anabranch, decreasing flood frequency, such that the effects cancelled each other. However, supply of water for stock and domestic use from the Anabranch (and associated storage of water behind small structures), plus the occasional practice of excluding floods from recently cropped lake beds, have locally altered the flow regime in the Anabranch. Complex in-channel benches have been partially eroded by relatively constant regulated high flows. The habitat value of intact benches is reduced because of unseasonal inundation from flows used to supply South Australia’s entitlement. The pools behind the structures on the Great Anabranch create a heterogeneous bed environment, with higher than natural levels of sedimentation. Regulation has increased the risk of algal blooms through the combination of low flows and weir pools that create stratified conditions favourable for algal growth. The relatively constant flow levels and unseasonal flows have resulted in an apparent lack of macrophytes in the lower Darling River. The reduced frequency of flooding has detrimentally impacted the health of riparian and floodplain vegetation, and reduced the input of organic matter to the river. The northern sections of the Great Anabranch appear to have reduced flooding frequency due to operation of the Menindee Lakes Scheme. The near-stable pools upstream of the structures on the Great Anabranch create conditions conducive for the growth of macrophytes, such as the invasive species Typha. While the fish assemblages in the lower Darling River are healthy, fish movement, recruitment and recolonisation are adversely affected by a number of barriers, relatively constant flow levels, and reduced access to floodplain habitat. Unlike the River Murray, the Darling River has a predominantly native fish fauna. Although diversions have markedly reduced the annual flow volumes in the Darling River, the impact of regulation on flow variability and flooding regimes is less than it is on the River Murray. Macroinvertebrate communities in the lower Darling River do not appear to have been impacted by regulation. Loss of important habitat through cropping of ephemeral lakes is not a direct flow regulation impact, but the reduced flood frequency, and the possibility of excluding flows when necessary, helps to facilitate this practice.

Fluvial Systems Pty Ltd xi Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Summary of Environmental Impacts due to regulation

Summary of hydrological impacts of regulation on the River Murray and lower Darling River. Direction of change is indicated by Ã and Ä symbols, with magnitude of change semi-quantitatively indicated by size of symbols; : is data unavailable; } is no appreciable change.

y a g Dartmouth- Hume Hume- Yarrawon Yarrawonga- Torrumbarr Torrumbarry- Mildura Mildura-Lock 3 Lock 3 - Wentworth Wentworth- mouth Lower Darling River Annual volume } Ã Ä Ä Ä Ä Ä Ä Inter-annual variability : Ä } Ã Ã Ã Ã Ä

Winter/spring volume Ä Ä Ä Ä Ä Ä Ä Ä Summer/autumn volume Ã Ã Ã Ä Ä Ä Ä Ä Low flow duration Ä Ä Ä Ã Ã Ã Ã Ã

Capacity flow duration Ã Ã Ã Ã Ä Ä Ä Ä

Rise and fall control Ã Ã Ã Ã Ã Ã Ã Ã Mid-range flood frequency Ä Ä Ä Ä Ä Ä Ä Ä Mid-range flood duration Ã Ä Ä Ä Ä Ä Ä Ä

Fluvial Systems Pty Ltd xii Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Summary of geomorphological and water quality impacts of flow regulation on the River Murray and lower Darling River. Direction of change is indicated by Ã and Ä symbols, with magnitude of change semi- quantitatively indicated by size of symbols; : is data unavailable; } is no appreciable change; ? denotes high level of uncertainty.

ume Dartmouth- H Hume- Yarrawonga Yarrawonga- Torrumbarry Torrumbarry- Mildura Mildura-Lock 3 Lock 3 - Wentworth Wentworth- mouth Lower Darling River Bank/in-channel bench erosion Ã Ã Ã5 Ã Ã Ã ÄÃ9 Ã Bed degradation (Ä) Ã ÄÃ1 ÄÃ6 Ã ÄÃ7 ÄÃ7 Ã sedimentation (Ã) Ã Sediment transport : Ä2 : : : : Ä10 : Water temperature Ä Ä } } } } } } Turbidity Ã?3 Ã?3 : Ä Ã8 Ã8 Ã8 :

Nutrient load/concentration Ã?4 Ã?4 : : : : Ã?4 :

Algal bloom likelihood : : : Ã Ã Ã Ã Ã

1. Degradation close to the dam, sedimentation downstream of Albury 2. Dams have high trap efficiency, but channel erosion has increased 3. Uncertain, based on observations of increased bank erosion 4. Uncertain, based on observations of increased bank erosion or higher turbidity. Unusual temporal trends in nutrients observed downstream of Hume Dam 5. Widening through over-cut benches in the River Murray, but extensive channel enlargement in the upper Edward River 6. Bed degradation in the upper Edward River. Sedimentation in the River Murray, and the Edward River closer to Deniliquin 7. A stepped profile has developed, with degradation downstream of weirs and sedimentation in the weir ponds 8. Overall, regulation has increased turbidity by increasing the proportion of turbid flows from the Darling River, but the problem is less marked in years of higher flow 9. Expansion of Bird Is. flood tide delta, and some prograding shorelines in sheltered areas, but shoreline erosion is more dominant 10. Speculation, based on trapping by headwater dams and reduced flows

Fluvial Systems Pty Ltd xiii Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Summary of ecological impacts of flow regulation on the River Murray and lower Darling River. Direction of change is indicated by Ã and Ä symbols, with magnitude of change semi-quantitatively indicated by size of symbols; : is data unavailable; } is no appreciable change.

awonga Dartmouth- Hume Hume- Yarr Yarrawonga- Torrumbarry Torrumbarry- Mildura Mildura-Lock 3 Lock 3 - Wentworth Wentworth- mouth Lower Darling River Native fish diversity/abundance Ä Ä Ä Ä Ä Ä Ä Ä Alien fish numbers Ã Ã Ã Ã Ã Ã Ã Ã Macroinvertebrate diversity/abundance Ä Ä Ä Ä Ä Ä Ä } Littoral plant diversity/abundance : Ä : : Ä Ä Ä Ä

Waterbird numbers : : Ä : : : Ä Ã

Wetland quality : : Ä Ä Ä Ä Ä Ä

Implications for environmental flow options While several previous reports have reviewed the environmental impacts of flow regulation in the River Murray, this report is unique in that it: • Covers the entire area of interest from Dartmouth Dam to the mouth • Deals solely with regulation impacts, and makes no attempt to recommend environmental flows • Attempts to isolate, and report on flow regulation impacts, as opposed to other types of environmental disturbance, • Concentrates on reporting quantified information that is reasonably well established and supported by appropriate data • Carefully references all data and information • Highlights gaps in knowledge regarding flow regulation impacts This review was prepared with three main applications in mind: • To assist all interested parties to further develop their ideas concerning the need for environmental flows in the River Murray • By quantitatively establishing the level of impact, location of impact and cause of impact, assist in prioritising areas for environmental flow options • To provide a quality review of information that can be used as a resource document by the Community, the Commission, Agencies and other stakeholders to aid the process of formulating flow options for the River Murray. The environmental objectives and hydrological options for the River Murray should be supported by strong, defensible arguments.

Fluvial Systems Pty Ltd xiv Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

1 Introduction

1.1 Environmental Flows and Water Quality Objectives for the River Murray Project Over the past decade, there have been numerous discussions, investigations and decisions made regarding aspects of managing flows in the River Murray to provide environmental benefits. The Murray-Darling Basin Commission (The Commission) has implemented a number of initiatives addressing environmental flow needs of the River. In March 1999 the Environmental Flows and Water Quality Objectives for the River Murray Project (the Project) was established. The objective of the Project is to advise the Ministerial Council regarding the future operation of the River Murray system. The Project will formulate actions aimed at achievement of a sustainable river environment and water quality, in accordance with community needs. The Plan will include an adaptive approach to management and operation of the River. Two important secondary products of the Project are: • Environmental flow objectives and water quality objectives to provide the basis for ongoing review and modification to management of the River Murray system. • Implementable options for providing flows and water quality that will enhance the River Murray riverine environment through achievement of environmentally beneficial outcomes. The River Murray is a large river with a catchment area of around 1,000,00 km2 (Figure 1). The River and its catchment have a long history of exploitation and modification, with flow regulation (Figure 1) and increased salinity commonly recognised as the two main factors that have impacted (and continue to impact) negatively on the environmental health of the river. Although flow regulation is not necessarily the main cause of salinity problems, flow and river salinity are intimately linked, so it makes sense to co-ordinate their management through an integrated program.

1.2 Objectives of this report An important step in developing environmental flow options for the River Murray is to clearly establish the nature of the flow-related problems. A credible account of the scope and scale of the flow-related environmental problems provides agencies, communities and various interest groups with the basis for developing a common understanding of the issues. If solutions to environmental problems are to be sought through new ways of managing river flows, then the links between environmental condition and river flow should be known with a reasonable level of confidence. Most of the available information is concerned with describing degradation of the natural environment as a result of past and current patterns of flow regulation, rather than predicting how the river might respond to different regulated flow patterns. However, this knowledge provides a reasonable basis for developing innovative ways of managing river flows in order to halt environmental degradation, and improve environmental condition where possible.

Fluvial Systems Pty Ltd 1 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

AUSTRALIA

Murray Darling Basin QLD

BRISBANE

er iv S A R

Upper Darling Flow to SA

g N S W

l r

a D

River Murrumbidgee R. M SYDNEY u r r um b i d R Yarrawonga g e e M u r Albury CANBERRA ra y ACT V I C R. Murray Snowy below Jindabyne outflow Goulburn R.

Kiewa R. Ovens 0 200 km MELBOURNE R. Snowy outflow

Changes in median annual discharge

Natural median annual discharge Current as percentage of natural Lost as percentage of natural Excess as percentage of natural 500 2000 5000 10 000 GL / year

Figure 1. River Murray catchment, showing distribution of, and impact of regulation on, median annual discharge for River Murray and major tributaries.

The Commission contracted Fluvial Systems Pty Ltd to conduct a review of the environmental impacts of flow regulation and other forms of water resource development along River Murray system, which includes the main stem of the River Murray, the Edward/Wakool system and the lower Darling River. The objective of the review is to collate the key references that document the decline, or otherwise, of the in-channel, riparian and floodplain environments of the River Murray. The review concentrates on data, reports, and other credible publications that use scientifically defensible information or argument that links changes in the environmental values of the river to changes in the flow volume or pattern caused by regulation.

Fluvial Systems Pty Ltd 2 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

One major problem faced by researchers of this topic has been the difficulty in isolating changes in the aquatic environment that are due to flow regulation from those that are due to changes in other factors such as catchment land use, fishing pressure, introduced species, riparian vegetation cover, large woody debris distribution, and natural variations in flow regime (the pattern of floods, droughts and seasonal flow variability). The difficulty arises from the fact that changes to these factors can produce similar ecological responses, and the changes in most of these factors have been overlapping or simultaneous with flow regulation. Another difficulty for interpretation of the data is that when changes in the controlling factors occur together, the effects are not necessarily a simple additive function of the impact of each factor. Changes to some controlling factors may take a period of time before they manifest as degradation of the aquatic environment. Some impacts are marked and easily detected, while other more subtle impacts may be very difficult or expensive to measure. Another major problem in determining the environmental impacts of regulation is that regulation of the River Murray occurred progressively over a long period of time, as water resources were developed to meet demands. Maheshwari et al. (1993) identified 7 main phases of regulation, beginning in the early 1900s and finishing with the closure of Dartmouth Dam in 1979. In reality, the degree of regulation continued to increase after Dartmouth Dam was closed. On 30th June 1995 an agreement was reached between the Commonwealth and the states with borders that contain the Murray-Darling Basin to place a cap on water diversions from the basin. This decision was made in the wake of a major audit of water use, which found that water diversions in the six years from 1988 to 1994 had increased by 8% (MDBMC, 1995). An example of the difficulty in attributing cause to an observed environmental problem is the case of Murray cod numbers in the River Murray. Various theories have been explored to explain the apparent decline in Murray cod abundance since 1960: reduced larval recruitment due to flow regulation, increased rate of fall in water level downstream of regulating structures, alienation of floodplains, over-fishing, competition and predation from alien fish species, creation of barriers to movement, degradation of in-channel habitat through de-snagging and channelisation, altered water temperatures downstream of dams, and siltation of habitat. While all of these factors have been implicated to some extent in the apparent decline of the Murray cod, the importance of flow regulation has been emphasised (Rowland, 1989; Walker and Thoms, 1993; McKinnon, 1997). The terms pre-regulation and post-regulation, when applied to the River Murray, must be interpreted with caution. While availability of data allows the hydrology of the system to be investigated in terms of the 7 phases of regulation, the ecological consequences cannot be investigated in such chronological detail. This is partly because collection of suitable data did not begin until the regulated flow regime was well established, but also because ecological responses are complex, often delayed, and can manifest in a location that is distant from the site of the hydrological disturbance. When the terms pre- and post-regulation are used in this report without reference to a specific regulating structure or practice, it can be assumed that the comparison is between natural (unregulated, i.e. pre- 1900), and post-Dartmouth Dam conditions. Most of the ecological data on the River Murray were collected post-Dartmouth, so in making pre- and post- regulation comparisons, ecological studies generally resort to speculation, anecdotal reports, inference from other studies, theoretical knowledge, or extrapolation from observed trends.

Fluvial Systems Pty Ltd 3 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Given the problems outlined above, it must be remembered that attributing certain aspects of environmental degradation to flow regulation alone, or assigning a percentage of the degradation to flow regulation, can be an extremely difficult problem, and in many cases it is not possible. For this type of problem, scientists usually support their local data with information or understanding gained from other less complicated situations, or established theory, to generate the most likely explanation of the cause of degradation and to suggest ways of correcting it. This is the case with determining the impacts of flow regulation on the River Murray. This document summarises the known changes in the natural environment of the River Murray that are thought to be largely due to flow regulation and other forms of water resource development. One important outcome of this review is identification of the major issues that can potentially be addressed by the Environmental Flows and Water Quality Objectives for the River Murray Project.

1.3 Study area This review is concerned with the River Murray System, defined as the Hume catchment including the Mitta Mitta River downstream from Dartmouth Dam, the main stem of the River Murray including the Edward/Wakool Rivers, Menindee Lakes and the Lower Darling and Great Anabranch, the Murray mouth and the Coorong (Figure 2). The many tributaries and their catchments are excluded from this review, mainly because the Project Board is currently concentrating its effort on the geographic scope of the Murray-Darling Basin Agreement (1992). This Agreement has considerable history, beginning in 1915 as the River Murray Waters Agreement, and was originally aimed at regulation of the main stream of the Murray to ensure that each of the three riparian states, and especially South Australia, received their agreed shares of the Murray's water. As the Commission’s structures are located within the River Murray System, this is the area of the Basin where flow is most directly under the Commissions control. The Environmental Flows and Water Quality Objectives for the River Murray Project is being driven by the Commission, hence, this review concentrates on impacts of regulation due to Commission structures. There is no doubt that activities in many of the tributary catchments contribute to regulation of flows in the main stem of the River Murray. Sometimes this is effected through operation of a major dam (such as Eildon Dam on the Goulburn River) (Figure 2), but more widespread is direct abstraction of water from streams and rivers, even though many of these waterways might be commonly referred to as unregulated. Environmental flows for the River Murray is really a Basin-wide issue, and in this respect it is relevant that Schedule F, which concerns the Cap, agreed upon by the Ministerial Council in August 2000, covers the entire Basin. The River Murray flow options will generally be limited to what can be achieved through alteration of operating rules at, or modification of, flow control structures that are located within the River Murray system (i.e. the geographic scope of the Agreement). This area includes numerous and significant regulating structures, so there is considerable scope for generating flow options that will enhance the River Murray riverine environment. However, it must be remembered that there are probably many other opportunities for providing environmental flows in tributary streams and rivers, and design of these flow regimes could be co-ordinated to also provide benefits for the main stem of the River Murray. This would be a large-scale undertaking, and is clearly beyond the scope of the current Project. However, environmental flow investigations are

Fluvial Systems Pty Ltd 4 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission underway in many rivers and streams located within the River Murray Basin, and in the future it could be worthwhile to review these individual efforts and integrate them using a catchment-wide approach.

r e i v Menindee R Lakes Menindee Weir 32

h c n a r b a

n SOUTH A NEW SOUTH WALES e r Ri v

AUSTRALIA g Pooncarie t n a i e l r r

G a Lock 6 D Lock 5 Lake Lock 2 Victoria n l a Lock 3 h Lock 10 c a Lock 11 L Renmark Mildura Blanchetown Lock 9 R i r r u m b i v M u d g y Euston e Lock 1 Lock 8 e e a r r Lock 7 r Lock 4

M Lock 15 Adelaide R i W E v e r dw a a ko rd Wellington o l R R Deniliquin M u r Barmah-Millewa Forest r a y To c u m w a l Lake Torrumbarry Yarrawonga

Alexandrina Weir Weir R The Coorong Hume Albury R Echuca O Reservoir i ve v n e R s C n r

d n R a VICTORIA r d u o b t l Dartmouth L c u Dam

o h G m N e n Lake t b Eildon o 050km u n d a r y

Mouth Wellington Lock 3 Mildura Torrumbarry Yarrawonga Hume Dartmouth River Management 76 5 4 3 2 1 Zone 8 Murray Menindee Junction (Darling River)

Figure 2. Study area for this report: Mitta Mitta River from Dartmouth Dam to Hume Dam, and main stem of River Murray to the mouth.

1.4 Flow regulation and environmental flows There are various definitions of the term ‘flow regulation’. For the purposes of this review flow regulation is considered to include any activity that alters the hydrological behaviour of the system from its pre-regulation state. Direct forms of flow regulation include large headwater dams that exert a very strong control on flow in the river for some distance downstream, lowland weirs that control stage height over long sections of the river, offline flow control structures, and direct pumping (abstraction) of water from the river. The other main direct hydrological modification is through practices and structures that disrupt river- floodplain connectivity. There are other activities that indirectly impact characteristics of the flow environment in rivers. A good example is the way de-snagging alters the hydraulic habitat conditions. Environmental flow studies normally consider only hydrological aspects of river management, while the hydraulic environment falls within the domain of river rehabilitation (e.g. meander re-instatement, riffle construction or snag re-introduction), but this may be an unhelpful separation. Implementation of environmental flows is only one of the tools that can be used to rehabilitate rivers. River rehabilitation can involve flow issues, as well as floodplain issues, riparian issues, catchment management and in-channel works. It would be rare for rehabilitation of a degraded river to require consideration of flow issues only. Ideally, clearly defined rehabilitation objectives should guide the direction of the environmental flow evaluation.

Fluvial Systems Pty Ltd 5 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Gordon et al. (1992, p. 424) defined ‘in-stream flows’ as those that are retained within their natural setting, as opposed to those diverted for off-stream uses, such as industry, agriculture, and human consumption. In-stream flows can also be used for human activities, such as recreation, or hydro-power generation. In- stream flows also support wildlife. The flows themselves can stimulate breeding cycles, or supply food from upstream, while the hydraulic conditions created by flows are important in providing habitat for wildlife, both within the channel and on the floodplain. In an unregulated river, all components of the natural flow regime impact on the aquatic biota. However, ecologists generally believe that certain components of the natural flow regime are more critical than others with respect to maintaining normal ecological functioning. The term ‘environmental flows’ is used in the context of managing flows in regulated rivers (including those affected water abstraction and floodplain disconnection) to describe the component of in-stream flows that should be maintained in order to achieve environmental objectives. Environmental flows vary from river to river because the environmental objectives vary, and because the ecological requirements of the flora and fauna that inhabit rivers also vary. Rivers with a very high conservation value are likely to have environmental flow needs that are closer to the natural flow regime than a degraded river, where the prospects for enhancing environmental values are low, and most of the water is probably already committed for off-stream uses. Along with providing the ecological requirements of native species, Kinhill (1998) also included in their definition of environmental flows: enhancing aesthetic values, maximising production of recreational and commercial species, and protecting features of scientific and cultural interest. Given that environmental flow regimes are different to the natural flow regime, habitat quality or availability are likely to be lower than under unregulated conditions. Even when the environmental flow regime is manipulated to provide natural levels of habitat for some species, this regime may disadvantage other species. Operational constraints often mean that the temporal pattern of habitat availability is different to the natural pattern. There may be opportunities to offset these differences by enhancing the habitat through channel reshaping or addition of large woody debris or other physical structures. In rivers that have suffered loss of riparian and in-stream vegetation, excessive sedimentation, channel erosion, degraded water quality, or other disturbance, rehabilitation of these components could add value to any environmental flows that were provided. Indeed, there may be little benefit in providing environmental flows alone to highly disturbed rivers. This is an important question that can only be resolved through implementation and careful monitoring of environmental flows. In this regard, the Environmental Flows and Water Quality Objectives for the River Murray Project will make an important contribution to the understanding of regulated river processes. There is no denying that the River Murray is a severely regulated river, with a high level of commitment of the water resource for off-stream use. Despite its history of exploitation, the river does retain some highly valuable ecological resources that warrant maintenance, and in many cases enhancement. The Environmental Flows and Water Quality Objectives for the River Murray Project aims to identify alternative operating strategies that meet the needs of off-stream users, but which enhance or maintain these ecologically important values of the river.

Fluvial Systems Pty Ltd 6 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

1.5 Report structure There are a number of issues that are worth summarising on a basin-wide scale. Alteration of the hydrology due to regulation is one such issue, and comparison of simple indices for various stations provides an overall perspective on the pattern of regulation along the length of the river. Some general principles relating flow and ecological and geomorphological response have been identified in the literature. This knowledge, together with more specific information, forms a general model of the major environmental processes operating in the River Murray (Young, 2001). The main elements of the flow response model are worth tabulating because these principles allow qualitative prediction of the impact of regulation across the full range of situations that occur in the River Murray. In other words, these principles allow prediction of the direction of change likely under any particular regulation situation. This model is an important tool, because some ecological and geomorphic responses are slow, variable or otherwise difficult or expensive to measure directly. The specific impacts of regulation on the River Murray environment are so numerous and complex that they have only been partially characterised. However, the scientific research effort undertaken so far has produced a good understanding of some of the major regulation impacts. Individual scientific investigations tend to focus on recognised field sites, or sites that are representative of larger areas of the river. Rivers are known to have distinct downstream (longitudinal) gradients in many environmental characteristics, often marked by discontinuities at sites where the flow regime or geomorphology abruptly alter. It is logical then to structure a review of the scientific literature in terms of distinct spatial river zones. While Young (2001) classified the rivers of the Murray Darling Basin on the basis of primary physical attributes, it was also recognised that when considering responses to alteration of the flow regime, it is more appropriate to divide the river into management zones rather than geomorphic zones (Young, 2001, p. 112). Young (2000, p. 101) divided the River Murray from Hume to the Darling River junction into three zones, with boundaries at Tocumwal and the Wakool River junction. Thoms et al. (2000) also divided this area into three zones but placed the boundaries at Tocumwal and Torrumbarry Weir. In defining the entire river from Dartmouth to Wellington, Thoms et al. (2000) included three additional zones. The full geographical scope of the Murray-Darling Basin Agreement extends to the mouth of the river, so an additional zone downstream of Wellington has also been defined (Jensen et al., 2000). The zones adopted in this report are consistent with the management zones defined by the Environmental Flows and Water Quality Objectives for the River Murray Project. These zones are as defined by Thoms et al. (2000) and Jensen et al. (2000), except that the downstream limit of the zone downstream of Hume Dam, set by Thoms et al. (2000) at Tocumwal, is located further downstream at Yarrawonga, and the zone from Mildura to Wellington is divided into two zones at Lock 3: 1. Dartmouth Dam to Hume Dam 2. Hume Dam to Yarrawonga 3. Yarrawonga to Torrumbarry Weir (including Barmah Choke and Edward River system) 4. Torrumbarry Weir to Lock 11 Mildura 5. Lock 11 Mildura to Lock 3 6. Lock 3 to Wellington

Fluvial Systems Pty Ltd 7 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

7. Wellington to the Mouth 8. Lower Darling River downstream of Menindee Lakes and the Great Anabranch The boundaries of these eight zones coincide with important regulating structures but some are also related to natural boundaries. For example, Lock 3 is located just upstream of Overland Corner, which marks the head of the Murray Gorge. Across the Mallee as far as Overland Corner, the River Murray trench is 10 km wide, being cut into the relatively soft Parilla Sands, while downstream as far as Mannum the River cuts into tough limestone to produce a narrow gorge 400- 1,600 m wide (Rutherfurd, 1990, p. 30). A great deal of information is available on the environmental condition of the River Murray, and the impact of flow regulation. This information is available in a variety of forms: refereed scientific journal articles based on rigorous research in a specific location on the Murray; management and consulting reports and expert knowledge. This review is focussed on quantifiable changes to the environmental condition of the river, and peer reviewed journal articles were preferred. However, there is a great deal of information available that does not appear in journals but which was considered worthy of inclusion. All scientifically generated information has a level of uncertainty associated with it. The information appearing in peer reviewed journals has the highest level of reliability and certainty. Expert opinion based on visual inspection, or consideration of available information, is less reliable and less certain, because the scientist is not basing his/her opinion on the results of specific experiments or field measurements that were carried out to address the particular question being asked. Rather, the opinion is based on experience with experiments or measurements in this or other rivers, information gathered from reading the results of other experiments and surveys, or from general discussions with other scientists working in the field. Scientists often regard a statement of expert opinion as being akin to an untested hypothesis (Young, 2001), i.e. a general prediction deduced on the basis of best available information, but stated with reservation and a degree of uncertainty that can only be progressed through a process of detailed scientific testing. To allow readers to judge the veracity of the reviewed documents, a scale of reliability/certainty of information has been devised (Table 1-1). This report does not rate each item of information presented according to the scale. The purpose of the scale is simply to remind readers that information comes in various forms, some of which is sufficiently reliable and accurate to be translated directly into confident management decisions, but the bulk of which contains a level of uncertainty. Some information is little more than speculation.

1.6 Key references

1.6.1 Hydrology There have been previous efforts to collate information concerning the impacts of regulation on the River Murray, and this review has drawn heavily on this work. The previous work has followed various approaches. In a major review of the hydrological impact of regulation on the River Murray, Maheshwari et al. (1993) undertook original analysis of hydrological data from 8 River Murray gauging stations, and also analysed water level fluctuation data from 6 Locks. Most of this work was later published as a peer reviewed journal article (Maheshwari et al., 1995). Although titled The Impact of Flow Regulation on the Hydrology of the River Murray and its Ecological Implications, Maheshwari et

Fluvial Systems Pty Ltd 8 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission al. (1993) did not explore the environmental consequences of flow regulation in much detail [the later paper, Maheshwari et al. (1995), did include a short review of the environmental consequences of regulation]. Maheshwari et al. (1993) is a key reference on the basis of its detailed and comprehensive original analysis, but it was not the first attempt to quantify the hydrological impact of regulation. Maheshwari et al. (1993) was preceded by the hydrological analysis of Baker and Wright (1978), Close (1990) and Thomson (1992). The reviews of Thoms et al. (2000) and Young (2001) draw heavily on the work of these authors.

Table 1-1. Levels of information, ranking in order of highest level of reliability and certainty to lowest. Form of information Rank of reliability level Locality-specific, peer reviewed scientific journal article 1 Conference paper or trade journal, not reviewed, but includes 2 data from the site, statistically analysed, plus reference to available information from other experts Consulting report, or internal report (grey literature) based on 3 visual inspection – may or may not include new data from the site, and including a formal review of available data and/or information from other experts, with supporting references Expert opinion without visual inspection, formed on the basis 4 of formal review of available information from other experts, with supporting references Expert opinion with visual inspection, but no formal review of 5 available information from other experts Expert opinion without visual inspection and no formal review 6 of available information from other experts

1.6.2 Geomorphology A comprehensive review of the impact of flow regulation on the geomorphology of the River Murray has not been published, but several studies have investigated different aspects of the river’s geomorphology. Rutherfurd (1990) reviewed changes in planform, channel width and thalweg elevation of the River Murray from Albury to the South Australian border, while ID&A (1993) and Tilleard et al. (1994) examined in greater detail the channel changes between Hume Dam and Lake Mulwala. Gippel and Lucas (2002) recently investigated the geomorphology of the River from Yarrawonga to Echuca. Thoms and Walker (1991) modelled changes in sediment transport rates from Dartmouth Dam to Tocumwal due to regulation. Thoms and Walker (1992a, b, 1993) investigated channel changes associated with weirs in the lower River Murray (South Australia), while the geomorphology of the lower lakes and mouth of the river were examined by Bourman and Barnett (1995). Thoms et al. (2000) and Young (2001) provide good general reviews of the geomorphic impacts of regulation.

Fluvial Systems Pty Ltd 9 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

1.6.3 Water quality A River Murray water quality monitoring program was initiated in 1978. The first eight years of its operation were reviewed by Mackay et al (1988), and although written 13 years ago, this report remains the most up-to-date and comprehensive consideration of the impact of regulation on water quality. Shafron et al. (1990) reproduced much of this material as a chapter in Mackay and Eastburn (1990). The review paper by Walker (1992) also relied heavily on the work of Mackay et al (1988). Walker (1985) provided additional information on the impact of dams on downstream water quality. In this review we group the issue of algal blooms with water quality. While the general subject of phytoplanton (algae) is undoubtedly an ecological issue, from the management perspective, an algal bloom is considered principally to be a problem of water quality.

1.6.4 Ecology There is no ecological equivalent to the structured, detailed and comprehensive hydrological analysis undertaken by Maheshwari et al. (1993). The hydrological analysis was possible only because of the availability of long-term gauging data (pre- and post- the various phases of regulation). Ecological data are much more diverse, and generally more difficult to collect. Until recently there was no integrated program for collection of ecological data. The Sustainable River Audit (MDBC, 2002) will overcome this deficiency. Walker (1985) and Walker (1992) provide excellent reviews of available information regarding the impacts of flow regulation on the ecology of the River Murray. Various chapters in Mackay and Eastburn (1990) contain information regarding the impacts of regulation on different aspects of the ecology of the River Murray. The reviews of Thoms et al. (2000) and Young (2001) draw on a wide range of literature concerning the ecological impacts of regulation.

1.6.5 Scientific Panel studies Small research teams or individuals from universities or government agencies have undertaken most of the detailed ecological and geomorphic investigations on the River Murray. These studies are generally concerned with a specific aspect of ecology or geomorphology, with data being collected from a limited area. Scientists who undertake this work often publish their results in refereed journals. Publication involves a review and editorial process that ruthlessly excludes non-essential information, so scientists usually possess considerably more knowledge about their areas of interest than is conveyed in their research papers. Scientists also possess a wealth of related knowledge, gained through review of previous studies, and comparison of results with similar studies from other areas. The Expert Panel Approach to assessment of environmental flows uses the collective knowledge of a selected group of experts (they may be scientific and non-scientific) to provide opinion on the environmental condition of a river system, generate hypotheses concerning the links between flow and environmental condition, and make recommendations on flow management (Swales and Harris, 1995). The Scientific Panel Approach (Thoms et al., 2000, pp. 17-21) is a more sophisticated version of the Expert Panel Approach, with the core group composed only of scientists, engineers, modellers or other technical experts. An associated Steering Committee may contain management or community representatives. The Scientific Panel Approach does not necessarily involve commissioning detailed field based scientific investigations,

Fluvial Systems Pty Ltd 10 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission but field inspections, detailed data and literature review, and hydrological modelling are normally conducted. Two important Scientific Panel investigations have been undertaken on the main stem of the River Murray: Thoms et al. (2000) reported on the River Murray from Dartmouth Dam to Wellington, including the Lower Darling River (downstream of Menindee Lakes), and Jensen et al. (2000) reported on the Lower Lakes below Wellington, including the Coorong estuary. The methodology employed by these Expert Panel studies was to collectively pool available data and theoretical knowledge, inspect a range of sites, glean local information, form judgements through workshop interaction, conduct necessary flow modelling, and then make recommendations for priority actions (Thoms et al., 2000, p. 19-21; Jensen et al., 2000, p. 5-6). Obviously, one of the key tasks of the Scientific Panel studies was to review the environmental impacts of flow regulation on the River, so it is not surprising that much of the information presented in this report is also contained in Thoms et al. (2000) and Jensen et al. (2000). While the Scientific Panel reports are well written, and they have lengthy bibliographies, they do contain some unsupported and/or apparently speculative statements. This may be partly a function of the style of the reports which, rather than dwelling on detailed scientific argument and minutiae, focus on developing priority management actions based on the best available knowledge generated by the assembled panel of experts. The main objective of both Scientific Panel studies was to recommend environmental flows, while the main objective of this report is to provide a credible account of the scope and scale of flow-related environmental problems.

1.6.6 General reviews of regulation impacts Apart from the reviews contained within the Scientific Panel reports of Thoms et al. (2000) and Jensen et al. (2000), only a few publications have attempted an overview of the impact of regulation on the total environment of the River Murray. Mackay and Eastburn (1990) is a collation of chapters that address various aspects of the River Murray environment, but it is more a reference document of the sum state of knowledge than a critical examination of the role of regulation in degrading environmental values. Walker (1992) provides a comprehensive overview of the character of the River Murray, and also neatly summarises the main impacts of regulation. Walker and Thoms (1993) is an excellent review of the environmental effects of flow regulation on the lower River Murray (downstream of the Murray-Darling confluence). The case study of the lower River Murray presented in Wittington et al. (2000) is similar in content to Walker and Thoms (1993). Jensen (1998) provides a less detailed summary of the effects of regulation in this area. Bourman and Barnett (1995), while focusing on the geomorphology of the lower lakes and mouth area, also summarises the ecological implications of regulation. Young (2001) undertook the most recent review of scientific knowledge of the interactions between riverine ecology and flow regimes in the Murray Darling Basin. This is a major work that attempts to develop a comprehensive conceptual model of river function for the major rivers of the basin. As well as collating existing published and unpublished material, the authors added their own expert opinions and those of other scientists who were interviewed. The style of Young (2001) is somewhat inconsistent, with some sections meticulously referenced, others unreferenced, and others making liberal use of “pers. comm.”, even when appropriate published sources are known to exist. This is partly a reflection of the mix of authors used for the various chapters, and partly due to the method of information collection (which included interviews with experts). Also, the book

Fluvial Systems Pty Ltd 11 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission is intended primarily as a reference guide for those engaged in river management, an audience that is generally more interested in obtaining applicable knowledge than delving into the complex scientific detail. Thoms et al. (2000), Jensen et al. (2000) and Young (2001) are undeniably the key references used in the preparation of this report. Jensen et al. (2000) and Young (2001) are both edited volumes that contain contributions from several other authors who were not editors, but here we cite the report rather than the individual authors. These three key reports contain a mixture of existing published information, existing unpublished data and knowledge, and new hypotheses generated throughout the course of their production (consultation with and interaction between scientific experts was integral to the process of preparing these manuscripts). Thus, the information they present covers the full spectrum of reliability and certainty (Table 1-1). Thoms et al. (2000) and Young (2001) were formally and externally reviewed, while Jensen et al. (2000) was simply circulated among the members of the scientific panel and project steering committee. External review helps maintain a certain quality standard for consulting reports, but generally the process is less stringent than that employed by the most highly regarded journals. Where possible we have examined the primary reference sources listed in these three key reports. We have also searched the literature for additional material. Our approach was to favour information that specifically addressed the impacts of flow regulation and was well supported by data. Where we included less reliable or apparently speculative information, it was identified as such.

Fluvial Systems Pty Ltd 12 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

2 River Murray Flow Regulation Background

2.1 Brief history of regulation The historical and current operations of the River Murray as a water resource have been well-documented elsewhere (e.g. Baker and Wright, 1978; Crabb, 1988; Jacobs, 1990; Walker, 1992; Young, 2001). Close (1990) and Thoms et al. (2000) detailed the main structures that are used to control discharge and/or water level in the system and described the hydrological impact of the operation of those structures. Flow regulation began with the construction of levees in the 18th century. Commercial navigational use of the river commenced in 1853. In the 1870s, Victoria started to use the river for irrigation in the Kerang Region. At the time, diversion of water for irrigation conflicted with navigation requirements, but by the 1880s the success of railways led to rapid demise of the importance of navigation (Jacobs, 1990). Expansion of irrigation was limited to some extent by the variability of flows in the river, and serious droughts in the late 1890s and early 1900s raised the issue of drought protection. The response to this problem was to exert a high level of control over river flow. The River Murray Waters Agreement, ratified in 1915, remains essentially unchanged today: • Flow at Albury is shared equally between New South Wales and Victoria; • Victoria and New South Wales retain control of their tributaries below Albury; • Victoria and New South Wales supply South Australia with a minimum agreed ‘entitlement’. The Agreement also provided for construction of a system of storages and locks and weirs. The large storages were intended to increase security of supply, while the smaller structures were to make the river permanently navigable and provide pools for irrigation diversions. In 1924 the Agreement was amended to give preference to building structures for irrigation rather than navigation (Jacobs, 1990). Hume Dam, with a storage capacity of 3,038,000 ML, is the principal (but not the largest) operating storage for the River Murray system (Figure 2). Construction of Hume Dam began in 1919, with the first stage completed in 1936, and enlargement completed in 1961. During this period weirs were constructed for irrigation purposes at Mildura (constructed in 1927), Torrumbarry (constructed in 1924) and Yarrawonga (constructed in 1939). Construction of Barrages at the Murray Mouth in 1940 was intended to prevent ocean intrusions and allow local landholders to use fresh water for irrigation (Jacobs, 1990). The next major storage involved construction of banks, weirs and channels on the natural Menindee Lakes (constructed 1949-1968), to increase their capacity and prevent drainage back to the Darling River after filling. The Snowy Mountains Scheme (constructed 1949-1974) increased inflows to Hume Dam, beginning in 1967, by diverting water from the Snowy River. At around this time it was recognised that rapid expansion of irrigation would require an additional storage to ensure supplies in dry years. Construction of Dartmouth Dam (1973-1979) on the Mitta Mitta River (Figure 2) made additional supplies available to Victoria, increased security of supply, and increased South Australia’s minimum entitlement (Jacobs, 1990).

Fluvial Systems Pty Ltd 13 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Between 10,000 GL and 11,000 GL of water is diverted per year from the rivers of the Murray-Darling Basin. This is twice the current average flow at the South Australian border. Over 90 major storages are located in the Murray-Darling Basin – the total storage is approximately 29,500 GL (more than double the average natural annual discharge at the river mouth). Fifteen locks and weirs have been constructed on the River Murray and more are located on its tributaries (Figure 2). Levees are located throughout the flood-prone reaches of the river, particularly around Swan Hill and Cobram.

2.2 Hydrological impacts The impacts of flow regulation on the hydrological behaviour of the Murray- Darling system have been presented by a number of authors, including Baker and Wright (1978), Close (1990), Maheshwari et al (1993; 1995) and Thoms et al. (2000).

2.2.1 Annual flow volume Annual flows in the River Murray are so variable that, despite diversions on average halving annual flows to South Australia, Cunningham et al. (1984) were unable to detect a statistically significant change in mean annual flow using historical data from the period 1901 to 1983. Baker and Wright’s (1978) limited analysis of historical data from Hume and Swan Hill appeared to demonstrate the impact of diversions on annual flows, but their results were certainly confounded by natural flow variations. To avoid the problem of natural flow variability, Mackay et al. (1988) used data from inflowing tributaries to construct a double mass plot that demonstrated the impact of regulation on annual flows to South Australia. Data on tributary inflows form the basis of the MDBC Monthly Simulation Model used by Close (1990) and Maheshwari et al. (1995) to determine the extent of the impact of water resource development (post-Dartmouth) on annual flows in the River Murray (Table 2-1). The analyses of Close (1990) and Maheshwari et al. (1995) produced similar results, but Maheshwari et al. (1995) examined more gauging stations and analysed more recent data. The data of Maheshwari et al. (1995) demonstrate that catchment and river diversions have reduced the flow in the lower River Murray (below Euston) to half the natural levels, or less (Table 2-1). Inflows from the Darling River do not improve the situation because the current (regulated) average annual discharge is only 57% of the natural level (Maheshwari et al., 1983, p. 17). The reduction in mean annual discharge due to regulation is less than the reduction calculated using the median annual discharge (Table 2-2), because the mean is strongly influenced by large discharges in the very wet years.

2.2.2 Flow variability Flow variability can be defined at various temporal scales, from annual flow variation to the rates of water level variation during flood events. Regulation has generally reduced flow variability. Variations in the volume of annual flow are still largely determined by the natural long-term pattern of rainfall, but the large storages have the capacity to carry water over from one year to the next, thereby dampening this natural year-to-year variation. For example, the natural annual coefficient of variation at Albury was 0.49, but the combined influence of Hume and Dartmouth dams reduced this to 0.34 (Maheshwari et al., 1995). In contrast, downstream of Yarrawonga, the annual coefficient of variation increased steadily as the river became more regulated, with the effect increasing downstream

Fluvial Systems Pty Ltd 14 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

(Maheshwari et al., 1993, p. 17, 33). Maheshwari et al. (1993, 1995) did not comment on this observation, but it can be explained by the fact that while annual discharge in wet years has been unaltered by regulation (large floods, >1 in 20 year ARI, are not noticeably affected by regulation), annual discharge in other years is reduced through diversions. Thus, while the spread (standard deviation) of annual discharge has been only slightly reduced by operation of the large impoundments, below Yarrawonga the mean flows have been drastically reduced, thereby increasing the coefficient of variation. Variation of flow through the year is reduced through the release of relatively constant flow volumes during the periods of water harvesting (relatively constant low flows are released) and irrigation water supply (relatively constant channel capacity flows are released). This is reflected in dramatic changes in the distribution of the monthly coefficient of variation. At Albury the regulated monthly coefficient of variation is lowest in February (0.24) and highest in June (1.27), whereas natural flows were least variable in September (0.50) and most variable in April (1.09) (Maheshwari et al., 1995). Weirs are managed to maintain the water level at a fairly constant level for long periods, and this further reduces natural flow variability (Maheshwari et al., 1993; Young, 2001, p. 74). Natural fluctuations in river levels are severely modified by impoundments, which are managed according to rules for release rates that are not necessarily based on natural rates of rise and fall. For example, water level below Hume Dam cannot be drawn down at a rate that exceeds 152 mm/day (the six-inch rule), but this traditional rule was not based on observations of natural recession rates (Green, 1999). Weirs on the lower River Murray have created distinctive patterns of rise and fall of water level, with low rates of change upstream of weirs and high rates downstream (Maheshwari et al., 1993, p. 150).

Table 2-1 Comparison of Natural and Benchmark (or Current based on 1990 levels of diversions) mean annual flows for selected locations on the River Murray. Data are from Maheshwari et al. (1993; 1995), based on MDBC MSM modelled flows from 1990. Location Natural mean Current Current as flow mean flow percentage of (GL/yr) (GL/yr) natural Albury 4,748 5,218 110% Yarrawonga 6,202 4,661 75% Euston 11,965 6,154 51% Upstream of Darling 12,438 5,851 47% River confluence South Australian Border 13,445 6,172 46% Lock 1 13,219 5,733 43% Barrages 12,654 4,867 38% Murray Mouth 12,498 4,712 38% Darling River at 3,442 1,957 57% Burtundy

Fluvial Systems Pty Ltd 15 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 2-2 Comparison of Natural and Benchmark (or Current, based on 2001 levels of diversions) median annual flows for selected locations on the River Murray and for major tributaries (at end of system), including the Snowy River, from which water is transferred to the River Murray System. (source: MDBC monthly simulation model results from 2001) (see also Fig 1). Location Natural Benchmark Current as (listed upstream to median flow (current) percentage of downstream) (GL/yr) median flow natural (GL/yr) Murray @ Albury 4,324 4,832 112% Kiewa 566 560 99% Ovens 1,399 1,395 100% Murray @ Yarrawonga 5,590 3,904 70% Goulburn 3,208 1,035 32% Broken 90 159 176% Campaspe 242 77 32% Loddon 188 50 27% Murrumbidgee 2,454 764 31% Upper Darling 1,780 1,164 65% Murray @ S.A. border 12,385 4,827 39% Murray @ Mouth 11,084 2,857 26% Snowy @ below 1,104 9 1% Jindabyne Snowy @ mouth 1,960 1,160 59%

2.2.3 Seasonality The seasonality of regulated flows varies along the river depending on the degree and timing of diversions, and the influence of natural inflows (Walker, 1992). Under pre-regulation conditions, peak flows occurred in spring and then rapidly receded to low levels in late summer and autumn. The construction of a large volume of storage on the system has altered this seasonal variation, by holding winter and spring inflows to the major storages in the upper reaches (Hume and Dartmouth Dams) and releasing water to meet peak irrigation demands in the summer and autumn (Close, 1990). For example, historical data show that the average March flows at Albury increased from approximately 100,000 ML pre- Hume to almost 400,000 ML post-Hume (Baker and Wright, 1978), while modelled data suggest an even higher average regulated March flow of 600,000 ML (Close, 1990). The increase in summer-autumn flows is less marked at Yarrawonga, below the diversion weir that impounds Lake Mulwala (Walker, 1992). Downstream of Yarrawonga, other major diversions (e.g. Torrumbarry Weir and Stevens Weir on the Edward River) act to further reduce total flows (Table 2-1), especially in summer. At Euston and further downstream, the combination of reduced summer flows due to irrigation diversions and unregulated inflows are sufficient to restore the natural pattern of higher flows in the late winter to early spring period (Close, 1990; Walker, 1992; Maheshwari et al., 1995).

Fluvial Systems Pty Ltd 16 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

2.2.4 Large floods The size of extreme floods, such as those that occurred in 1917 and 1956, have not been changed much by water resources development. Maheshwari et al. (1995) found that at all stations along the River Murray the frequencies of peak flows with recurrence intervals of 20 years or more did not change appreciably with regulation. Close (1990) explained that this was because major floods are usually preceded by a wet period that fills storages and reduces demand for diversions. The runoff from very large events simply overwhelms the storages, so that their influence becomes negligible.

2.2.5 Mid-range flows The major hydrological impact of the construction of storages has been to reduce the frequency of occurrence of mid-range flows, or minor-medium floods (Close, 1990). Under pre-regulation conditions, at the South Australian border, a monthly flow in excess of 1,000 GL occurred in 45% of months, while under current conditions, this would only be expected in only 15% of months. The frequency of monthly flows in excess of 1,500 GL decreased from 25% to 5% of months under regulation (Close, 1990). Thoms et al. (2000, p. 59) demonstrated that for the entire River Murray, floods that had a natural recurrence interval in the range 1 in 10 years to 1 in 2 years currently have a much reduced frequency of occurrence. Floods that used to occur every second year now occur every 6 to 8 years, while floods that used to occur every 10 years now occur every 25 to 30 years. It is this disruption to the flow regime that is manifest as reduced frequency flooding of wetlands and riparian vegetation (Close, 1990; Thoms et al., 2000). Regulation has also reduced the duration of mid-range floods. For example, since construction of Hume Dam the duration of seasonal floods in the river downstream to the Barmah Forest has reduced by 1-2 months (Walker, 1992).

2.2.6 Low flows The River Murray flows through a semi-arid environment, so it is not surprising that prior to regulation, during times of extreme drought, it was reduced to a chain of saline ponds. The Darling River also ceased to flow during protracted droughts (Jacobs, 1990). Close (1990) determined from modelling that the frequency of drying in the middle reaches of the Murray was about once every hundred years. Under regulated conditions, there is always some flow in the river. At Albury and Yarrawonga, low flows (0-500 GL/month) increased in magnitude noticeably after completion of Hume Dam, and they also became less variable from year to year (Maheshwari et al., 1995). Between Euston and the South Australian border, the low flow characteristics have been only slightly altered by regulation, but below Lock 1 to the mouth, for a given annual non- exceedance probability, low flows are generally lower under regulated conditions (Maheshwari et al., 1995). At the South Australian border, monthly flows less than 500 GL occurred 7% of the time under natural conditions, but under current conditions they can be expected to occur 66% of the time (Close, 1990). Near the Murray mouth, prior to regulation there was flow in the river for more than 95% of the time, keeping Lake Alexandrina quite fresh for extended periods. Under the regulated regime, flow through the Mouth effectively ceases (i.e. <10 GL/month) for around 20% of the time (Maheshwari et al, 1995). The Mouth now experiences periods of up to four or five years with little outflow (Close, 1990). Where the lakes once supported a Murray cod fishery, by the 1930s they were supporting a salt water fishery. Construction of the barrages in

Fluvial Systems Pty Ltd 17 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

1940 prevented saline inflow, and since then the lakes have remained fresh (Close, 1990).

Fluvial Systems Pty Ltd 18 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

3 General Ecological and Geomorphic Responses to Flow Regulation This report is structured and focused on specific, defined zones of the River Murray. However, there are a number of issues that are worth presenting on a spatial scale larger than the individual zone, either because they are basin-wide issues or because research has been published on a particular issue in a number of locations in the basin. Young (2001) tabulated general ecological and geomorphic responses to flow regime change for the Murray-Darling Basin (Table 3-1 to Table 3-5). The responses listed by Young (2001) are not always supported by data in the specific case of reaches of the River Murray. Rather, they are general principles that were sourced from the literature, interviews with experts, and specific studies where available. A similar list of ecological responses to flow regulation appears in Thoms et al. (2000, p. 52). Part I of Thoms et al. (2000) is an overview of scientific knowledge of the River Murray and the lower Darling River. Information presented in the overview of Thoms et al. (2000) supports the general principles tabulated by Young (2001), and also elaborates on the impacts of altered water quality (Table 3-6) and barriers (Table 3-7). Both of these publications were based on comprehensive reviews of the extensive body of literature concerning the River Murray, and the opinions of noted experts in the field, so it is not surprising that they developed similar conclusions regarding general ecological responses to flow regulation.

Fluvial Systems Pty Ltd 19 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 3-1. General ecological and geomorphic responses to changes in flow magnitude (source: Young, 2001) Likely changes to river processes: • Reduced range of flow leads to reduced hydraulic diversity • Reduced flow depth lowers near-channel groundwater profiles, thus affecting deep-rooted riparian vegetation • Reduced flow leads to reduced bedload and suspended sediment transport, while increased flow has the opposite effect • Reduced flow leads to increased sedimentation and drowning of coarse sediment by fines • Reduced flow leads to a reduction in floodplain-channel connectivity • Reduced flow leads to a slow reduction in the size of the active channel, and sedimentation reduces the depth and variability of the bed. Increased flow has the opposite effect, and the process is more rapid • Reduced flow causes a reduction in the wetted perimeter of the channel, and a consequent reduction in available habitat • Reduced flow reduces habitat diversity through altered bed character, hydraulics, vegetation and channel morphology • Increased flow causes bank erosion, and the associated loss of riparian trees reduces cover and shading • Changes in flow volume alter the carrying capacity of the system, i.e. the total biomass it will support, by changing the amount of habitat that is available • Changes in flow volume alter the character of habitats which often leads to adjustments in the species composition. When habitat diversity is reduced, species diversity is usually also reduced. Likely ecological changes following flow reduction: • Invasion of slow water areas by aquatic plants • Changes in invertebrate fauna from typically lotic species to species more typical of lentic areas • Increases in relative abundance of introduced fish species with wide habitat tolerances • Reductions to the health of riparian trees due to reductions in the level of groundwater Likely ecological changes following flow augmentation: • Replacement by biofilms of filamentous algae and submerged plants in faster flowing areas • Loss of plant communities along the bank (due to erosion) • Changes in invertebrates to a predominance of species able to survive higher hydraulic stress • Changes in fish species to a predominance of species preferring faster- flowing water

Fluvial Systems Pty Ltd 20 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 3-2. General ecological and geomorphic responses to changes in flow variability (source: Young, 2001) Likely changes to river processes: • Decreased flow variability leads to a decrease in temporal variability of hydraulic conditions • Short-term variability of freshes and minor floods determines the frequency of lateral connection with the riparian zone • Short-term variability of freshes and minor floods is an important control on bed disturbance (affecting sediment transport and the disturbance regime for bed-dwelling organisms) • Short-term variability of freshes and minor floods determines the frequency of movement of organic material from floodplain and backwaters to the main channel • Changes in seasonal variability of flows alters the seasonal pattern of material transport, affecting seasonal nutrient availability and suitability of bed material for spawning • Reduction in short-term flow variability can lead to bank erosion • Reduction in short-term flow variability can reduce channel cross-sectional complexity, with the erosion or degradation of in-channel features like benches • Reduction in short-term flow variability alters the diversity of bedforms and substrate types • Reduction in short-term flow variability leads to a reduction of habitat diversity • Alteration of the seasonality of flow changes the seasonal availability of in- stream habitat • Changes in the temporal availability of different habitats disrupts the life cycles and behaviour of fish which have different preferences for resting, feeding and spawning • Reduction in frequency of depth changes is likely to alter the character of the habitat for deep-rooted riparian plant communities Likely ecological changes: • Reduced flow variability is likely to alter species composition and reduce species diversity • Reduced flow variability is likely to alter the composition of riparian plant communities and benthic fauna • Changes in seasonal flow patters may alter the levels of in-stream primary production • Changes in seasonal flow patters may reduce the availability of life cycle cues and migration opportunities for higher organisms such as fish • Changes in seasonal flow patters may alter growth pattern of riparian vegetation • Changes in seasonal flow patters may disrupt the breeding responses of some bottom-dwelling invertebrates • Morphological changes associated with reduced flow variability, such as bank slumping and degradation of in-channel features, alter the rates of recruitment of woody debris and grossly affects littoral and riparian plants

Fluvial Systems Pty Ltd 21 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 3-3. General ecological and geomorphic responses to alteration in the rate of flow changes (source: Young, 2001) Likely changes to river processes: • Reduced rate of change of flow leads to a reduced rate of change of hydraulic parameters (e.g. depth and velocity) • Rate of fall affects the pattern of post-flood sediment deposition, which influences bedforms • Rapid rate of fall can cause bank slumping (in areas of dispersive clays) • Rate of fall determines the rate at which floodplain and backwater habitats are disconnected from the channel Likely ecological changes: • Some native fish use the rate of rise in water level as a cue for breeding. When depth drops rapidly they can be stranded on the floodplain • Invertebrates take feeding cues from rates of change in depth and velocity. Invertebrates can be stranded by rapid de-watering, while unnaturally rapid rises can cause drift • Bank erosion causes loss of riparian vegetation and increases large woody debris recruitment • Plants may not be able to respond quickly enough to rapid rates of depth increase • Rapid rates of fall in wetlands can cause waterbirds to abandon their nests

Fluvial Systems Pty Ltd 22 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 3-4. General ecological and geomorphic responses to changes in extreme flows (source: Young, 2001) Likely changes to river processes: • Flood flows determine the maximum depths, velocities and shear stresses, which are responsible for the major system disturbances • Droughts and periods of very low flow also act as disturbances • Changes to the frequency of flood flows lead to a change in the transport of sediment, to the form of the channel and the distribution of large woody debris • Flooding determines the volume of sediment transported to the floodplain • Floods transport carbon-rich material from the floodplain to the channel which is an important process in confined floodplain channels, but less important in open floodplain channels • Drought periods on the floodplain prepare soils for release of nutrients during floods • Large floods determine overall channel dimensions and planform, while the major bedforms are a response to the full range of floods • As a result of their impact on channel form, floods determine the character of the channel inherited by the following period of low flow • Floods are responsible for floodplain construction • Through the control they exert over channel morphology, floods determine the structure of habitats, the disturbance of habitats, the dynamics of large woody debris, and the frequency of overbank flooding • Increased inundation of floodplains through irrigation changes some temporary wetlands into near-permanently flooded areas, with consequent changes to their habitat and species diversity • Any change to the health and composition of floodplain vegetation induced by changed flooding and drying patterns also affects birds, reptiles and marsupials • Bed topography (determined by flood regime) affects the availability and character of pools for use as refuge during droughts Likely ecological changes: • The lowered frequency of disturbance associated with reduced flooding typically causes species diversity to drop. Predation and competition for resources may become more important determinants of community structure • Reduction of flood frequency or magnitude will lower the productivity of the floodplain • A reduction in flooding may lead to a reduction in the successful recruitment of native fish • Seeds on the floodplain have a finite viability, so germination may fail to take place if flooding frequency is sufficiently reduced • Flooding of wetlands is a key trigger for the breeding of ibis and many species of duck, and the duration of the flood is important for allowing them to complete their breeding cycle • Drought periods cause high stress in aquatic communities, but many native species have developed methods to survive dry periods, which provides them with a competitive advantage over introduced species. The regulation of flows to provide continual base or irrigation flows leads to the loss of this advantage, and is likely to encourage introduced species

Fluvial Systems Pty Ltd 23 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 3-5. General ecological and geomorphic responses to changes in seasonal predictability of flows (source: Young, 2001) Likely changes to river processes: • By world standards, Australian rivers generally have unpredictable flows. Regulation generally increases the seasonal predictability of flows • Changed seasonal predictability also affects the predictability of in-stream hydraulics, flooding, nutrient fluxes, channel bed conditions, channel and floodplain habitats Likely ecological changes: • The seasonal predictability of flow conditions determines the extent to which organisms can adapt to rely on seasonal cues for breeding, migration and to some extent growth. Where predictability of flows is low, organisms are typically opportunistic in their responses to flooding, irrespective of season. An increase in predictability may favour non-native species (more likely to be adapted to predictable seasonal flows) over native ones

Fluvial Systems Pty Ltd 24 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 3-6. General water quality responses to regulation (source: Thoms et al., 2000, pp. 31-37, 47-49; Young, 2001, p. 75-78; Walker, 1992; Mackay et al., 1988) Likely changes to river processes: • Anecdotal evidence suggests that summer turbidity has increased under regulated conditions. Summer flows in the lower River Murray are mostly sourced from the Darling River, and generally are highly turbid (Walker, 1992) • Turbidity and nutrient concentration are flow dependent, being controlled by re-suspension and sedimentation processes • Headwater storage dams cause substantial decreases in downstream water temperature and a delay of 1-2 months in the timing of the summer maximum • Headwater storage dams can cause increases in nutrient load and concentrations of natural toxicants such as hydrogen sulphide and heavy metals • Water released from storages can have lower levels of dissolved oxygen • Water resources development is intimately associated with the expansion of irrigation. Irrigation has led to an increase in the accession of salt to the River Murray, such that average salinities have increased (Walker, 1992). • Flow regulation has reduced the range of river salinity by preventing cease- to-flow. Salinity at Morgan in 1914/15 reached 10,000 µS/cm, while post- Hume Dam salinities have not exceeded 1,500 µS/cm (Mackay et al., 1988, p. 16). • In general, there is an inverse relation between river salinity and flow rate (Mackay et al., 1988, p. 16). Likely ecological changes: • Water temperature determines the rate of biological processes and, or, acts as a trigger for their onset • Primary production (plant and algae) and decomposition are both temperature dependent functions • For macroinvertebrates, the number of days taken for a particular growth age is inversely proportional to the degrees of temperature. Sexually reproducing aquatic plants may be limited in a similar way • Macroinvertebrate assemblages below dams may favour those adapted to depressed dissolved oxygen levels • Fish often reproduce according to specific water temperature thresholds. Native fish may be unable to breed in rivers with depressed temperatures, with trout the only large fish that can successfully recruit. This effect, combined with competition-predation pressures, is likely to dramatically reduce native fish populations • Most native riverine species of plant and animal can tolerate high salinites and have not been impacted by rising salinity levels. Exceptions are in wetlands used as evaporation basins, and floodplain areas where rising saline groundwater has caused decline of riparian vegetation, especially red gum and black box (Walker, 1992) • Increased turbidity may have provided some compensatory relief from problems of algal growth (Walker, 1992)

Fluvial Systems Pty Ltd 25 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 3-7. General ecological responses to barriers (source: Thoms et al., 2000, p. 50; Young, 2001, p. 74) • Adults and juvenile fish species are known to migrate, and this process is disrupted by barriers such as dams and weirs • Some species that are denied migration may be able to survive and reproduce in the short term but their long term survival prospects are poor • Aggregations of migratory fish below barriers are highly susceptible to capture by anglers • Impounded waters can trap larvae and prevent their distribution downstream • Still water bodies impounded by structures in rivers promotes organisms specialised for those habitats, notably algae, but also zooplankton

Fluvial Systems Pty Ltd 26 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

4 Dartmouth Dam to Hume Dam (Mitta Mitta River)

4.1 Summary of impacts and causes The Mitta Mitta River is an upper tributary of the River Murray that flows into the Hume Reservoir. Dartmouth Dam is a large storage that has altered the pattern of low flows, flood flows, as well as seasonal and daily flow variations in the Mitta Mitta River. Accelerated channel erosion has been reported as a problem. Long periods of relatively constant regulated flow appears to be the main cause of this erosion, rather than drawdown, which has long been the conventional wisdom. The channel bed has less pronounced pool and riffle morphology than expected. This has been attributed to excessive sedimentation during the dam construction period, with the regulated flow conditions maintaining the poorly defined morphology, and degrading the habitat quality of the substrate. The cold water released from the base of the dam prevents spawning of native fish. The cold water favours alien species (trout) at the expense of native fish. Also, the macroinvertebrate communities have changed their composition in response to the regulated flows, degraded substrate, and the cold water releases. The environmental impacts of Dartmouth Dam can extend downstream as far as Lake Hume, depending on flow conditions.

4.2 Regulation influences

4.2.1 Dartmouth Dam Dartmouth Dam, on the Mitta Mitta River, is the primary regulating influence on the Dartmouth to Hume Zone (Figure 2). There are relatively few direct irrigation demands in this area. Lake Dartmouth is a 3,906 GL storage formed by the closure of Dartmouth Dam in 1979. The storage capacity is approximately ten times the mean annual flow of the river at the dam site, so the dam exerts a very strong hydrological influence on the river. The impoundment is 170 m deep, with two offtakes, one at 62 m and one at 121 m depth (Walker, 1985). When the storage is less than 30% full, only the low level outlet can be used. Because the dam water is subject to thermal stratification, releases from the low level outlet are hypolimnetic, and releases from the high level outlet can also be hypolimnetic depending on water level (Lawson and Treloar et al., 2001). The dam controls flow into the reach downstream as far as Lake Hume, 100 km downstream. The main purpose of the dam is to supplement Lake Hume as a carry-over storage for drought security, but it also provides a level of flood protection for the Mitta Mitta valley (Lawson and Treloar et al., 2001). Water from Dartmouth Dam is used when Lake Hume is unable to meet consumptive demands, so it does not have a regular annual cycle of operation (Lawson and Treloar et al., 2001). During dam filling mode the minimum release is 200 ML/d at Colemans gauge (immediately downstream of the Dam), and during release mode the flow is up to 10,000 ML/d (corresponding to channel capacity). Lower releases are also made at times to maximize the storage in Lakes Hume and Dartmouth, and pre-releases are sometimes made to maintain airspace to help in flood mitigation (Lawson and Treloar et al., 2001). Terrazzolo and Erskine (1995), HDDORRP (1998) and Lawson and Treloar et al. (2001) quantified various aspects of the hydrology of the Mitta Mitta River downstream of Dartmouth Dam. Snowy Creek contributes a significant discharge to the Mitta Mitta River, but even below this tributary the flow record is still dominated by operation of the Dam (Lawson and Treloar et al., 2001). The

Fluvial Systems Pty Ltd 27 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission hydrological impacts of regulation on the Mitta Mitta River between Dartmouth Dam and Hume Dam were summarised by Thoms et al. (2000), but the descriptions were qualitative. Lawson and Treloar et al. (2001) provided a quantitative analysis for flows at Tallandoon (downstream of Snowy Creek) (Table 4-1).

Table 4-1. Identified impacts of regulation on the hydrology between Dartmouth Dam and Hume Dam (source: Lawson and Treloar et al., 2001; Thoms et al., 2000) Variable Change due to flow regulation Flow volume • No appreciable change in long term annual discharge Flow • Extended periods of flows at near channel capacity (8,000- variability 12,000 ML/d) for much of summer/autumn, with event duration increased by up to a factor of 3 • For flows between 5,000 ML/d and 10,000 ML/d average event duration has increased by 55%-168%, and overall flow duration has increased by 16%-40% • Above 10,000 ML/d there are fewer events per year (up to 80% reduction) and overall flow duration has reduced (by 43%-83%), but event duration has generally increased (by up to 36%) • More prolonged periods of low flow (over the winter) • Power station releases causing a twice daily fluctuation in flow • Maximum drawdown rates higher and drawdown rates more variable since regulation • Recession limbs of hydrographs are often stepped Flow • Reversed seasonal flow pattern, with more flow in seasonality summer, and autumn and less winter-spring flooding Floods • Flood frequencies decreased for floods greater than 8,000 ML/d peak • Reduced peak flood levels, and increased flood duration (due to attenuating effect of the spillway)

4.3 Identified impacts of regulation on environmental values between Dartmouth Dam and Hume Dam

4.3.1 Channel morphology Banks The conventional wisdom of landowners is that rapid drawdown is the key cause of bank slump failure and erosion in the reach of the Mitta Mitta River below Dartmouth Dam. Green (1999) carried out a detailed study of bank erosion in this reach, and concluded that there was no evidence that the rates of drawdown had changed significantly since regulation commenced or that drawdown was a significant factor in the observed bank failure. Fairly widespread bank erosion was observed, but Green (1999) considered that it was the result of either scouring during sustained higher regulated flows (as evidenced by the location of the scour on the outside bends of meanders) or seepage failure as a result of the

Fluvial Systems Pty Ltd 28 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission sustained elevated stage. The bank failures were generally located close to irrigation head channels and were driven by a local increase in water table elevation of up to 0.4 m above the river level, which caused a head difference sufficient to cause bank failure. Lawson and Treloar et al. (2001) re-examined Green’s (1999) plots and decided that since regulation, maximum drawdown rates were higher and drawdown rates were more variable, and that this had contributed to bank erosion. Thoms et al. (2000) identified channel degradation throughout this zone, evidenced by the exposure of once-buried bridge piers and active bank erosion. Thoms et al. (2000) was in agreement with Green (1999) that the erosion was the result of sustained periods of high flow. In fact, bank erosion is a normal geomorphic process that occurs in rivers whether they are regulated or not, so claims of accelerated rates of bank erosion are difficult to defend unless data are available to allow comparison of the natural and the current rates of erosion. Thoms et al. (2000) did not provide such data, but they speculated that the observed rate of bank erosion was greater than would occur under unregulated conditions, and that this accelerated erosion was responsible for reducing the amount of available habitat. Lawson and Treloar et al. (2001), relying heavily on Terrazzolo and Erskine (1995), compared recent river surveys with those from the 1930s and concluded that in the upper section of the river, close to the Dam outlet, the river was stable. The reduction in duration of high flows may have acted to mitigate bank erosion. Bedrock imparts stability to the upper parts of this section. In the middle sections of the river there was localised erosion, and the lower section, downstream of Tallandoon, had more widespread bank erosion. However, bank erosion was much more extensive in the 1930s than it is at present (Lawson and Treloar et al., 2001, p. 22) (Note: erosion rates were not quantified). The alluvial banks of the river are naturally prone to erosion, and bank protection works undertaken post- Dartmouth Dam construction appear to have mitigated the extent of erosion. However, increased saturation of the banks due to flow regulation, combined with the general lack of bank vegetation, has decreased the resistance of the banks to erosion (Lawson and Treloar et al., 2001, p. 27). Reduced sediment supply, due to trapping by Dartmouth Dam, may also exacerbate erosion of the channel (Lawson and Treloar et al., 2001, p. 30). Bed Thoms et al. (2000) described the bed morphology as being relatively featureless, with poorly developed pool-riffle sequences and a layer of gravel armouring, beneath which the gravel sediments were choked by fines. These conditions were attributed to sustained periods of high flow. Doeg (1984) reported that the Mitta Mitta River was subject to high levels of sedimentation during and immediately following construction of the dam, which may have resulted in the in-filling of pools and drowning of coarser sediment. Koehn et al. (1995) also attributed an observed change in the availability of pool habitat in the Mitta Mitta River to construction and operation of Dartmouth Dam. Koehn et al. (1995) found that there was a 46 % reduction in the area of pools in the river downstream of the dam since 1977. Bedload in-filling of pools was identified as the cause of this morphological change. Sites below the dam also showed an increase in fines, compared with both sites upstream of the dam and with conditions prior to dam construction (Koehn et al., 1995). In contrast to the above studies, Terrazzolo and Erskine (1995), who compared cross-section surveys from the mid-1980s and mid-1990s, found evidence for increases in mean channel depth. Lawson and Treloar et al. (2001, p. 26)

Fluvial Systems Pty Ltd 29 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission postulated that this could have been due to pool deepening, as they found no appreciable change in the bed level through time at the Tallandoon gauge. Repeated cross-section surveys by ID&A (1997) indicate no significant change in bed levels between 1977 and 1997. Lawson and Treloar et al. (2001) concluded that while scour and fill during floods, and pool deepening, may have occurred, there is no evidence for a major change in bed level associated with regulation.

4.3.2 Water quality Water released from Dartmouth Dam is fairly constant at around 9-11ºC throughout the year (Ebsary, 1990, cited in Thoms et al., 2000, pp. 32-34). The effect of these releases is to depress the temperature range in the Mitta Mitta River downstream of the dam below that which would occur under unregulated conditions. A stilling pond is located downstream of the dam, which provides some opportunity for water released during the summer to be warmed by the atmosphere and radiation. However, the flow rate during major releases is generally sufficiently rapid that the residence time in the pond is too short for significant changes to the temperature of the water. However, the rate of change of the temperature of water discharged from the pond can vary dramatically in a short space of time, which can have fatal effects on fish and other animal populations (Thoms et al., 2000). The concentration of nutrients being removed from Dartmouth Dam depends on the depth of the off-take relative to the bottom of the chemocline (where dissolved oxygen levels are zero). Elevated concentrations of filterable reactive phosphorous have been measured in the hypolimnion, while the surface water is nutrient depleted (Thoms et al., 2000, pp. 34-35). Under high flow conditions the release water is a mixture of bottom and surface water, while under low flow conditions, proportionately more nutrient-rich bottom water is released (Thoms et al., 2000, p. 35). The impact of Dartmouth Dam on water quality can persist as far downstream as Lake Hume. Walker (1985) cited evidence that when the low-level offtake was used exclusively during the drought of 1982-83, flows reaching Lake Hume were distinctly cooler, more turbid and enriched in nutrients and heavy metals.

4.3.3 Ecology Fish The re-survey of Tunbridge’s (1977) Mitta Mitta River sites by Koehn et al. (1995) concluded that the construction and operation of Dartmouth Dam caused a substantial change in the downstream biota. Koehn et al. (1995) attributed the disappearance of the warmwater fish to their inability to recruit due to coldwater releases during their spawning season. Murray cod requires a spawning temperature (about 20ºC) that greatly exceeds the regulated water temperature, but the cold temperature also prevents spawning of trout cod and Macquarie perch. In addition to the reduction in native fish populations, the modified temperature regime in the Mitta Mitta River has favoured introduced species such as brown trout. Optimal temperatures for the recruitment of these (and other introduced species) are in the range 4-19ºC, which is significantly below the preferred range for native fish. Regulation has seen a fourfold increase in the population of brown trout (introduced coldwater fish), which was found to comprise 71% of total fish numbers and 63% of fish biomass (Koehn et al., 1995). Native fish

Fluvial Systems Pty Ltd 30 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission species comprise less than 7% of the fish biomass in the Mitta Mitta River below Dartmouth Dam, falling as low as 4% at sites close to the dam (Thoms et al., 2000, p. 85). The higher water temperatures prevalent under pre-dam conditions would have limited the population of introduced species, but that situation has been reversed since the closure of Dartmouth Dam. Macroinvertebrates A significant decline in the macroinvertebrate fauna of the Mitta Mitta River below Dartmouth Dam was noted by Koehn et al. (1995). The majority of the decline occurred during construction of the dam. The fauna is now depauperate, and little or no recovery has been observed since closure. Populations are dominated by a few species, and show little seasonal variation (Koehn et al., 1995). Mayfly assemblages on the Mitta Mitta River were compared to those on the unregulated Snowy Creek by Pardo et al. (1998), who concluded that the cold deep releases from Dartmouth Dam altered the assemblages. Several species present in the Snowy Creek were absent from the Mitta Mitta River, including Centroptilum sp. and Tasmanophlebia lacuscoerulei. The relative abundance of the dominant species also differed between the two streams. The differences in assemblages were attributed to the physical changes induced by flow regulation: reduction in low flows; changes in bed material composition; and depressed summer temperatures (Pardo et al., 1998).

Fluvial Systems Pty Ltd 31 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

5 Hume Dam to Yarrawonga

5.1 Summary of impacts and causes The section of the River Murray, immediately downstream of Hume Dam (Figure 2), receives the full hydrological impact of flow regulation. For a distance of 218 km downstream to Lake Mulwala (the 15 km long pond formed by Yarrawonga Weir), the channel conveys all regulated flow destined for downstream uses. Hume Dam is operated to store winter-spring floods and release water for irrigation during summer-autumn. This effectively reverses the natural seasonal flow pattern, and severely reduces the occurrence of small and medium-sized floods. The River Murray is a highly stable river when compared with rivers of a similar size and catchment area in other parts of the world. Downstream of Hume Dam there has been localised bend migration of the order of 1,000 mm/yr, but there is no evidence to suggest that the rates of migration have altered in response to regulation. While rapid drawdown is a threat to bank stability in anabranches, the main cause of bank erosion in the main channel is the extended periods of regulated near-to-banktop flow. Bank erosion is prevalent throughout the reach, occurring at an average rate of 160 mm/yr. Downstream of Hume Dam the bed has deepened by as much as 24%, but downstream of Albury the river has shallowed. It is important to note that the observed rates of morphological change are low in comparison with many rivers of a similar size and catchment area in other parts of the world. There has been a reduction in water temperatures and dissolved oxygen levels downstream of Hume Dam. Data suggest unusual seasonal trends in nitrate and phosphorous concentration. Turbidity and associated nutrient concentrations may have increased due to accelerated bank erosion, but data are lacking. Primary production, native fish and macroinvertebrate populations have been adversely affected by the altered flow regime and associated changes in water quality. However, towards the lower end of this zone, fish populations are in good condition, probably due to the positive influence of Lake Mulwala.

5.2 Regulation influences

5.2.1 Hume Dam Hume Dam is located on the River Murray, 16 km upstream of Albury – Wodonga. It is the primary regulating structure on the River Murray. Construction of Hume Dam commenced in 1919. In 1924, the Commission agreed to enlarge it from its planned capacity of 1,360 GL to 2,470 GL, but this was later reduced so that when construction was finished in 1936 it was 1,540 GL. The dam was enlarged to a capacity of 3,038 GL in 1961 to handle increased flows in the River Murray from the Snowy Mountains scheme (MDBC, 2002). This capacity has remained unchanged to the present day, and represents approximately 58% of the annual flow at the Dam site. The Lake has a surface area of 20,190 ha and an average depth of 15 m when full. Being and old dam, it is of the deep release design. The capacity of the dam allows it to capture small to medium sized floods, which means that the dam is the primary control on flows in this zone of the Murray (Thoms et al., 2000, p. 89). The primary purpose of Hume Dam is to conserve water in periods of high flow for later release during periods of low flow. The principal use of the water is for

Fluvial Systems Pty Ltd 32 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission irrigation but significant quantities of water are diverted from the River Murray for domestic uses, industrial uses and to help supply entitlement flows to South Australia, and the dam also fulfils a flood mitigation role (MDBC, 2002). This pattern of operation effectively reverses the natural seasonal flow pattern, and severely reduces the occurrence of small and medium-sized floods (Table 5-1). At the end of the irrigation season the river level is drawn down to a winter supply level to allow dam re-filling. The maximum drawdown rate at this time, and at other times when drawdown is appropriate (for example, after heavy rain, when water is not required by irrigators) is six inches (0.15 m) per day (Green, 1999).

Table 5-1. Identified impacts of regulation on the hydrology between Hume Dam and Yarrawonga Variable Change due to flow regulation Flow volume • Average annual flow 10% higher than natural conditions, due to the input of the Snowy Mountains Scheme (Maheshwari et al., 1993, p. 17) Flow • Annual flows greater than natural for 55% of years variability (Thoms et al., 2000, p. 27) • Flow at or near channel capacity for much of year (effectively constant near-bankfull flows during irrigation season) (Thoms and Walker, 1991) • Annual coefficient of variation reduced from 0.49 (natural) to 0.34 at Albury (Maheshwari et al., 1993, p. 17) • Minimum monthly discharge increased by 156% over natural level at Albury (Maheshwari et al., 1993, p. 17) Flow • Natural pattern reversed (Close, 1990, p. 68, Thoms et al., seasonality 2000, p. 28) Floods • Unseasonal summer floods due to “rain rejection” events (Thoms et al., 2000, p. 62) • Reduced frequency/duration of winter/spring flooding. • The recurrence intervals of floods from 32,000– 55,000 ML/d magnitude have more than doubled (Thoms et al., 2000, p. 59) • Frequency of floods with return period exceeding 1 in 20 years is unaffected by regulation (Thoms et al., 2000, p. 97)

5.3 Identified impacts of regulation on environmental values between Hume Dam and Yarrawonga

5.3.1 Channel morphology Banks In general the planform of the River Murray has not changed since the 1860s, when the river was first surveyed. Some bends below the Hume Dam have moved about 30 m, but Rutherfurd (1990, p. 33) regarded these changes as localised and rare. Dating the age of trees on the banks of the river from Albury to Wentworth confirms this conclusion, by suggesting that the position of the

Fluvial Systems Pty Ltd 33 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission channel has been stable for at least 200 years (Rutherfurd, 1990, p. 33). By world standards, the River Murray is one of the most stable rivers of its size described in the literature. The River Murray from Albury to Wentworth has shortened by 65 km (5% of river length) since European settlement through a process of meander cut-off (Rutherfurd, 1990, p. 34). Young (2001, p. 62) acknowledged that most avulsions pre-dated regulation, and proposed that they were probably a response to increased runoff and sediment loads due to catchment clearing. This contrasts with the explanation of Rutherfurd (1990, p. 34), who proposed that most of the cut-offs were initiated by the record highest flood of 1870 (a natural event). ID&A (1993) measured post-regulation migration rates for active bends on the river between Hume and Yarrawonga. The active bends constituted only 13% of the total bends on the river, so most of the bends were considered stable. The bends identified as active migrated at average rates of around 1,000 mm/yr (although migration was episodic), with the highest rate being 2,840 mm/yr. The pattern of bend migration was highly variable in space and time. ID&A (1993) speculated that the long-term average bend migration rate for the river was 100 mm/yr, but the derivation of this figure was not explained. Meander migration is a normal geomorphic process, and ID&A (1993) did not compare the observed rates with what would be expected under natural (unregulated) conditions. Channel migration is known to be a function of bank resistance as well as flow regime, and the resistance of the banks in this area has been reduced due to replacement of much of the native riparian vegetation with pasture and exotic plants. Free stock access to the stream bank can also reduce bank stability. ID&A (1993) concluded that the data could not be used to support the hypothesis that channel migration rates had increased due to flow regulation. Arnott’s (1994) field observations found that drawdown (under the six inch rule), while not obviously impacting the banks of the River Murray itself, did cause accelerated erosion of the banks of anabranches, particularly Ryans Creek, which leaves the Murray 6.5 km downstream of Hume Dam. This difference was attributed to the characteristics of the anabranches (clayey banks, and steeper gradients) and the hydraulic impact of the horizontally-crested sheet pile regulating structures on the entrance to the anabranches enhancing the rate of drawdown. This study did identify accelerated erosion on the River Murray channel, but attributed it to extended periods of regulated near-to-banktop flow. Green’s (1999) review of literature found very few observations of drawdown failures, and also found a lack of consensus on the importance and magnitude of drawdown induced bank failures. Rutherfurd (1990, p. 34) reported that the River Murray from Albury to South Australia widened on average by 7 m between the 1860s and the 1970s (approx. 60 mm/yr average), but it was also noted that the erosion was highly variable and in places the river had narrowed. Analysis of historical cross-section surveys from Albury to Yarrawonga by ID&A (1993) revealed that river widened in the period between 1977 and 1992 at an average rate of 160 mm/yr. Unlike bend migration, which was variable, widening was prevalent throughout the reach. Widening occurred in straight reaches as well as on bends. Channel widening appeared to be greater in the aggrading area downstream of Howlong. Under regulation the channel has developed distinctive a bank profile: an erosional notch has formed at the level of the long-duration high irrigation flows and a lower bank facet has formed at a level associated with long-duration winter low flow releases (ID&A, 1993, Tilleard et al., 1994). While regulation is a likely contributor to these channel changes, clearing and grazing of bank vegetation,

Fluvial Systems Pty Ltd 34 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission snag management, and waves generated by boats are other possible explanations (ID&A, 1993). Bed Analysis of historical cross-section survey data by ID&A (1993) suggested that the river bed between Hume Dam and Albury deepened by as much as 24% in the period between 1977 and 1993. Hume Dam has a high sediment trap efficiency (estimated at more than 90%) so the bed sediment that is normally transported out of this reach is not replaced by incoming sediment, leading to the observed deepening (Tilleard et al., 1994; ID&A, 1993). For the first few kilometres downstream of the dam the bed has developed an armour layer of coarse imbricated cobbles, which has probably arrested the deepening, but incision may propagate further downstream until limited by bed armouring. ID&A (1993) speculated that this process could impact on a further 50 km of river length. Rutherfurd (1990, p. 35) reported shallowing (by up to 2 m) of the River Murray from Albury to Yarrawonga between 1876 and 1981. Later analysis of more recent survey data by ID&A (1993) confirmed this trend, with channel depth decreasing up to 16% during the period between 1977 and 1993 in the Howlong to Lake Mulwala reach. Aggradation occurred from Corowa Throat almost to the Lake Mulwala backwater. ID&A (1993) did not establish the cause of the aggradation. The observed changes in channel morphology could be due to several factors, including natural floods and flow regulation, but it is important to note that these changes are minor by world standards for rivers of a similar size. Rutherfurd (1990, p. 35) explained that the River Murray responds very slowly to changed energy and material inputs due to its characteristically low energy regime. Given this, morphological adjustments to the past 150 years of disturbance are likely to continue into the future (Rutherfurd, 1990). Sediment transport The sediment transport regime of the River Murray is currently dominated by in- channel sources. Over 69% of the average annual load is derived from in-channel sources, primarily identified as areas of bank erosion (Thoms and Walker, 1991). The dominant discharge is that which transports the largest fraction of the annual sediment load, averaged over a long time period. In the River Murray, sustained discharges at or near channel capacity dominate the sediment regime. Thoms and Walker (1991) considered this to be atypical of semi-arid rivers, and therefore an artefact of flow regulation.

5.3.2 Water quality Hume Dam is shallower than Dartmouth Dam, so it does not stratify to the same extent. In addition, the reservoir is drawn down considerably in some years, which reduces the depth of the offtake. Consequently, the temperature depression of waters in the reaches downstream of Hume is significantly less than that experienced downstream of Dartmouth (Thoms et al., 2000). However, water temperature has been depressed, expressed as a reduction in maximum surface water temperature of 4-6ºC downstream of the dam (Thoms et al., 2000). Also, the time of maximum temperature has shifted from the January-February period to the March-April period (Thoms et al., 2000). In addition to the reduction in summer water temperature, the range of temperature variation in the reach downstream Hume Dam has also been reduced (i.e. the minimum temperatures during winter are increased) (Walker, 1985).

Fluvial Systems Pty Ltd 35 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Walker (1985) compared oxygen levels up- and downstream of the Hume Dam for the 1974–1982 period, and found that the downstream oxygen levels were severely depressed in comparison with the upstream levels. The maximum difference was 55% saturation (40% saturation at the downstream site and 95% at the upstream site). Walker (1985) suggested that the low summer oxygen levels in the reach downstream of Hume Dam reflected the discharge of anoxic bottom water. Walker (1985) also noted that the impact of Hume Dam on water quality persisted for many kilometres downstream. Water quality data from Heywoods Bridge (located several hundred metres downstream of Hume Dam) show unusual seasonal trends in nitrate and phosphorous concentration, which Thoms et al. (2000, pp. 35-37) hypothesised were related to release of hypolimnetic water from Hume Dam. The higher rates of sediment transport that have occurred as a result of increased channel erosion have likely increased turbidity (caused by fine suspended particles) and associated nutrients but there are no data available that quantify this (Young, 2001, p. 76). Fluctuation of water levels above and below bank-top level controls the dynamics of nutrient flows to and from wetlands, so disturbance of the mid- range flood pattern has probably changed the dynamics of this process (Hillman et al., 2000).

5.3.3 Ecological impacts Plants Irrigation releases cause unseasonal summer flooding along the channel margins, and the pattern of wetting and drying is different to the natural pattern. Nielsen et al. (1996, cited in Young, 2001, p. 77) found one response was a lower diversity of plant species inundated during unseasonal summer flooding compared with natural winter/spring flooding conditions. The seasonal growth of littoral plants is influenced by flow patterns, and according to Young (2001, p. 113), the depauperate nature of plant communities in this zone is indicative of the impact of the regulated flow pattern. Hillman (unpublished data, cited in Young, 2001, p. 115) speculated that in the River Murray downstream of Hume, with its depressed water temperatures, sexually reproducing aquatic plants may reach reproductive maturity later than those in adjacent unregulated streams. Young (2001, p. 115) speculated that increased turbidity from accelerated bank erosion would act to limit benthic primary production (micro-organisms and macrophytes) and the germination of sexually reproducing aquatic plants. These are reasonable hypotheses, but determination of the magnitude and significance of these impacts awaits more detailed scientific investigation. Fish Young (2001, p. 116) suggested that the unseasonality of high flows may compromise the recruitment (i.e. spawning and subsequent survival) of fish. Reduced flooding and lower summer temperatures have significant effects on the spawning cycle of warmwater species, such as the Murray cod. Upstream movement of some native species, including golden perch, can be triggered by small increases in water level, while recruitment of large native fish appears to benefit from high flows (Young, 2001, p. 79). This suggests that alteration of flow variability and flooding characteristics due to regulation may have adversely affected the migration and recruitment of some native species.

Fluvial Systems Pty Ltd 36 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Towards the lower end of this zone, fish populations are in good condition. Thoms et al. (2000, p. 95) explained that this was probably due to the increased snag density providing habitat, and because Lake Mulwala (which is warmed to some extent by radiation) appears to act as a nursery ground for native fish. Macroinvertebrates Depressed water temperatures retard the rate of growth and development of the majority of cold-blooded aquatic fauna. For example, freshwater prawns in the reach downstream of Hume Dam reach reproductive maturity 2-4 weeks after those in adjacent streams (Hillman, unpublished data, in Young, 2001, p. 78). The impacts of temperature reduction are coincident with flow reversal and flow velocity changes. Bennison et al. (1989) found that downstream of Hume Dam, macroinvertebrate diversity was lower than expected, and some species had adapted to depressed oxygen levels (a side effect of cold water releases).

Fluvial Systems Pty Ltd 37 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

6 Yarrawonga to Torrumbarry Weir (includes Edward-Wakool system)

6.1 Summary of impacts and causes The influence of Hume Dam reduces with distance downstream, but it still exerts a profound influence through the Yarrawonga-Torrumbarry Weir Zone (Figure 2). Additional regulation is exerted through diversion of water from Yarrawonga Weir, regulators on side-channels through the Barmah-Millewa Forest to help protect against unseasonal flooding, and offtakes on the Edward River and Gulpa Creek distributary system. Major tributaries of the River Murray upstream of Torrumbarry Weir include the Kiewa, Ovens, Goulburn and Campaspe rivers. The weir pool level at Torrumbarry determines the river level as far upstream as Echuca (74 km upstream) during the fully regulated mode of operation. Downstream of Yarrawonga, annual flow is 25% less than under natural conditions, but summer flow is 19% greater than natural. Flow variability has been decreased and water level is held at relatively constant near capacity discharge for much of the year. Seasonality has been altered, and the frequency and duration of winter/spring flooding has been reduced. Regulation has reduced the frequency of flooding of Barmah-Millewa Forest, but this ecologically important area also suffers from unseasonal flooding. The Edward River has also been affected by regulation, with flows through the Millewa Forest section at or near channel capacity for much of the year. In this section of the River Murray, persistently high summer flows characteristic of the regulated regime have led to widening of the upper part of the cross- section through the creation of in-channel over-cut benches, but this appears to have slowed or ceased over much of the zone since 1980. There has been no appreciable long-term change in bed elevation. Dramatic channel widening and bed degradation in response to regulation has been documented on the Edward River. It appears that the influence of cold water releases from Dartmouth and Hume dams is insignificant downstream of Yarrawonga. Regulation severely impacted waterbird populations. The river is dominated by alien fish species such as carp, which appear to be using unseasonally flooded wetlands for breeding. Despite this, the River Murray between Lake Mulwala and Barmah Choke contains excellent native fish populations, including the only remaining truly natural population of the endangered Trout cod. Macroinvertebrate communities may be adversely affected by unseasonally high summer and autumn flows. The quality of habitats and the distribution of vegetation types in the Barmah-Millewa Forest have been significantly affected by the changed flooding regime.

6.2 Regulation influences

6.2.1 Hume Dam Hume Dam is the primary regulating structure on the River Murray, and its influence extends throughout the Yarrawonga-Torrumbarry Weir zone.

6.2.2 Yarrawonga Weir Yarrawonga Weir is located near the townships of Yarrawonga, in Victoria, and Mulwala, in NSW, 233 km downstream of Hume Dam. Construction of Yarrawonga Weir began early in 1935 and was completed in 1939. A privately

Fluvial Systems Pty Ltd 38 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission owned hydroelectric station and a fish lift were added to Yarrawonga Weir in 1994 (MDBC, 2002). The primary role of the Weir is to raise water level in the River Murray so that gravity diversion is possible via major channels to irrigate land in both New South Wales and Victoria. Yarrawonga Weir is the site of major water diversions. The Weir is used to regulate flows downstream, including the river reaches through the Barmah–Millewa forest and Echuca, and it plays a minor role in flood mitigation (MDBC, 2002). The combined effects of regulation by Hume and Yarrawonga Weir have had a dramatic impact on the hydrology of the River Murray in this zone (Table 6-1). Frequency distributions of stage height at Tocumwal, plotted by Gippel and Lucas (2002), show that when the river was essentially unregulated, the stage heights were widely and fairly evenly distributed, although there was a peak at around 1.6-2.0 m stage. After Hume Dam was first constructed the peak shifted up the bank by around 0.4 m to 2.0-2.4 m stage, and became more focussed. The distribution of stage height was not greatly affected by enlargement of Hume Dam, but Dartmouth Dam had the effect of shifting and focussing the peak up the bank by about 0.6 m to 2.6-3.0 m stage.

6.2.3 Torrumbarry Weir pool Torrumbarry Weir is located 74 river kilometres downstream of Echuca, and 133 river kilometres upstream of Swan Hill. The weir pool level at Torrumbarry determines the river level as far upstream as Echuca during the fully regulated mode of operation (MDBC, 2002). The Goulburn and the Campaspe Rivers are the major tributaries entering the River Murray through this zone (Figure 2).

6.2.4 Side channel regulators Twenty-eight regulating structures have been constructed on side channels from the River Murray in this zone (Leitch, 1989). These regulators are designed to minimise unseasonal flooding of the Barmah and Millewa riparian forests during the high flow irrigation season.

6.2.5 Distributary offtakes Under regulated conditions, all of the regulating structures are closed, apart from the Edward River and Gulpa Creek offtakes. The Edward River offtake structure (constructed in 1959) is used to control flows in the channel as it passes through the Millewa Forest and then on to the Wakool Irrigation District. Stevens Weir (constructed in 1935) is located downstream of Deniliquin and diverts water to the Wakool Canal. The hydrology of the Edward-Wakool River system has been altered directly by the offtake weir and Stevens Weir, and indirectly by regulation of the River Murray, which feeds this distributory system (Table 6-2). Thoms et al. (2000, p. 28) graphically summarised the major hydrological change in flow in the Edward River at Deniliquin.

6.2.6 Hydrological change in the Barmah-Millewa Forest The Barmah-Millewa Forest is the largest river red gum (Eucalyptus camaldulensis) forest in Australia (and the world). The key hydrological changes due to regulation that have impacted this area are (Leitch, 1989): • Relatively constant high summer-autumn river flow levels • Seasonal reversal of flooding and the impact of rain rejection floods • Reduction in the frequency of flooding (particularly during winter-autumn)

Fluvial Systems Pty Ltd 39 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 6-1. Identified impacts of regulation on the hydrology of the River Murray between Yarrawonga and Torrumbarry Weir. Variable Change due to flow regulation Flow volume • Annual flow downstream of Yarrawonga Weir under current conditions is 25% less than under natural conditions as a result of diversions for irrigation (Yarrawonga Main Channel and Mulwala Canal) (Maheshwari et al., 1993, p. 17) • Summer (Dec-Feb) regulated discharge Downstream of Yarrawonga is 19.4% greater than unregulated summer discharge (Gippel and Lucas, 2002) • Summer (Dec-Feb) regulated discharge at Picnic Point is 19.3% greater than unregulated summer discharge (Gippel and Lucas, 2002) Flow • Downstream of Yarrawonga, annual flows are greater than variability natural for 8% of years (Thoms et al., 2000, p. 27). • Reduced flow variability (particularly winter/spring). Under natural conditions, average monthly flows vary between 100 GL/month and 980 GL/month, whereas under current conditions the average monthly flows vary between 110 GL/month and 400 GL/month (MDBMC, 1995) • Flow at/near channel capacity of 330-350 GL/month (Barmah Choke limit) for approximately eight months of the year (MDBMC, 1995) • Annual coefficient of variation unchanged at Yarrawonga (Maheshwari et al., 1993, p. 17) • Minimum monthly discharge increased by 99% over natural level at Yarrawonga (Maheshwari et al., 1993, p. 17) Flow • Lowest flows now occur in winter rather than summer, seasonality and pronounced spring peak has been eliminated by regulation (Thoms et al., 2000, p. 28). The average flow for September under natural conditions was approximately 980 GL/month, while under the present conditions it is less than 400 GL/month (MDBMC, 1995, Thoms et al., 2000, p. 28) Floods • Frequency/duration of winter/spring flooding is reduced. The recurrence intervals of Barmah-Millewa floods from 42,000–78,000 ML/d magnitude have more than doubled (Thoms et al., 2000, p. 97) • Frequency of floods with return period exceeding 1 in 20 years is unaffected by regulation (Maheshwari et al., 1995) • Unseasonal rain rejection flooding in Barmah-Millewa Forest (Thoms et al., 2000, p. 62, 100)

Fluvial Systems Pty Ltd 40 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 6-2. Identified impacts of regulation on the hydrology of the Edward River system below the offtake weir to Deniliquin (source: Gippel, 1998). Note: The flow records for the Edward River are not of sufficient length to allow full analysis for the pre-Hume Dam era, therefore most comparisons are for pre/post Dartmouth Dam closure only. Variable Change due to flow regulation Flow volume • Annual flow increased 7% post-Dartmouth Dam closure Flow • Flow at/near channel capacity for 8 months of the year variability • Post-Dartmouth Dam, channel capacity flow (1600 ML/d) was exceeded 49% of the time, compared to 38% pre- closure • Coefficient of variation of annual flows decreased from 0.43 to 0.19 post-Dartmouth Dam closure Flow • Closure of Hume Dam increased autumn flows but seasonality decreased spring flows. This change in seasonality was exacerbated by the closure of Dartmouth Dam Floods • Reduced magnitude of high return period events. The 1 in 10 year event decreased from 15,000 ML/d prior to the closure of Dartmouth Dam to 3,000 ML/d post-closure

Inundation of Barmah-Millewa Forest begins at a flow of 12,000 ML/d, and the forest is fully inundated at 68,500 ML/d (Bren et al., 1987). Dexter et al (1986) identified “effective flooding” as 30 days of flows above 24,500 ML/d (750 GL/month), which inundates about 80% of the forest. Using modelled data for Yarrawonga, Close (1990, p. 70) calculated that such an event can now be expected in 35% of years, compared with 80% of years under natural conditions. Using historical data from Tocumwal, Leitch (1989) reported similar results for frequency of “effective flooding”, and for floods above a threshold of 590 GL/month (40% of forest flooded) the calculated frequency was 92% of years under natural conditions and 59% of years under the post-Dartmouth regulated regime (Leitch, 1989). Flows of 550 GL/month and 912 GL/month at Yarrawonga were identified as being critical for watering of key parts of the Barmah-Millewa Forest (MBDMC, 1995). Modelling of a range of water resource development levels at Yarrawonga determined that regulation caused a marked reduction in the percentage of years that flows of these critical magnitudes were experienced in the Barmah-Millewa Forest (MBDMC, 1995) (Error! Not a valid bookmark self-reference.).

Table 6-3. Watering characteristics of the Barmah-Millewa Forest based on flows at Yarrawonga (source: MDBMC, 1995, with the “Full development” scenario not included here as it is no longer relevant). Level of Percentage of years with Percentage of years with development flow greater than flow greater than 550 GL/month 912 GL/month Natural conditions 94% 69% 1988 development 50% 35% 1994 development 48% 26%

Fluvial Systems Pty Ltd 41 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Leitch (1989) collated information from studies by Bren and Gibbs (1986) and Bren (1987; 1988a; 1988b) on the hydrological changes experienced by the main vegetation communities in the Forest. These studies developed relationships between flooding extent and flood magnitude. These relationships were then combined with a hydrological analysis to provide a long-term comparison of natural and regulated flooding characteristics. Across all vegetation communities, flood frequency and duration decreased after regulation, while the duration of dry periods increased (Table 6-4).

Table 6-4. Altered flooding characteristics of the main vegetation communities in the Barmah-Millewa Forest (source: Leitch, 1989). Nat. is natural (unregulated flow regime) and Reg. is regulated (post-Dartmouth) flow regime. Rushlands Grasslands High quality Low quality red gum red gum Index Nat. Reg. Nat. Reg. Nat. Reg. Nat. Reg. Annual 100% 78% 100% 70% 92% 57% 46% 25% flood freq. (% of years flooded) Inundation 8.6 3.6 7.5 3.0 5.2 2.1 1.2 0.7 duration (mean no. months per year) Dry period 3.9 10.7 4.4 13.0 7.4 17.1 24 45 duration (mean no. months dry) Longest dry 8 45 11 45 20 116 129 201 period in record (days)

6.3 Identified impacts of regulation on environmental values from Yarrawonga to Torrumbarry Weir

6.3.1 Channel morphology River Murray Banks The most recent geomorphic investigation of this river zone was conducted by Gippel and Lucas (2002). The greatest and most consistent channel widening between the 1876 and 1976 channel surveys occurred in the section from Yarrawonga to Bullatale Creek, i.e., upstream of the Barmah Fan. Here the channel widened about 30-40 m throughout. Further downstream the average widening between 1876 and 1976 was of the order of 10-20 m. The 2002 survey suggested that widening appears to have slowed, or ceased since the 1970s, except for the section downstream of the Goulburn River junction to the Campaspe River junction.

Fluvial Systems Pty Ltd 42 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Regulation was the main cause of river widening, with waves from water skiing being a secondary cause (Gippel and Lucas, 2002). Had the banks been covered in Phragmites over the last century, the rate of erosion would probably have been less. However, it is possible that loss of Phragmites was partly related to bank retreat caused by regulation, so a negative feedback loop may have been operating. Phragmites currently in the channel may be the product of recolonisation, rather than representing remnants of pre-regulation stands. Remnant stands of Phragmites would be expected in sections of the channel that have not widened and which have had low grazing pressure. Erosion upstream of Yarrawonga occurs through bank slumping and bank collapse, while downstream of Yarrawonga the process is one of erosional benches over-cut into the banks. The difference relates to the higher percentage of silt and clay in the banks (making them more resistant) downstream of Yarrawonga. However, the exact process of bench cutting is unknown. Bed Overall there has been little net aggradation of the bed of the River Murray since the 1870s (average of 0.35 m from Yarrawonga to Torrumbarry Weir) (Rutherfurd, 1991; Gippel and Lucas, 2002). There has been virtually no scour below Yarrawonga Weir, due to development of a resistant armour layer, which is evident on point bars. This armouring material is ancestral channel gravels eroded out of lenses in the banks. Degradation is also limited by the hard clay bed (Rutherfurd, 1991; Gippel and Lucas, 2002). Bed morphology Pools and riffles on the River Murray are largely erosional features cut into the Riverine Plains clays. They are unlikely to have been impacted by a reduction in frequency of bankfull flows due to regulation (Rutherfurd, 1991, p. 355). Surveys show that the pool-riffle sequence has been remarkably stable over the past century, with only a few pools infilling with sand (Rutherfurd, 1991). Floodplain Thoms (1995) reported that elevated rates of floodplain sedimentation in areas of high to very high flood frequency were due to poor land and water management. Thoms et al. (2000, p. 101) warned that sedimentation can have a feedback effect by altering the distribution of floodwaters across the floodplain. Kenyon (2001) and Kenyon and Rutherfurd (1999) disputed the idea of high modern rates of floodplain sedimentation in the Barmah-Millewa Forest area. Their data indicate a rate of 7 mm/10 years compared with assumed long-term background rates of 3 mm/10 yrs. While this rate is low, it still represents a doubling of the floodplain sedimentation rate since European settlement. Edward River Gippel (1998) conducted a detailed investigation of geomorphic changes in the Edward River from the offtake to just upstream of Deniliquin since regulation, using historical accounts, and comparison of cross-sections surveyed through time. A comparison of the 1965/66 and 1998 cross-sections revealed that, over this period, the top width of the channel consistently enlarged from the offtake near the River Murray to chainage 5,000 m. This increase in width, combined with the bed degradation that occurred in this reach, produced a large increase in channel cross-sectional area. From chainage 13,500 m to 16,000 m the channel cross-sectional area increased, largely through a width increase. From the Tocumwal Road Bridge to chainage 11,000 m the channel cross-sectional area contracted substantially, largely due to aggradation of the bed. Heavy deposition of sand was present in the bed of this part of the river. The channel changes were

Fluvial Systems Pty Ltd 43 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission thought to be a response to flow regulation, but it was not known whether the channel had reached a stable geometry (Gippel, 1998). The Edward River channel from the offtake to Taylors Bridge has the potential to become a very large channel because of the large volume of water that is available in the River Murray for diversion into the Edward River (Gippel, 1998). Erosion of the channel could result in sediment deposition in other sections of the channel, smothering of habitat, and degradation of water quality. Expansion of channel capacity will direct a greater proportion of the total flow through the Edward River system into the river channel. This could have the effect of reducing the frequency or duration of flooding in the Millewa Forest, and this could alter the distribution of habitat in the forest (Gippel, 1998).

6.3.2 Water Quality Temperature The longitudinal (downstream) plot in median water temperature of the River Murray (Mackay et al., 1988, p. 51) indicates that while cooler temperatures prevail in the upper part of the river, from Yarrawonga to Tailem Bend in South Australia the median temperature is essentially constant. Downstream of Yarrawonga the maximum and minimum temperatures occur at the same time of the year at each location. There is no evidence of a phase shift; that is, the temperature at any location is dependent on local and current air temperature conditions rather than reflecting the temperature of the water at locations upstream (Mackay et al., 1988, p. 50). It appears then that the influence of cold water releases from Dartmouth and Hume Dams is insignificant downstream of Yarrawonga. Organic carbon Reduced flood frequency means that there is ample time for the labile forms of carbon to be processed and integrated into the floodplain food web, leaving the less tractable forms to accumulate between floods. Young (2001) hypothesised that the organic load of the river would therefore be dominated by less tractable forms of carbon.

6.3.3 Ecology Waterbirds Regulation severely impacted waterbird populations, with Briggs and Lawler (1989) reporting that egrets did not breed in the Barmah Forest for the previous 20 years. Chesterfield et al. (1984, cited in Briggs and Lawler, 1989) attributed this to a lack of deep, spring floods. Conversely, permanent inundation of some wetlands may have enhanced the breeding of great cormorants by providing nesting sites in dead trees and a regular food supply in the form of fish. The overall effect of regulation on waterbirds is likely to be reduced recruitment of young, but enhanced survival of adults. Fish Gehrke et al. (1995) examined the distribution of fish populations throughout the Murray-Darling Basin by determining the relative dominance of native and species between different subcatchments. The study was based around a fish catch of 11,010 individuals, caught during high and low flow seasons. Gehrke et al. (1995) made the general conclusion that the Murray was dominated by alien species such as carp. For the sampling site located within the Yarrawonga to Torrumbarry Weir zone, the ratio of native:alien fish was 0.04, compared with a

Fluvial Systems Pty Ltd 44 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission ratio of 13.35 for the less regulated Darling River system. Thoms et al. (2000, p. 101) noted that carp appear to be using unseasonally flooded wetlands for breeding. Despite the high abundance of alien fish, the River Murray between Lake Mulwala and Barmah Choke contains excellent native fish populations, including the only remaining truly natural population of the endangered Trout cod (Thoms et al., 2000, p. 95). Macroinvertebrates Thoms et al. (2000, p. 101) reported that macroinvertebrate populations within the main channel were predominantly found on the few snags that were present. Their abundance was considered to be low. Thoms et al. (2000, p. 101) implied that macroinvertebrate communities were adversely affected by unseasonally high summer and autumn flows. Barmah-Millewa Forest The Barmah-Millewa Forest is a particular area where impacts of the changed flooding regime have been well documented. Leitch (1989) summarised the impacts of changed flooding regime on the main vegetation communities of the Forest (Table 6-5). There has also been a general reduction in connectivity between the river channel and the floodplain in the Yarrawonga to Torrumbarry Weir zone. This is the result of changes in the flood characteristics combined with the increasing alienation of the floodplain through the construction of levees (Young, 2001). Breakdown of this linkage is thought to have resulted in decreased inoculation of the river with micro-organisms and micro-invertebrates sourced from billabongs and other wetland areas. There are significant differences between the response of macroinvertebrates in permanent and temporary wetlands in the Barmah-Millewa Forest following flood events (Quinn et al., 2000). In comparison with the relatively unregulated Ovens River catchment, the composition and abundance of macroinvertebrate populations is significantly different in permanent and temporary wetlands following flooding, and the differences persist for up to six months after the event. The lack of taxa typical of lentic water bodies in the sites surveyed in the Barmah-Millewa Forest suggests that the occasional summer drying is not sufficient for a true temporary wetland fauna to develop. There is a significant difference in the mechanism by which the contrasted systems become inundated. The Ovens River is subjected to rapid overbank flows and flooding, while flows into the Barmah-Millewa Forest are controlled by regulating structures that allow water in relatively gently (Quinn et al., 2000). The key difference between the systems is the level of regulation, with overbank flows uncommon in the Barmah-Millewa Forest, which ensures that temporary wetlands remain dry for longer periods than the less regulated Ovens River system. Heavy rain in the summer can cause irrigation demand downstream of Hume Dam to be suddenly reduced, but the current drawdown rule means that flow cannot be cut back quickly. The excess water that is transferred down the river can cause unseasonal flooding, and possibly environmental degradation of the Barmah-Millewa Forest (Arnott, 1994) The maximum regulated flow in the Edward River, as it passes through the Millewa Forest, has been as high as 2,000 ML/d. This discharge allowed a degree of floodplain inundation through several of the natural effluent channels that occur in this area. Excessive inundation was thought to be causing die-back of red gum forest. This problem was overcome by construction of regulators on

Fluvial Systems Pty Ltd 45 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission the largest of these effluents, and lowering the maximum regulated flow to about 1,600 ML/d (Gippel, 1998).

Table 6-5 Impact of hydrological changes on vegetation composition of Barmah Forest (source: Leitch, 1989) Vegetation Impact due to flow regulation community Rushlands • Giant rush (Juncus ingens) has established over 1.5% of Barmah Forest, in some areas that previously were grasslands. Chesterfield (1986, cited in Leitch, 1989), suggested that this was due to regular summer inundation, and reduced frequency/period of inundation winter/spring flooding (rush seeds cannot withstand total submergence for an extended period) • Leitch (1989) suggested that some of the wetlands are prone to siltation and drying out more readily than they were 20 years ago Grasslands • Grassland area has declined from 13.5% in 1930 to 5.2% (1500 ha) in 1979 (Chesterfield, 1986, cited in Leitch, 1989). This decrease is strongly linked to regulation. Bren (1992) investigated the invasion of the largest grass plain in the Barmah-Millewa Forest, and found that there had been substantial invasion by red gums. Bren (1992) suggested that this decline was a direct result of changes to water levels following regulation Red gum forest • Natural regeneration of red gum is dependent on occurrence and timing of flooding. The ideal conditions are on the spring recession of winter flooding • High river levels associated with summer irrigation supplies have led to tree death due to waterlogging.

Fluvial Systems Pty Ltd 46 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

7 Torrumbarry Weir to Lock 11 Mildura

7.1 Summary of impacts and causes The primary impact of regulation in the Torrumbarry Weir to Lock 11 Zone (Figure 2) is the significant decrease in flow volume due to large upstream diversions, but weirs have the effect of maintaining relatively constant water levels in the pools upstream. Mean annual flow at Euston reduced by 49% from natural levels. Periods of prolonged low flow are more frequent. Frequency, duration and magnitude of all but the largest floods have been reduced. There appears to be an accelerated loss of in-channel benches due to prolonged constant regulated flows and rapid falls in water level, although de-snagging is also implicated. Between 1927 and 1981, the river aggraded up to 3 m in some locations along this zone, with the average being around 1.5-2.0 m. There is an unequivocal increase in turbidity between Torrumbarry and Euston Weir pool, followed by a decrease in turbidity at Euston and a further decrease to Mildura. This is probably partly due to the regional geomorphic change (from Riverine Plain to Mallee tract), but reduced velocities in the weir pool, and saline inflows may also be involved. The Mallee tract showed consistent bed aggradation between surveys undertaken in 1927 and 1981. The three weir pools in this zone can potentially be on the threshold of cyanobacterial bloom problems during dry periods, with Torrumbarry and Mildura Weir pools having the highest risk. Wetland and floodplain habitats have been adversely impacted by reduced frequency of inundation, while the artificial elevation of water levels upstream of weirs has led to the permanent inundation of some wetlands, reducing their productivity. The diversity and abundance of most aquatic biota (fish and macroinvertebrates) is comparatively poor immediately below Torrumbarry Weir, but improves significantly further downstream. The improvement was associated with increasing water clarity downstream which appears to promote richer development of benthic algae and bacterial biofilm on hard surfaces, which in turn improves the situation for macroinvertebrates and fish. The altered flooding pattern is thought to have reduced the effectiveness of inoculation of the river with micro-organisms and micro-invertebrates, and hence reduced the availability of larval fish food. Weirs create severe restrictions on faunal movement, which leads to population discontinuities. Flow regulation has removed cease-to-flow conditions from the flow pattern but such events were very rare (1 in 100 years) prior to regulation.

7.2 Regulation influences The Torrumbarry Weir to Wentworth zone (upstream of the Murray-Darling confluence) is downstream of the major irrigation diversions and receives water from a number of unregulated tributaries. For these reasons the seasonality of flow in this zone is closer to the pre-regulation regime than it is for the reaches further upstream. The primary impact of regulation in this zone is the significant decrease in flow volume due to large upstream diversions (Table 7-1).

7.2.1 Torrumbarry Weir The original Torrumbarry Weir was constructed in 1924. Between 1993 and 1996, a new Torrumbarry Weir was designed and constructed. The Torrumbarry

Fluvial Systems Pty Ltd 47 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Weir pool has a capacity of 38 GL at a full supply level. This volume is one percent of the capacity of Dartmouth Dam. Like Yarrawonga Weir, Torrumbarry Weir must be full to allow gravity diversion. Torrumbarry Weir is used to provide sufficient hydraulic head to all water to be diverted into the Torrumbarry irrigation system. The Weir is also operated to maintain a minimum release to meet the minimum flow requirement of approximately 1,600 ML/day (0.6 m on the gauge) at Swan Hill. The water level is elevated from mid-August to mid- May (Thoms et al., 2000). The new Weir operates year round, including during floods (compared to the old weir where the trestles were removed during floods). As such, the upstream pool level is retained except for drawdowns in early winter for maintenance purposes (MDBC, 2002). Torrumbarry Weir has a fish ladder to allow migratory fish to pass the structure. The fish ladder can be operated with a cage to trap fish as they leave the ladder. This allows the weir keepers to record the size, species and abundance of fish using the ladder, which can be used in scientific studies. Indigenous species of fish that are caught in the cage are released, while introduced species of fish (eg. carp) are destroyed (MDBC, 2002). The original fish ladder was replaced as part of the construction of the new Weir (MDBC, 2002).

7.2.2 Euston Weir Euston Weir is located in the Sunraysia region on the River Murray near Euston (NSW) and Robinvale (Victoria), 1,110 km upstream of the river mouth (MDBC, 2002). Euston Weir was originally installed in 1937 to maintain a water level sufficient for navigation along this zone of the Murray. It was subsequently used to maintain sufficient flow or water level for the Sunraysia diversions. Water is pumped from the Euston Weir pool to supply the Robinvale Irrigation District in Victoria, private diverters in NSW and urban supplies. Euston Weir raises the water in the River Murray to a level so that it permanently inundates the Euston Lakes, Dry Lake and Lake Benanee. Lake Carrringay would also be inundated if it were not for levee banks isolating it, which allows the lakebed to be used for agriculture (MDBC, 2002).

7.2.3 Mildura Weir (Lock 11) Mildura Weir, constructed in 1927, is located near Mildura (river kilometre 878), in north western Victoria (Figure 2). The purpose of Mildura Weir is to hold a steady elevated water level for irrigation diversions and navigation. The weir pool can also be used for recreational pursuits (MDBC, 2002). The River Murray is permanently navigable to the top of the Mildura Weir pool, near Nangiloc, a distance of 978 km from the Mouth. The Murray upstream of Nangiloc is only navigable during periods of high flow. Wentworth Weir, constructed in 1929, is also known as Weir 10. Mildura Weir os primarily a navigational structure that has little impact on flows (Thoms et al., 2000). The Murrumbidgee, Edward- Wakool and Loddon Rivers join the River Murray through this zone (Figure 2).

Fluvial Systems Pty Ltd 48 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 7-1. Main identified impacts of regulation on hydrology between Torrumbarry Weir and Lock 11 Mildura (source: Thoms et al., 2000) Variable Change due to flow regulation Flow volume • Mean annual flow at Euston reduced by 49% from natural levels. Upstream of the Darling River junction the reduction is 53% (Maheshwari et al., 1993, p. 17) Flow • Coefficient of variation of annual flows at Euston variability increased from 0.56 under natural conditions to 0.92 (Maheshwari et al., 1993, p. 17) • Periods of prolonged low flow much more frequent (Thoms et al., 2000, p. 106) Flow • Seasonality of monthly flows at Euston unaltered seasonality (Maheshwari et al., 1993, p. 27) Floods • Frequency, duration and magnitude of all but the largest floods reduced due to effects of major storages on Murray and tributaries including Eildon, Blowering and Burrinjuck Reservoirs (Thoms et al., 2000, p. 106) • Under natural conditions the average flow for the wettest month exceeded 20,000 ML/d in at least 94% of years, while under regulation this fell to 49% of years at Torrumbarry and 65% of years at Euston (Thoms et al., 2000, p. 109) • Under regulated conditions, the highest monthly flow, in September is 15,000 ML/d, while under natural conditions it was 35,000 ML/d (Cooling et al., 2001). • Flows to Gunbower Forest of 15,000 ML/d or more naturally occurred for 5.8 months per year, while under regulated conditions this has reduced to 2 months. Similarly flows >30,000 ML/d have declined in duration from 3.2 months to 0.9 months per year (Cooling et al., 2001). • Inflows to Gunbower Forest (at 15,000 ML/d or more in the River Murray) naturally occurred 98% of years, while under regulated conditions this has reduced to 57%. Similarly, overbank flows at >30,000 ML/d have declined in frequency from 83% of years to 32% (Cooling et al., 2001). • Hattah Lakes National Park, downstream of Euston can expect flood inflows in 38% of years under regulated conditions (1994 level of development), compared with 85% of years under natural conditions (Close, 1990, p. 70; MDBMC, 1995, p. 16).

Fluvial Systems Pty Ltd 49 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

7.3 Identified impacts of regulation on environmental values from Torrumbarry Weir to Lock 11 Mildura

7.3.1 Channel morphology Banks The river in this zone has quite variable in-channel morphology. Around Tooleybuc, bench complexity increases, and like the zone above Torrumbarry, there appears to be an accelerated loss of these features due to prolonged constant regulated flows and rapid falls in water level, although de-snagging is also implicated (Thoms et al., 2000, p. 108). At the Wakool junction, a notch was observed at the regulated flow level, and recent riverbank erosion was also observed (Thoms et al., 2000, p. 108). Bed Rutherfurd (1990, p. 35) reported that between 1927 and 1981, the river aggraded up to 3 m in some locations along this zone, with the average being around 1.5-2.0 m. There is a suggestion that this was related to the onset of regulation, because the river did not aggrade between the 1876 and 1927 surveys (in contrast, there was slight degradation of the bed) (Rutherfurd, 1990, p. 35).

7.3.2 Water Quality Turbidity The data of Mackay et al. (1988) show an unequivocal increase in turbidity between Torrumbarry and Euston Weir pool (median increases from 20 NTU to 40 NTU). There is a decrease in turbidity at Euston and a further decrease to Wentworth (median decreases from 40 NTU to 30 NTU at Euston then declines to 20 NTU at Wentworth). The increase in turbidity across the Riverine Plain (comprising fine clay-rich sediments) is caused by turbid inflows from tributaries (Ovens, Goulburn, Wakool and Murrumbidgee Rivers). However, just downstream of the Murrumbidgee River junction the river enters the Mallee, a geomorphic region that is composed largely of coarse sands of marine origin. Also, at this location the river enters the Euston weir pool where the flow velocity slows considerably. The bed elevation data in Rutherfurd (1990) indicate that while bed aggradation did occur upstream of Euston Weir between the 1927 and 1981 surveys, the degree of shallowing was greater and more consistent in the Mallee tract. These data suggest that the combination of geomorphic conditions and regulation of flow levels by Euston Weir act to reduce turbidity levels from Euston to the junction of the Darling River. There is also a suggestion that saline river and groundwater inflows, by flocculating fine sediment, may be involved in the declining turbidity (Mackay et al., 1988, p. 30; Thoms et al., 2000, p. 108). Algal blooms The decreased current velocity during summer flow conditions means that the four weir pools in this river zone can potentially be on the threshold of cyanobacterial bloom problems during dry periods (Thoms et al., 2000, p. 109). Although the current regulated regime is low risk for major cyanobacterial blooms at Euston and Wentworth (1 year in 20), the risk is higher at Torrumbarry and Mildura. However, less serious cyanobacterial problems, but which still require treatment, should be expected to occur more frequently (Jones, 1997, cited in Thoms et al., 2000, p. 111-112). It must be remembered that low flows conducive to algal blooms occurred under natural flow conditions

Fluvial Systems Pty Ltd 50 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission approximately 1 in 20 years, and Thoms et al. (2000, p. 113) speculated that this process may be important to the long-term functioning of the river.

7.3.3 Ecology Wetlands and floodplains - general The reduction in flood frequency due to regulation (combined with levee construction) means that the riparian and floodplain zones with their associated billabongs and other wetlands, are inundated less frequently, and Thoms et al. (2000) proposed that this has resulted in degradation of habitat. As a general process, reduced flood frequency also leads to less frequent inputs of organic material from the floodplain to the channel (Thoms et al., 2000). The artificial elevation of water levels upstream of weirs has led to the permanent inundation of some wetlands, reducing their productivity (Thoms et al., 2000). Hattah-Kulkyne Lakes, Koondrook and Perricoota Forest, Werai Forest and Gunbower Forest Some high value wetland systems exist within this river zone. Gunbower Forest (19,450 ha) and Hattah-Kulkyne Lakes (1,018 ha) are Ramsar listed, and Koondrook and Pericoota Forest and Werai Forest are listed as internationally important (MDBC, 2002). Hattah-Kulkyne Lakes (48,000 ha) are a series of interconnected lakes joining onto the Murray River just south of Mildura. The Werai Forest covers an area of 11,234 ha and is located along the Edward and Niemur Rivers between Yadabal Lagoon and Morago. The Koondrook and Perricoota Forests are located on the floodplain of the central Murray River from approximately 40 km downstream of Moama to Barham. They cover an area of 31,150 ha. The River Red Gums which characterise the higher floodplain areas represent a substantial proportion of the River Red Gum forest in NSW (DLWC, 2002). Gunbower Forest extends from Torrumbarry Weir to Koondrook weir and is the second largest Red Gum forest in Victoria (Cooling et al., 2001). The mid- range flooding hydrology of these important wetland areas have been affected by regulation. Gunbower Forest has been affected by a decline in the frequency of flooding associated with regulation and diversions. This has led to a decline in the extent of wetlands important for colonial waterbirds to breed. It is believed that Black Box woodland is encroaching on formerly frequently flooded areas in response to the drier water regime (Cooling et al., 2001). The extent of flooding in Gunbower Forest is determined by the height of the River Murray upstream of Torrumbarry Weir. Although the wetlands commence to flow values at quite low river levels, the capacity of the inlets is low, and substantial flooding requires overbank flows for more than four weeks (Cooling et al., 2001). The duration and frequency of flooding has declined substantially (Table 7-1). A similar reduction in flooding frequency has occurred at Hattah- Kulkyne Lakes (Table 7-1). Fish and invertebrates Thoms et al. (2000, p. 108) observed that the diversity and abundance of most aquatic biota (fish and macroinvertebrates) is comparatively poor immediately below Torrumbarry Weir, but improves significantly further downstream. The improvement was associated with improved in-stream habitat, and inflows from the Edward-Wakool Anabranch. Increasing water clarity downstream of the Murrumbidgee River junction appears to promote richer development of benthic

Fluvial Systems Pty Ltd 51 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission algae and bacterial biofilm on hard surfaces, which in turn improves the situation for macroinvertebrates and fish (Thoms et al., 2000, p. 108). Flooding of billabongs helps inoculate a river with micro-organisms and micro- invertebrates. The altered flooding pattern on the River Murray is thought to have reduced the effectiveness of this process, and hence reduced the availability of larval fish food (Thoms et al., 2000). Weirs create severe restrictions on faunal movement, which leads to population discontinuities (Thoms et al., 2000, p. 108). Flow regulation has removed cease-to-flow conditions from the flow pattern. While cease-to-flow would have been rare in the middle-River Murray prior to regulation (Close, 1990), very low flow would not have been uncommon in dry years. It is not clear what the ecological impact of this change has been. Young (2001) postulated that lack of sustained periods of very low flow (or cease-to- flow) would be more favourable to carp.

Fluvial Systems Pty Ltd 52 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

8 Lock 11 Mildura to Lock 3

8.1 Summary of impacts and causes The Lock 11 to Lock 3 Zone, being located downstream of the major irrigation diversions and being fed by a number of unregulated tributaries (Figure 2), has a flow seasonality that is similar to that of the pre-regulation regime. However, the volume of flow has been much reduced, as has the frequency, duration and magnitude of flooding. The variability of mid-range flows has been reduced, so that the present regime is dominated by prolonged low flows, with the occasional high flows. Lake Victoria is used to re-regulate surplus flow in the River Murray, and ensure that South Australia is supplied with its entitlement flow. The role of the locks on the River Murray between Mildura and Lock 3 is to aid navigation and facilitate the diversion of water by maintaining a constant level in the weir pools. The planform of the river did not alter between 1906 and 1988, reflecting the low stream energy and cohesive bank material, but significant changes in the internal dimensions and slope of the channel have occurred following weir construction. Bank retreat of over two metres per year downstream of weirs is associated with extremely rapid flood recessions. Bank slopes have also increased. Width-depth ratios have increased by an average of 32% between Locks 3 and 4. Prior to regulation, the bed sediments were predominantly coarse sand, while now they are comprised mainly of fine silts and clays. The channel is developing a stepped gradient associated with the weirs. Due to the operation of Lake Victoria, the length of the period of highly turbid Darling River water impacting significantly on the turbidity of the River Murray has historically been extended from two months to approximately seven months. In more recent seasons (since 1989) Lake Victoria has been filled with lower turbidity water originating from the Murray-Murrumbidgee catchment (although this decision was driven by the lower salinity of Murray-Murrumbidgee water). This had the effect of increasing the growth of macrophytes and macroinvertebrates in wetlands and in the main river channel. While for an algal bloom to fully develop, low flow conditions must be combined with other favourable conditions of warm temperatures and stable weather, a flow at least as low as 6,000–7,000 ML/d was found to be critical to the process. This flow is similar to that currently used to supply the minimum entitlement flow to South Australia, suggesting that the current operating conditions may be conducive to development of algal blooms. Increased salinity and higher groundwater levels, combined with the regulated flow regime, has had an adverse effect on the growth and regeneration of floodplain trees, including black box and Lignum. The impact of water level management has changed the nature of the littoral zone by limiting exchanges between the river and its floodplain. The wider range of water level variation in the upper part of the weir pools (compared to the lower end) is reflected in the wider elevational range over which specific littoral flora are observed. Artificially raised water levels have led to a general shift in wetland type from temporary to permanent. Reducing floodplain inundation frequency through regulation probably severely reduces the reserve of invertebrates that can contribute to the floodplain foodweb following inundation. Stabilisation of the water level through flow regulation has promoted the growth of filamentous green algae. This shift in phytoplanton composition has had repercussions at higher levels in the food chain, with the

Fluvial Systems Pty Ltd 53 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission loss of two species of gastropod. The riverine freshwater mussel, Murray crayfish, and the river snail have either declined or become regionally extinct. Although there are few historical data, a decline in the populations of many native fish, including Murray cod, trout cod, golden perch and river blackfish has been reported under regulated flow conditions. Regional extinctions are well advanced for five native species in the lower Murray, and another two are threatened. Native fish represent only about 5% of the total fish biomass. The main problem is reduced opportunities for recruitment because of the elimination of small floods, but sudden changes in water levels below weirs can strand fish eggs and cause fish to abandon nests.

8.2 Regulation influences The Lock 11 Mildura (river kilometre 878) to Lock 3 (river kilometre 381) Zone, being located downstream of the major irrigation diversions and being fed by a number of unregulated tributaries, has a flow seasonality that is similar to that of the pre-regulation regime. The major external influence on the hydrology in this zone is inflows from the Darling River (Figure 2). The volume of flow in this zone has been much reduced, as has the frequency, duration and magnitude of flooding. The variability of mid-range flows has been reduced, so that the present regime is dominated by low flows, with the occasional high flows (Wittington et al., 2000) (Table 8-1).

8.2.1 Locks and weirs between Wentworth and Blanchetown The first lock and weir on the Murray was completed in 1922 at Blanchetown in South Australia. The decline in the river trade led to the abandonment, in 1934, of the construction of further weirs and locks purely for navigation purposes (MDBC, 2002). The ten weirs with locks between Wentworth and Blanchetown, each raising the water level behind it by an average of 3.1 m, create a continuous series of stepped pools (MDBC, 2002). The locks aid navigation and facilitate the diversion of water by maintaining a constant level in the weir pools. The water levels are maintained even during low flows (e.g. the major drought of 1967/68) (Wittington et al., 2000). Weir 9 raises the water level high enough to allow gravity diversion to Lake Victoria (MDBC, 2002). Locks 1 and 2 are located downstream of this river Zone.

8.2.2 Lake Victoria Lake Victoria (capacity 680 GL), located in south-western New South Wales, is one of the four major water storages on the Murray-Darling River system. The Lake Victoria storage was constructed in the late 1920s by the River Murray Commission to provide a reliable water supply for the development of the Lower Murray region in South Australia and to mitigate and augment flood peaks (MDBC, 2002). Water enters Lake Victoria upstream of Lock 9, often with highly turbid floodwaters from the Darling River, and water is discharged via the Rufus River downstream of Lock 7. Lake Victoria is located immediately upstream from South Australia. This means the Commission can deliver a uniform water supply to South Australia, in accordance to water entitlements defined in the Murray-Darling Basin Agreement. South Australia is entitled to receive between 3,000 and 7,000 ML/d, depending upon the time of the year (MDBC, 2002).

Fluvial Systems Pty Ltd 54 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Lake Victoria is used to reduce the problems associated with constraints on channel capacity imposed by the Barmah choke. This narrow section of the River Murray in The Yarrawonga to Torrumbarry Zone (Figure 2) makes it impossible to move large volumes of water downstream from the main Commission’s storages of Hume and Dartmouth without causing environmental degradation due to the unseasonal flooding of the River Red Gum forests. The water stored in Lake Victoria is used to meet the shortfall resulting from the constraints of the Choke (MDBC, 2002). In this way, operation of the Lake Victoria storage could be said to have a positive effect on the environmental values of the River Murray.

Table 8-1. Identified impacts of regulation on the hydrology between Lock 11 Mildura and Lock 3. Variable Change due to flow regulation Flow volume • Reduction in mean volume of flow at the South Australian border by 54% of the natural volume (Maheshwari et al., 1993, p. 17) Flow • Median annual natural flow exceeded only 8% of the variability time, compared with approximately 50% under natural conditions (Wittington et al., 2000) • Prolonged periods of medium-low flow. This is illustrated by the fact that entitlement flow is received on average in 55% of months, and that flows in the range 500- 1500 GL/month have increased by a factor of approximately two (Thoms et al., 2000) • Low flows (<500 ML) occur 66% of the time under regulation, but only 7% of the time under natural flow conditions (Wittington et al., 2000) • Coefficient of variation of annual flows at the South Australian border increased from 0.55 to 1.00 (Maheshwari et al., 1993, p. 17) Flow • Seasonality of flows unaltered (Thoms et al., 2000, p. 116) seasonality Floods • Reduced frequency, duration and magnitude of flooding. Inundation of the Chowilla floodplain now occurs at a frequency of 1 in 13 years compared to the natural frequency of 1 in 4 years (Sharley, 1992) • Flows of 3,000 GL/month occurred 1 in 3.6 years under natural conditions, but occur only 1 in 10.5 years under regulated conditions (Thoms and Walker, 1993). • Increased rates of flood recession (Thoms and Walker, 1992a)

Fluvial Systems Pty Ltd 55 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

8.3 Identified impacts of regulation on environmental values from Lock 11 Mildura to Lock 3

8.3.1 Channel morphology Banks The planform of the river did not alter between 1906 and 1988, reflecting the low stream energy and cohesive bank material (Thoms and Walker, 1992b). However, significant changes in the internal dimensions and slope of the channel have occurred following weir construction (Thoms and Walker, 1992a, b). Under the regulated regime, the banks in the lower Murray are subjected to increased duration of near bankfull flow and more rapid changes in water level (Thoms and Walker, 1991). Thoms and Walker (1989) reported bank retreat of over two metres per year as a result of apparent block failure and bank scour. The channel cross-section at Locks 3-4 and 8-10 has stabilised 30-40 years after regulation, while at locks 5-7 erosion has continued (Thoms and Walker, 1992b). It was proposed that the main process of erosion was large-scale bank slumping accelerated by extremely rapid flood recessions. Width-depth ratios have increased by an average of 32% between Locks 3 and 4. In the weir pools at the downstream end of the sequence, in-channel benches that provide valuable habitat areas have been largely eroded, while in the upper areas, pools have been flooded by increased water levels (Walker and Thoms, 1993). The slope of the river banks in the lower Murray is thought to have increased as a result of erosion promoted by rapid water level recession rather than scouring by high discharges. Walker et al. (1992) found that the slope of the banks immediately downstream of Lock 4 increased from 65º to 72º during 1988, and from 72º to 81º during 1989. Smaller more frequent changes in water level associated with routine weir operations may be undermining the toe of the banks, so that they are vulnerable to slumping following the more significant falls in water level (Walker et al., 1992). Bed Prior to regulation, the bed sediments were predominantly coarse sand, while now they are comprised mainly of fine silts and clays. This change represents a decrease of approximately 30% in median grain size, which is likely to have significant impacts on benthic organisms (Walker and Thoms, 1993). The channel is developing a stepped gradient associated with the weirs. Thoms and Walker (1992b) explained this in terms of bed degradation downstream of weirs and deposition of material in the weir pools. From 1906 to 1988 the mean river bed level aggraded by an average of 0.58 m in Pool 3, although there were also areas of bed degradation in this weir pool (Thoms and Walker, 1993).

8.3.2 Water quality Salinity The irrigation areas of South Australia are threatened by river salinities that exceed the tolerances of crop plants (Walker, 1985). Approximately 40% of salt in the lower river enters within South Australia, and a quarter of this is contributed by the weirs that are located in this zone (Walker, 1985). The saline groundwater downstream is forced into the river by the hydraulic pressure of the artificially elevated levels in the weir impoundments. Chowilla Creek, the main outflow from the Chowilla wetlands, contributes up to 145 tonne/day of salt.

Fluvial Systems Pty Ltd 56 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Lake Victoria plays an important role in reducing the salinity levels of water flowing along the River Murray to South Australia. This is achieved by coordinating operation between Lake Victoria and the Menindee Lakes Storage on the lower Darling River (MDBC, 2002). Evaporation loss is greater in the Menindee Lakes than in Lake Victoria, as a result water from Menindee Lakes is used in preference to Lake Victoria under a set of detailed operating procedures. The water saved by the "harmony operation" is used to decrease salinity levels in the lower part of the Murray by increasing the volume of water flows. The reduction in salinity is significant for domestic (including Adelaide), industrial and agricultural water use in South Australia. Lake Victoria is also used to buffer the flow to South Australia against brief periods of high salinity water, particularly after thunderstorms that result in the inflow of heavily salinised water. Under these situations much of the saline water in the River Murray is diverted into Lake Victoria where it mixes with water of a lower salinity. Releases are made from Lake Victoria to maintain the flow to South Australia. The Lake is flushed during periods of high flow with low saline water to reduce the Lake’s salinity (MDBC,2002). Turbidity Regulation of flows in the upper Murray has significant impacts on the turbidity characteristics in the lower reaches of the system. The length of the period of highly turbid Darling River water impacting significantly on the turbidity of the River Murray has historically been extended from two months to approximately seven months (Suter et al., 1993). During the El Nino episode of the 1980s, the demand for irrigation water was such that flows to the lower Murray were comprised mainly of Darling River water stored and released from Lake Victoria (Walker et al., 1992). In more recent, wetter seasons (since 1989), Lake Victoria has been filled with lower turbidity water originating from the Murray- Murrumbidgee catchment (Thoms et al., 2000, p. 68-69; Jensen, 1998). The effect of storing turbid floodwater from the Darling River in Lake Victoria for later summer release to the River Murray is to extend the duration of high turbidity flows. This practice (prior to 1989) had the effect of increasing median turbidity in the River Murray downstream of the Rufus River junction above the median value at Lock 9 (Mackay et al., 1988, p. 30). Algal blooms Burch et al. (1994) examined the relationship between flow level and blooms of toxic cyanobacteria (Anabaena circinalis) in the lower River Murray. While for a bloom to fully develop, low flow conditions must be combined with other favourable conditions of warm temperatures and stable weather, a flow at least as low as 6,000–7,000 ML/d was found to be critical to the process. This flow is similar to that currently used to supply the minimum entitlement flow to South Australia, suggesting that the current operating conditions may be conducive to development of algal blooms (Burch et al., 1994).

8.3.3 Ecological impacts Plants Increased salinity, combined with the regulated flow regime, has had an adverse effect on the growth and regeneration of floodplain trees, including black box and Lignum (Margules and Partners et al., 1990; O’Malley and Sheldon, 1990, both cited in Walker and Thoms, 1993; Craig et al., 1991; Jolly et al., 1993; Akeroyd et al., 1998). The regeneration rates of key riverine tree species (e.g. River red gum) are low, and the trees lack vigour on the floodplain below

Fluvial Systems Pty Ltd 57 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Wentworth (Margules and Partners et al., 1990). There is a striking paucity of plant (and animal) species in the middle and lower reaches of the River Murray in comparison with the upper reaches (Thoms and Walker, 1992a). Telfer and Overton (1999) proposed that the major cause of floodplain (including riparian areas) tree health decline in the area near Lock 4 was increased groundwater levels due to discharge of irrigation drainage water, but they did not rule out the role of reduced flood frequency in this process. The impact of water level management has changed the nature of the littoral zone by limiting exchanges between the river and its floodplain. In the lower weir pools, the water level is changing through most of the zone available for littoral plant growth. According to Walker et al. (1992), the steepening of river banks has acted to limit vegetation growth. It was further claimed that these changes degraded the littoral zone as a resource for fish. The range of water level fluctuations between weirs is reflected in the distribution of littoral plants (Blanch et al., 2000). The wider range of water level variation in the upper part of the weir pools (compared to the lower end) is reflected in the wider elevational range over which specific littoral flora are observed. Blanch et al. (2000) recorded a 4-6 m elevational range for the 48 investigated species in the upper reaches of weir pools in their study area in the lower Murray, where most fluctuation occurs, and a 1.0-1.5 m range in the lower pools, where levels are more stable. Forty-one of the 48 observed species occurred across much of the cone-shaped matrix, which indicated considerable tolerance of variable water levels, particularly Phragmites australia, Cyperus spp. and Sentripeda spp. Blanch et al. (2000) predicted that an increase in the amplitude of river level fluctuation during low flows, from the current 10-20 cm to 20-50 cm, would be sufficient to reinstate water regimes suitable to the majority of species surveyed. Thoms et al. (2000, p. 68) noted that the growth rate and vertical range of submersed plants such as Vallisneria, and also benthic algae biofilms, are highly sensitive to turbidity. More vigorous and abundant growth occurred during periods of lower turbidity when the flow in the river was dominated by clearer Murray-Murrumbidgee water as opposed to the more turbid Darling River water. Wetlands/floodplain habitats Artificially raised water levels have led to a general shift in wetland type from temporary to permanent, with the locking up of nutrient particles into the clay substrate under water-logged conditions reducing productivity (Briggs and Maher, 1985). Nielsen and Chick (1997) measured reduced productivity under conditions of long-term or permanent inundation. This process is obviously relevant for this Zone because wetlands are mainly (75%) “permanent wetlands connected to the river at minimum regulated flow (weir pool level)” (Pressey, 1990). The impact of releasing from Lake Victoria (since 1989) less turbid water sourced from the Murray-Murrumbidgee catchment had the effect of increasing the euphotic zone by 60-70% (Suter et al., 1993). Wetlands watered by flows that originate from Lake Victoria, that were previously regarded as having low value, were subsequently re-assessed as having high value due to growth of dense macrophyte beds and high waterbird populations (Jensen, 1998). Boulton and Lloyd (1992) conducted experiments that demonstrated much greater biomass and diversity of invertebrates that emerged from samples of dry floodplain sediment that were flooded annually compared with those that were flooded every 22 years. It was hypothesised that reducing floodplain inundation

Fluvial Systems Pty Ltd 58 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission frequency through regulation probably severely reduces the reserve of invertebrates that can contribute to the floodplain foodweb following inundation. Chowilla Wetlands Located near Renmark, the Chowilla floodplain covers 17,700 ha. It is a wetland in an arid environment, totally dependent on the Murray. It contains a diversity of habitats, with lakes, billabongs, islands, flowing creeks, levees and lunettes, and more than 100 km of anabranch creeks (MDBC, 2002). About 30,600 ha of the Chowilla Floodplain in South Australia constitute the 'Riverland' Wetland Complex which was listed under the Ramsar Convention in 1987 (ANCA 1996, p. 494-496). The construction of Lock 6 resulted in permanently higher water levels on the floodplain and continuous flows of water through the Chowilla anabranch system. The area is one of natural discharge of saline groundwater, but the salinity problems have been exacerbated by the higher groundwater levels resulting from Lock 6. River regulation has resulted in a reduction of flood frequency, significantly reducing the flushing of salt from the floodplain. This has affected the vegetation and resulted in the death of many trees, including the salt-tolerant black box (MDBC, 2002). In spite of these changes, the Chowilla Floodplain is one of the last remaining parts of the lower Murray floodplain that has not been used for irrigation and it retains much of the area's natural character and attributes (MDBC, 2002). Biofilms Periphytic biofilms are common on littoral surfaces in the lower River Murray. Burns et al. (1994) hypothesised that prior to regulation, high flow variability in the water level would have discouraged algal growth, with biofilms dominated by heterotrophic bacteria and fungi. Stabilisation of the water level through flow regulation has promoted the growth of filamentous green algae. This change has had repercussions at higher levels in the food chain (Burns et al., 1994). Macroinvertebrates The data of Bennison et al. (1989) showed that macroinvertebrate numbers downstream of Lake Victoria were lower by a factor of 10 compared with upstream of the Lake. In more recent years the lower turbidity during the summer-autumn period (due to a higher proportion of clearer Murray- Murrumbidgee water) resulted in a two- to three-fold increase in macroinvertebrate numbers, mostly crustaceans, especially the shrimp (Paratya australiensis) and prawns (Macrobranchium australiense) (Thoms et al., 2000, p. 68; Jensen, 1998, p. 224). The distribution of a number of species shows the effects of flow regulation (Walker, 1985). For example, the range of the riverine freshwater mussel Alathyria jacksoni has declined relative to the range of the lentic-adapted Velesunio ambiguus. The river-adapted Murray crayfish (Euastacus armatus) has been displaced by its floodplain counterpart, the yabbie (Cherax destructor) (Walker, 1985; Walker et al., 1992). The Murray crayfish is now virtually extinct in South Australia (Walker, 1985). The river snail Vivipara sublineata (Condrad) was common along the lower River Murray before weir construction, but live specimens have not been reported since the 1940s (Walker, 1985). There has been a regional extinction of the detritivorous prosobranch gastropods Notopala sp. and Thiara balonnenesis since the 1950s, thought to be related to the shift in phytoplanton composition caused by regulation (Sheldon and Walker, 1997).

Fluvial Systems Pty Ltd 59 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Fish Although there are few historical data, a decline in the populations of many native fish, including Murray cod, trout cod, golden perch and river blackfish has been reported under regulated flow conditions (Walker, 1985; Walker and Thoms, 1993). Although the year-to-year variation in fish catches is partly related to natural flooding patterns, there are significant correlations between increases in storage and diversions and the decline in catches of Murray cod and golden perch in the lower Murray. Walker and Thoms (1993) presented data collected by Cadwallader and Lawrence (1990) from professional anglers that showed a clear positive relationship between golden perch catch and river levels in part of the lower Murray between 1939 and 1979. Regional extinctions are well advanced for five native species in the lower Murray, and another two are threatened (Lloyd and Walker, 1986; Lloyd et al., 1991). The lower Murray fish community is severely depleted, with 29% of 55 native species absent and others present in very low numbers. Native fish represent only about 5% of the total fish biomass (Wittington et al., 2000), but this partly reflects the high numbers of carp (Gehrke et al., 1995). Flow regulation is implicated in declines in native fish because floods are vital for reproduction in most species, while less variable flows favour alien species (Walker and Thoms, 1993). The main problem is reduced opportunities for recruitment because of the elimination of small floods, but sudden changes in water levels below weirs can strand fish eggs and cause fish to abandon nests (Lloyd et al., 1989). Walker et al. (1992) considered the effects of flow regulation on fish populations to be significant, because floods enhance spawning and recruitment in several species, and suitable floods are now less frequent and less prolonged. For example, no recruitment of Murray cod was recorded in the period 1975-1989 (Jensen, 1996). Despite regulation, significant areas of floodplain are still occasionally inundated. However, Thoms and Walker (1992a) suggested that the rate of recession may be too fast for the plants and animals to adapt. Young fish risk being stranded if the flood recedes too quickly. Although a simplistic explanation for decline in fish populations, Thoms and Walker (1992a) noted that the essential point was that the regulated flow regime does not provide for maintenance of plant and animal communities.

Fluvial Systems Pty Ltd 60 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

9 Lock 3 to Wellington

9.1 Summary of impacts and causes Lock 3 to Wellington Zone contains the Murray Gorge (Figure 2). This zone has a similar degree of regulation as the Valley Zone upstream. The Darling River does not assist much in returning the River Murray to a less regulated regime, because it too is regulated. The cross-sectional area between Lock 2 and 3 has fluctuated through time. Weir pools have aggraded, and the bed has degraded downstream of the weirs. Littoral plant growth is limited in this Zone. In the absence of overbank flows (reduced in frequency through regulation), Gorge wetlands are more prone to sediment blockage than Valley wetlands, because they usually have only a single inlet channel.

9.2 Regulation influences The Lock 3 to Wellington Zone is distinguished from the Zone upstream by its geomorphology. Lock 3 is located just upstream of Overland Corner, which marks the head of the Murray Gorge. Across the Mallee as far as Overland Corner, the River Murray trench is 10 km wide, being cut into the relatively soft Parilla Sands, while downstream as far as Mannum the River cuts into tough limestone to produce a narrow gorge 400-1,600 m wide (Rutherfurd, 1990, p. 30). The degree of regulation in this Zone is similar to that in the Zone upstream (Table 9-1).

Table 9-1. Identified impacts of regulation on the hydrology between Lock 3 and Wellington Variable Change due to flow regulation Flow volume • Reduction in mean volume of flow at Lock 1 by 57% of the natural volume (Maheshwari et al., 1993, p. 17) Flow • As for the Zone upstream (Table 8-1) except for: variability • Coefficient of variation of annual flows at the Lock 1 increased from 0.56 to 1.08 (Maheshwari et al., 1993, p. 17) Flow • Seasonality of flows unaltered (Thoms et al., 2000, p. 116) seasonality Floods • Increased rates of flood recession (Thoms and Walker, 1992a)

9.3 Identified impacts of regulation on environmental values from Lock 3 to Wellington

9.3.1 Channel morphology Banks Width-depth ratios have decreased by 22% between Locks 2 and 3. (Walker and Thoms, 1993). Pool 2 cross-sections attained stability in <50 years, but the channel slopes are not yet stable, 63 years after weir construction (Thoms and Walker, 1993). The adjustments in cross-sectional area occurred independent of discharge changes, but there is some synchrony in the changes blow Locks 1 and

Fluvial Systems Pty Ltd 61 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

2, with the magnitude of change greatest below Lock 1 (Thoms and Walker, 1992b). Bed From 1906 to 1988 the mean river bed level aggraded by an average of 0.20 m in Pool 2, although there were also areas of bed degradation in this weir pool (Thoms and Walker, 1993). Although degradation has occurred below each weir, the maximum degradation does not necessarily occur in this area. For example, 32 km downstream of Lock to the river bed was lowered by 8.2 m following regulation.

9.3.2 Ecological impacts Plants Immediately below Lock 3 the range of daily water level fluctuations is comparable with the depth of the euphotic zone, so that the water level changes through most if not all of the zone for effective plant growth. Erosion has steepened the bank in this zone and plant growth is limited. Snags represent the most stable littoral microhabitat in this zone. Benches in the upper weir pools also have limited plant growth because they are typically inundated by >2 m of water. Most plant growth occurs in the middle reaches of weir pools (Walker and Thoms, 1993). Wetlands/floodplain habitats The main difference between the Mildura to Lock 3 (Valley) Zone and the Lock 3 to Wellington (Gorge) Zone are in the development of floodplain wetlands (Walker and Thoms, 1993). Many of the wetlands in both Zones were temporary before weir construction, and connected to the river only in times of flood. While the Valley wetlands are generally flooded via more than one connection, in the Gorge section the wetlands are usually connected by only one channel except during overbank floods (Walker and Thoms, 1993). In the absence of overbank flows (reduced in frequency through regulation), Gorge wetlands are more prone to sediment blockage than Valley wetlands.

Fluvial Systems Pty Ltd 62 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

10 Wellington to the Mouth

10.1 Summary of impacts and causes The Murray Barrages (Figure 2) comprise five low head weirs and earthen causeways linking the islands that once formed a previous shoreline. The barrages block 7.6 km of previously navigable channels, and prevent ingress of water to Lake Alexandrina. Regulation reduced mean volume of flow at the Barrages and Mouth by 62% of the natural volume (74% of the natural median volume). The frequency of drought conditions at the mouth has increased from 1 in 20 years to 2 in 3 years. Minor- to medium-sized floods (up to 1 in 7 year event) have been eliminated. Rates of flow recession following moderate/high events have increased. Currently there is very restricted flow through Mundoo and Boundary Creek Barrages, even when they are open. There is now an abrupt interface between the fluvial and tidal reaches, reducing the size of the estuarine component to 11% of its pre-Barrage scale. Increasing frequency and duration of periods of very low flow have contributed greatly to sedimentation at the Mouth and nearby channels. The sediment regime has also been impacted by the restriction of flow by the barrages causing a shift towards a more strongly depositional regime. The continuing growth of Bird Island has the potential to result in more frequent and more permanent blockage of the Mouth. Sedimentation upstream of Goolwa Barrage has been associated with a change from bioclastic sands to muds. The turbidity of the lakes has increased as a result of the relatively highly turbid Darling River water making a more significant contribution to the lower River Murray water budget. Although algal blooms did occur before regulation, there is general consensus that the incidence of cyanobacterial blooms in the Lower Lakes has increased with time. The Barrages have created an abrupt fresh-saline interface, whereas in its natural state there would have been a very large and transient interface, creating estuarine conditions. The management of the saline-freshwater interface over a very narrow range has effectively removed the habitats that represent the transition from saline to freshwater. As a consequence, flora adapted to this transition zone is poorly represented. The lack of many submerged plant species within the Lower Lakes appears to be correlated with the reduced light penetration associated with increased turbidity. The numbers of almost all species of waders and waterbirds using the wetlands of the Coorong and Lower Lakes have declined, particularly over the past 20 years. While this trend also applied on regional and national scales, the decline in total populations in the region between the Barrages and the Mouth are considered to be greater than the general levels of decline noted elsewhere. The confusion of ecological signals resulting from hydrological and geomorphic changes interferes with breeding/recruitment of fish and macroinvertebrates. The reduction in area of the highly productive estuarine habitat has affected the abundance of commercial and non-commercial fish species. Estuarine macroinvertebrates have also suffered reduced habitat area.

10.2 Regulation influences The Murray Barrages, built in 1940, were the last structures put in place by the River Murray Commission They comprise five low head weirs and earthen causeways linking the islands that once formed a previous shoreline. The barrages block 7.6 km of previously navigable channels, and prevent ingress of water to Lake Alexandrina. This maintains fresh water in the Lower Lakes and

Fluvial Systems Pty Ltd 63 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

River Murray (Jensen et al., 2000). Of course, flows at the Mouth are impacted by all the regulating structures upstream (Table 10-1).

10.3 Identified impacts of regulation on environmental values downstream of Wellington

10.3.1 Channel morphology Increasing frequency and duration of periods of very low flow have contributed greatly to sedimentation at the Mouth and nearby channels (Jensen et al., 2000, p. 78). The sediment transport regime has also changed, with Bourman and Barnett (1995) postulating a post-regulation reduction of sediment input from the River Murray. The sediment regime has also been impacted by the restriction of flow by the barrages causing a shift towards a more strongly depositional regime (Bourman and Barnett, 1995). The Bird Island flood tidal delta has developed and consolidated landward of the Murray Mouth, primarily due to lack of discharge through the Mundoo Barrage. (Bourman and Harvey, 1983). The Mundoo channel was closed in 1915 to restrict the ingress of saline water to Lake Alexandrina. The continuing growth of Bird Island has the potential to result in more frequent and more permanent blockage of the Mouth (Jensen et al., 2000, p. 27). Shoreline erosion in Lakes Albert and Alexandrina has been measured at rates up to 10 m/year in some locations, with an average of approximately 1 m/year. Bourman and Barnett (1995) attributed this mainly to a reduction of sediment input from the River Murray. Prograding shorelines have developed in sheltered areas of the lakes. The growth of these features is linked to the increased rates of erosion in other areas. The long-term net accumulation rate of 0.5 mm/year is significantly less than the short-term rate of 1.7 mm/year, which suggests that the sedimentation rate has accelerated since regulation (Bourman and Barnett, 1995). Further evidence is provided by the relatively recent change in facies from bioclastic sands to muds in the upper sediments of the lower channel. Sedimentation has occurred upstream of Goolwa Barrage at a rate of 4.5 mm/year over the last 50 years, with a change from bioclastic sands to muds (Bourman and Bennett, 1995). This is attributed to the capture of coarser material in the upstream storages.

10.3.2 Water quality The turbidity of the lakes has increased as a result of the relatively highly turbid Darling River water making a more significant contribution to the lower River Murray water budget (Mackay et al., 1988). Generally increased turbidity implies increased nutrient concentrations (Jensen et al., 2000, p. 19). Low summer turbidity in combination with reduced flow levels increases the risk of algal blooms (Jensen et al., 2000, p. 78). Although algal blooms did occur before regulation, there is general consensus that the incidence of cyanobacterial blooms in the Lower Lakes has increased with time (Jensen et al., 2000, p. 53).

Fluvial Systems Pty Ltd 64 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 10-1. Identified impacts of regulation on the hydrology downstream of Wellington Variable Change due to flow regulation Flow volume • Reduction in mean volume of flow at the Barrages and Mouth by 62% of the natural volume (Maheshwari et al., 1993, p. 17) • Median flow reduced by 80% (Jensen et al., 2000, p. 78) Flow • Since construction of the barrages there have been 20-30 variability periods of cease-to-flow (barrages closed for 100 or more consecutive days). The frequency of cease-to-flow at the mouth has increased from 1 in 20 years to 1 in 2 years (Close, 1990; Jensen et al., 2000, p. 78) • Pool levels are unnaturally high, so low summer water levels no longer experienced (Jensen et al., 2000, p. 78) • Minimum monthly flow at the Barrages reduced from 1,552 GL to 135 GL (Maheshwari et al., 1993, p. 17) • Coefficient of variation of annual flows at the Mouth increased from 0.60 to 1.32 (Maheshwari et al., 1993, p. 17) • Minimum monthly flow at the Mouth reduced from 1,383 GL to 35 GL (Maheshwari et al., 1993, p. 17) • The frequency of drought conditions increased from 1 in 20 (natural) to 2 in 3 years (regulated) (MDBMC, 1995) • Years with annual flows <5,000 GL occurred 7% of the time under natural conditions, but 66% of the time under regulated conditions (Thomson, 1992) Flow • Seasonality of monthly flows unaltered (Jensen et al., seasonality 2000, p. 15) Floods • Minor- to medium-sized floods (up to 1 in 7 year event) have been eliminated (Jensen et al., 2000, p. 78). • Threefold reduction in the frequency of medium–sized flood events (20,000 – 80,000 ML/d) (Jensen et al., 2000, p. 19) • Reduction in the duration of medium-sized flood events (Jensen et al., 2000, p. 19) • Increased rates of flow recession following moderate/high events (Jensen et al., 2000, p. 19) • Years with annual flow >25,000 GL occurred 5% of the time under natural conditions, and 2% of the time under regulated conditions (Thomson, 1992) Estuary • Significant changes in the flow distribution through the estuary channels. Currently there is very restricted flow through Mundoo and Boundary Creek Barrages, even when they are open (Jensen et al., 2000, p. 28) • Natural rates and patterns of ebb and flow have been constrained. There is now an abrupt interface between the fluvial and tidal reaches, reducing the size of the estuarine component to 11% of its pre-Barrage scale (Jensen et al., 2000, pp. 19, 28) • Barrage operation causes rapid changes in flow level (Jensen et al., 2000, p. 19)

Fluvial Systems Pty Ltd 65 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

• The water level in Bool Lagoon is now controlled by levee banks and regulator gates which have caused the water depth to be greater for longer and made the lagoon less likely to dry out (Denton and Graf, 1994) • Increased chance of mouth closure increases statistical risk of flooding (Jensen et al., 2000, p. 19)

The terminal lakes system would have previously offered a wide range of fresh, brackish, saline and hypersaline systems. The Barrages have created an abrupt fresh-saline interface, whereas in its natural state there would have been a very large and transient interface, creating estuarine conditions (Jensen et al., 2000, p. 19). The remnant estuary can change abruptly from saline to fresh conditions and back again in an unseasonal and unnatural pattern (Jensen et al., 2000, p. 20).

10.3.3 Ecological impacts Plants The management of the saline-freshwater interface over a very narrow range has effectively removed the habitats that represent the transition from saline to freshwater. As a consequence, flora adapted to this transition zone is poorly represented (Jensen et al., 2000, p. 31). The lack of many submerged plant species within the Lower Lakes appears to be correlated with the reduced light penetration associated with increased turbidity. This, in combination with the labile nature of the bed sediments, has acted to restrict vegetation growth to the near-shore areas, where it is subject to wind and wave forces (Jensen et al., 2000, p. 31). Experimental work suggests that plants are sensitive to sudden desiccation induced by rapid fall in water level (Jensen et al., 2000, p. 32). The recruitment of the previously widespread Melalueca halmaturorum in the Bool Lagoon, South Australia has been inhibited by the regulation of water levels (Denton and Graf, 1994). The plant has the ability to grow in water-logged soils, but it is unable to tolerate flooding of the above ground component because the supply of inorganic carbon is then restricted to an aqueous source. The lack of recruitment of juveniles into the population is likely to be a response to increased water levels. Birds Jensen et al. (2000, p. 36-37) reported that the numbers of almost all species of waders and waterbirds using the wetlands of the Coorong and Lower Lakes were considered by experts to be declining, particularly over the past 20 years. While it was acknowledged that this trend also applied on regional and national scales, the decline in total populations in the region between the Barrages and the Mouth were considered to be greater than the general levels of decline noted elsewhere (Jensen et al., 2000, p. 37). The decline in populations of waterbirds is thought to be related to habitat loss caused by operation of the Barrages and alteration of river flows by upstream storages (Jensen et al., 2000, p. 37). The drastic reduction in the area of estuarine conditions by the implementation of the barrages has greatly reduced the habitat available to waterbirds, which rely on estuarine-type habitats (Jensen et al., 2000). The current operating regime reduces the amount of shoreline around the lakes that is seasonally exposed when water levels drop.

Fluvial Systems Pty Ltd 66 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Fish and invertebrates The confusion of ecological signals resulting from hydrological and geomorphic changes interferes with breeding/recruitment of fish and macroinvertebrates. For example, greenback flounder and bream develop to advanced stages of reproduction, but are deprived of well-defined flood flow cues, and do not progress to spawning. The pattern of mulloway catches follows the pattern of freshwater outflows at the Barrages (Jensen et al., 2000, p. 45). Jensen et al. (2000, p. 47) reported that bream catches were consistently 100+ tonne/year in the 1970s, but are now as low as 3 tonne/year and declining. The reduction in area of the highly productive estuarine habitat has affected the abundance of commercial and non-commercial fish species. Similarly, estuarine macroinvertebrates have also suffered reduced habitat area (Jensen et al., 2000, pp. 46-47).

Fluvial Systems Pty Ltd 67 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

11 Lower Darling River and Great Anabranch

11.1 Summary of impacts and causes Mean annual flow in the Darling River (Figure 2) has been reduced by around 57% as a result of abstractions in the Barwon-Darling River system. The major impoundment on the lower Darling River, Menindee Lakes is an extensive online storage scheme, supplying approximately 40% of South Australia’s entitlement flow. Operation of the Menindee Lakes Scheme has shifted the seasonal flow pattern such that high flows now occur in summer. Winter flows are now less variable, and bankfull floods are less frequent. The Great Anabranch of the Darling River (Figure 2) leaves the River 55 km south of Menindee, but further downstream it is connected to the River by other flow paths, depending on flow level. The Great Anabranch is separate from the Darling River for a distance of 200 km, joining the River Murray 15 km west of the Darling River confluence at Wentworth. The Great Anabranch is an ephemeral channel that carries flood flows from the Darling River (every 2-3 years), but otherwise it does not flow. Early settlers lowered the main Anabranch off-take level over 130 years ago, thus lowering the threshold for commencement of flow. Operation of Menindee Lakes had the opposite effect on flow in the Anabranch, decreasing flood frequency, such that the effects cancelled each other. However, supply of water for stock and domestic use from the Anabranch (and associated storage of water behind small structures), plus the occasional practice of excluding floods from recently cropped lake beds, have locally altered flow regime in the Anabranch. Complex in-channel benches have been partially eroded by relatively constant regulated high flows. The habitat value of intact benches is reduced because of unseasonal inundation from flows used to supply South Australia’s entitlement. The pools behind the structures on the Great Anabranch create a heterogeneous bed environment, with higher than natural levels of sedimentation. Regulation has increased the risk of algal blooms through the combination of low flows and weir pools that create stratified conditions favourable for algal growth. The relatively constant flow levels and unseasonal flows have resulted in an apparent lack of macrophytes in the lower Darling River. The reduced frequency of flooding has detrimentally impacted the health of riparian and floodplain vegetation, and reduced the input of organic matter to the river. The northern sections of the Great Anabranch appear to have reduced flooding frequency due to operation of the Menindee Lakes Scheme. The near-stable pools upstream of the structures on the Great Anabranch create conditions conducive for the growth of macrophytes, such as the invasive species Typha. While the fish assemblages in the lower Darling River are healthy, fish movement, recruitment and recolonisation are adversely affected by a number of barriers, relatively constant flow levels, and reduced access to floodplain habitat. Unlike the River Murray, the Darling River has a predominantly native fish fauna. Although diversions have markedly reduced the annual flow volumes in the Darling River, the impact of regulation on flow variability and flooding regimes is less than it is on the River Murray. Macroinvertebrate communities in the lower Darling River do not appear to have been impacted by regulation. Loss of important habitat through cropping of ephemeral lakes is not a direct flow regulation impact, but the reduced flood frequency, and the possibility of excluding flows when necessary, helps to facilitate this practice.

Fluvial Systems Pty Ltd 68 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

11.2 Regulation influences

11.2.1 Lower Darling River The major impoundment on the lower Darling River, Menindee Lakes is an extensive online storage scheme, completed in 1960, which also performs a flood defence function. The pool behind the retaining weir can be surcharged to 2000 GL, and the lakes supply approximately 40% of South Australia’s entitlement flow. There are three low level weirs downstream of Menindee Lakes that provide town water supply and domestic and irrigation use. Early settlers lowered the main Anabranch off-take level over 130 years ago, thus lowering the threshold for commencement of flow. Operation of Menindee Lakes had the opposite effect on flow in the Anabranch, decreasing flood frequency, such that the effects cancelled each other in the southern end of the system (Irish, 1993) (Table 11-1).

11.2.2 Great Anabranch Supply of water for stock and domestic use from the Anabranch (and associated storage of water behind small structures), plus the occasional practice of excluding floods from recently cropped lake beds, have locally altered flow regime in the Anabranch (Thoms et al., 2000, p. 127) (Table 11-1). Before European settlement the Darling River used to spill into the anabranch along the main channel of the anabranch about 55 km south of Menindee (Jenkins, 1999). Under natural conditions the anabranch flowed 2 years in 3 in its upper reaches and about 8 years in 10 in the lower reaches (Irish, 1993). Control of flows in the anabranch began before 1885, by which time settlers had placed several dams along the Anabranch and had dug a channel linking the Darling with Coonalhugga Creek near Lignum Swamp. The Water Trust of the Great Anabranch of the Darling River was created in 1917 to manage flows for stock and domestic uses. By the late 1960s the Anabranch had been changed from a highly ephemeral stream to a permanent water body. Approximately 50,000 ML is released annually from Lake Cawndilla to the Anabranch through Tandou and Redbank Creeks. The release extends over 3 months, usually in mid- to late-winter. Regulators along the anabranch are operated to contain the water within the channel. Water is not specifically provided for the purpose of maintaining ecological values, but ponding of releases provides considerable, if unnatural habitat values (Briggs and Townsend, 1993). Flows past Menindee must exceed 10,000 ML/d before water can pass to the Anabranch from its southern intake, while at least 20,000 ML/d is required for inflows through Tandou Creek (Briggs and Townsend, 1993).

Fluvial Systems Pty Ltd 69 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Table 11-1. Identified impacts of regulation on the hydrology in the Lower Darling River and Great Anabranch (source: Thoms et al., 2000; Green et al., 1998; plus others indicated) Variable Change due to flow regulation Flow volume • In the Darling River, a reduction in mean annual discharge of 43% as a result of abstractions in the Barwon-Darling River system (Maheshwari et al., 1993, p. 17) • 50,000 ML/yr allocated for stock and domestic from the Great Anabranch • Flow volume in the Great Anabranch thought to be similar to pre-settlement levels (Irish, 1993) Flow • In the Darling River, the variability of winter flows is variability reduced (flows in the range 200-500 ML/d now occur more than 65% of the time) • In the Darling River, flows released in summer to supply South Australia are relatively constant • Low flows in the Darling reduced due to increased storage • Reduction in cease-to-flow in the Darling from 22 days per year to 3 days per year • Localised ponding occurs in the Great Anabranch due to operation of structures Flow • In the Darling River, seasonality of peak flows shifted seasonality from spring or autumn to summer due to operation of Menindee Lakes • Seasonality of flows unaltered in the Great Anabranch Floods • In the Darling River, the frequency of bankfull flows is reduced, with flows greater than 10,000 ML/d now occurring 10% of the time compared to 25% of the time pre-regulation • In the Darling River, floods that inundate high benches (15,000 ML/d), which occurred in 60% of years prior to regulation, now occur in 30% of years • Abstractions from the Darling River reduce the frequency of small to medium sized floods and reduce flood duration • Flood frequencies in the end lakes of the Great Anabranch thought to be similar to pre-settlement levels (Irish, 1993), but the northern floodplain may be suffering reduced flood frequencies (Thoms et al., 2000, p. 128). Briggs and Townsend (1993) claimed reduced Anabranch flood frequency for flows of 20,000 ML/d in the Darling from 2 in 5 years to 1 in 5 years. • Flooding pattern in the Great Anabranch similar to natural, except when a flood is excluded from a recently sown lake bed • Regulation has broadened and attenuated Anabranch flood peaks (Harriss et al., 1991)

Fluvial Systems Pty Ltd 70 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

11.3 Identified impacts of regulation on environmental values in the Lower Darling River and Great Anabranch

11.3.1 Channel morphology Banks Complex in-channel benches have been partially eroded by relatively constant regulated high flows. The habitat value of intact benches is reduced because of unseasonal inundation from flows used to supply South Australia’s entitlement. In addition, the banks have reportedly steepened due to constant high flows, which results in reduced micro-complexity and habitat value (Thoms et al., 2000, p. 127). Bed The pools behind the structures on the Great Anabranch create a heterogeneous bed environment, with higher than natural levels of sedimentation (Thoms et al., 2000, p. 128).

11.3.2 Water quality The Darling River is naturally turbid, high in phosphorous and low in nitrogen. Algal cell counts can be high, especially during spells of low flow. This problem particularly affects the Great Anabranch, which does not flow for much of the year, and in weir pools when summer flows are low. Thoms et al. (2000, p. 127) and Webster et al. (1997) argued that regulation increased the risk of algal blooms through the combination of low flows and weir pools that create stratified conditions favourable for algal growth.

11.3.3 Ecological impacts Plants The relatively constant flow levels and unseasonal flows have resulted in an apparent lack of macrophytes in the lower Darling River (Thoms et al., 2000, p. 127). The reduced frequency of flooding has detrimentally impacted the health of riparian and floodplain vegetation, and reduced the input of organic matter to the river (Thoms et al., 2000, p. 127). There is a flooding gradient on the floodplain of the Great Anabranch, such that the northern sections appear to have reduced flooding frequency due to operation of the Menindee Lakes Scheme. Field inspections by Thoms et al. (2000, p. 128) suggested that these northern areas may be slowly drying out. The near-stable pools upstream of the structures on the Great Anabranch create conditions conducive for the growth of macrophytes, such as the invasive species Typha. The reliable water level also appears to encourage growth of deep-rooted perennials such as riparian trees and shrubs (Thoms et al., 2000, p. 128). Black box trees are encroaching on the channel in some areas, and lignum may be spreading in some floodplain areas (Briggs and Townsend, 1993). Water birds Water birds, and wildlife generally, have greatly increased in numbers and in permanence in the Anabranch, reflecting the more reliable water supply (Briggs and Townsend, 1993).

Fluvial Systems Pty Ltd 71 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Fish and macroinvertebrates While the fish assemblages in the lower Darling River are healthy, fish movement, recruitment and recolonisation are adversely affected by a number of barriers, relatively constant flow levels (which apparently inhibit movement of some fish species), and reduced access to floodplain habitat (Thoms et al., 2000, p. 127). The weir at Menindee Lakes has allowed fish passage only twice since its construction (during very large floods). Unlike the River Murray, the Darling River has a predominantly native fish fauna (Gehrke et al., 1995). Although diversions have markedly reduced the annual flow volumes in the Darling River, the impact of regulation on flow variability and flooding regimes is less than it is on the River Murray. Macroinvertebrate communities in the lower Darling River do not appear to have been impacted by regulation (Thoms et al., 2000, p. 127). Thoms et al. (2000, p. 128) observed very little specific habitat for native fish in the upper areas of the Great Anabranch. It was implied that this problem was related to reduced flooding and the existence of migration barriers. In contrast, in the lower flooded areas of the Great Anabranch, fish abundance and diversity were observed to be high (King and Green, 1993, cited in Thoms et al., 2000, p. 128). The pools upstream of the structures on the Great Anabranch have macroinvertebrate and fish populations characteristic of billabongs (Thoms et al., 2000, p. 128). Twelve of the seventeen lakes on the Great Anabranch are opportunistically cropped. Lake Tandou and two ancillary lakes have been totally lost to cropping, which has destroyed their habitat value. Thoms et al. (2000, p. 128) speculated that these ephemeral lakes are highly significant in terms of their contribution to terrestrial and aquatic biodiversity. Loss of important habitat through cropping of ephemeral lakes is not a direct flow regulation impact, but the reduced flood frequency, and the possibility of excluding flows when necessary, helps to facilitate this practice.

Fluvial Systems Pty Ltd 72 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

12 Summary of Environmental Impacts due to Regulation

12.1 Hydrology The hydrology of the River Murray system has been characterised in a more consistent way than has the ecology and geomorphology. However, some aspects of the flow regime have not been characterised in detail. Flood characteristics have been examined at various levels of detail along the length of the river. Flood statistics are probably the most difficult to calculate. This is especially the case for the frequent floods that are of ecological interest, because they should be characterised using partial series analysis. The overall impact of regulation on the river’s hydrology can be summarised in terms of semi-quantitative changes in the major variables that seem to be related to ecological impact (Table 12-1).

Table 12-1. Summary of hydrological impacts of regulation on the River Murray and lower Darling River. Direction of change is indicated by Ã and Ä symbols, with magnitude of change semi-quantitatively indicated by size of symbols; : is data unavailable; } is no appreciable change.

Dartmouth-Hume Hume- Yarrawonga Yarrawonga- Torrumbarry Torrumbarry- Mildura 3 Mildura-Lock Lock 3 - Wentworth Wentworth- mouth Lower Darling River Annual volume } Ã Ä Ä Ä Ä Ä Ä Inter-annual variability : Ä } Ã Ã Ã Ã Ä

Winter/spring volume Ä Ä Ä Ä Ä Ä Ä Ä Summer/autumn volume Ã Ã Ã Ä Ä Ä Ä Ä Low flow duration Ä Ä Ä Ã Ã Ã Ã Ã

Capacity flow duration Ã Ã Ã Ã Ä Ä Ä Ä

Rise and fall control Ã Ã Ã Ã Ã Ã Ã Ã Mid-range flood frequency Ä Ä Ä Ä Ä Ä Ä Ä Mid-range flood duration Ã Ä Ä Ä Ä Ä Ä Ä

12.2 Geomorphology and water quality There has been no systematic geomorphic investigation of the whole river, so information on the impacts of regulation on geomorphology is incomplete and it comes from disparate sources. There is considerable uncertainty regarding some aspects of geomorphic change, either because data are lacking, or because detailed investigations have not been conducted. Water quality data are available from 1978, but lack of earlier data makes determination of the impact of

Fluvial Systems Pty Ltd 73 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission regulation difficult. However, knowledge of basic geomorphic, chemical and biological processes, and system modelling, has allowed formulation of general conclusions. The overall impact of regulation on the river’s geomorphology and water quality can be summarised in terms of semi-quantitative changes in the major variables that seem to be related to ecological impact (Table 12-2).

Table 12-2. Summary of geomorphological and water quality impacts of flow regulation on the River Murray and lower Darling River. Direction of change is indicated by Ã and Ä symbols, with magnitude of change semi- quantitatively indicated by size of symbols; : is data unavailable; } is no appreciable change; ? denotes high level of uncertainty.

Dartmouth-Hume Hume- Yarrawonga Yarrawonga- Torrumbarry Torrumbarry- Mildura 3 Mildura-Lock Lock 3 - Wentworth Wentworth- mouth Lower Darling River Bank/in-channel bench erosion Ã Ã Ã5 Ã Ã Ã ÄÃ9 Ã Bed degradation (Ä) Ã ÄÃ1 ÄÃ6 Ã ÄÃ7 ÄÃ7 Ã sedimentation (Ã) Ã Sediment transport : Ä2 : : : : Ä10 : Water temperature Ä Ä } } } } } } Turbidity Ã?3 Ã?3 : Ä Ã8 Ã8 Ã8 :

Nutrient load/concentration Ã?4 Ã?4 : : : : Ã?4 :

Algal bloom likelihood : : : Ã Ã Ã Ã Ã 1. Degradation close to the dam, sedimentation downstream of Albury 2. Dams have high trap efficiency, but channel erosion has increased 3. Uncertain, based on observations of increased bank erosion 4. Uncertain, based on observations of increased bank erosion or higher turbidity. Unusual temporal trends in nutrients observed downstream of Hume Dam 5. Widening through over-cut benches in the River Murray, but extensive channel enlargement in the upper Edward River 6. Bed degradation in the upper Edward River. Sedimentation in the River Murray, and the Edward River closer to Deniliquin 7. A stepped profile has developed, with degradation downstream of weirs and sedimentation in the weir ponds 8. Overall, regulation has increased turbidity by increasing the proportion of turbid flows from the Darling River, but the problem is less marked in years of higher flow 9. Expansion of Bird Is. flood tide delta, and some prograding shorelines in sheltered areas, but shoreline erosion is more dominant 10. Speculation, based on trapping by headwater dams and reduced flows

Fluvial Systems Pty Ltd 74 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

12.3 Ecology There has been no systematic ecological investigation of the whole river, so information on the impacts of regulation on ecology is incomplete and it comes from disparate sources. Some studies have involved highly detailed investigations in small areas, while others are based on more general surveys, or anecdotal evidence. The overall impact of regulation on the river’s ecology can be summarised in terms of semi-quantitative changes in the major variables (Table 12-3).

Table 12-3. Summary of ecological impacts of flow regulation on the River Murray and lower Darling River. Direction of change is indicated by Ã and Ä symbols, with magnitude of change semi-quantitatively indicated by size of symbols; : is data unavailable; } is no appreciable change.

Dartmouth-Hume Hume- Yarrawonga Yarrawonga- Torrumbarry Torrumbarry- Mildura 3 Mildura-Lock Lock 3 - Wentworth Wentworth- mouth Lower Darling River Native fish diversity/abundance Ä Ä Ä Ä Ä Ä Ä Ä Alien fish numbers Ã Ã Ã Ã Ã Ã Ã Ã Macroinvertebrate Ä diversity/abundance Ä Ä Ä Ä Ä Ä } Littoral plant diversity/abundance : Ä : : Ä Ä Ä Ä

Waterbird numbers : : Ä : : : Ä Ã

Wetland quality : : Ä Ä Ä Ä Ä Ä

Fluvial Systems Pty Ltd 75 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

13 Implications for Environmental Flow Options

13.1 Process for development of environmental flow options This document summarises the known changes in the natural environment of the River Murray that are thought to be largely due to flow regulation and other forms of water resource development. One important outcome of this review is identification of the major issues that can potentially be addressed by the Environmental Flows and Water Quality Objectives for the River Murray Project. In fact, the previous Scientific Panel reports prepared for the Murray- Darling Basin Commission (Thoms et al., 2000; Jensen et al., 2000) made numerous recommendations in this regard on the basis of the thorough research and review that was undertaken by the project teams. In developing flow Options, the emphasis is on accepting Objectives that relate to high priority sites or outcomes, and which involve quantitative, predictive models that link the hydrological event to ecological, geomorphic or habitat processes. Environmental flow Objectives are statements of long-term (preferably hydrologically-based) objectives that relate to predicted ecological, geomorphic or habitat outcomes. These objectives require specific flow targets that can be implemented by river operators. These targets are detailed specifications of how flows or structures are to be controlled or modified to produce the desired flow regime. Environmental flow needs of the River Murray were based upon the findings of the two earlier Scientific Panel studies. Another Expert Reference Panel, established specifically for the Project, adopted a risked-based approach to assessment of environmental flow options. Environmental flows can be delivered by making more water available, by altering the distribution of flows (operational changes) or by altering flow regulating structures (structural changes), or through some combination of these. It is recognised that recovering water from existing water users would likely involve significant social, economic, political and practical challenges. Ongoing work to investigate and resolve these aspects of the Project is being undertaken with the same level of rigour as applied to the development of flow scenarios (Gippel et al., 2001). Three critical aspects of the environmental flow setting process have proved to be the inclusive nature of the Project structure (emphasising relationship building between stakeholders and meaningful community involvement), system-wide hydrological modelling, and consideration of the dollar and social costs and benefits of flow changes. Also of great value has been the opportunistic implementation (as natural flows allow) and monitoring of certain environmental flow initiatives.

13.2 Relevance of this report to development of flow Options While several previous reports have reviewed the environmental impacts of flow regulation in the River Murray, this report is unique in that it: • Covers the entire area of interest from Dartmouth Dam to the mouth of the River Murray • Deals solely with regulation impacts, and makes no attempt to recommend environmental flows • Is focussed on regulation impacts, ignoring other types of environmental disturbance that cannot be addressed by flow management

Fluvial Systems Pty Ltd 76 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

• Concentrates on reporting quantified information that is reasonably well established and supported by appropriate data • Carefully references all data and information This review was prepared with three main applications in mind: • To assist all interested parties to further develop their ideas concerning the need for environmental flows in the River Murray • By quantitatively establishing the level of impact, location of impact and cause of impact, assist in prioritising areas for environmental flow options • To provide a quality review of information that can be used as a resource document by the Community, the Commission, Agencies and other stakeholders to aid the process of formulating flow options for the River Murray. The environmental objectives and hydrological options for the River Murray should be supported by strong, defensible arguments.

Fluvial Systems Pty Ltd 77 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

14 References Akeroyd, M.D., Tyerman, S.D., Walker, G.R. and Jolly, I.D. 1998. Impact of flooding on the water use of semi-arid riparian eucalypts. Journal of Hydrology, 206: 104-117. ANCA 1996. A Directory of Important Wetlands in Australia. Second Edition. Australian Nature Conservation Agency, Canberra. Arnott, C. 1994. Rates of stage fluctuations on the Murray River between Hume Reservoir and Albury. BSc (Hons) thesis, Department of Geography, The University of Melbourne, Parkville (unpublished). Baker, B. W. and Wright, G.L. 1978. The Murray Valley. Its hydrological regime and the effects of water development on the river, Proceedings of the Royal Society of Victoria, 90: 110-130. 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, ACT. Blanch, S.J., Walker, K.F. and Ganf, G.G., 2000. Water regimes and littoral plants in four weir pools of the River Murray, Australia. Regulated Rivers: Research and Management, 16: 445-456. Boulton, A. J. and. Lloyd, L.N. 1992. Flooding frequency and invertebrate emergence from dry floodplain sediments of the River Murray, Australia. Regulated Rivers: Research and Management, 7: 137-151. Bourman, R. P., and Barnett, E.J. 1995. Impacts of river regulation on the terminal lakes and mouth of the River Murray, South Australia. Australian Geographical Studies, 33: 101-115. Bourman, R. P. and Harvey, N. 1983. The Murray mouth flood tidal delta, Australian Geographer, 15: 403-406. Bren, L. J. 1987. The duration of inundation of flooding in a river red gum forest. Australian Forest Research, 17: 191-202. Bren, L. J. 1988a. Effects of river regulation on flooding of a riparian red gum forest on the River Murray, Australia. Regulated Rivers: Research and Management, 2: 65-77. Bren, L. J. 1988b. Flooding characteristics of a riparian red gum forest. Australian Forestry, 51: 57-62. 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. Bren, L. J. and. Gibbs, N.L. 1986. Relationships between flood frequency, vegetation and topography in a river red gum forest. Australian Forestry Research, 16: 357-370. Bren, L. J., Gibbs, N. L. and O’Neill, I.C. 1988. The Hydrology of the Barmah River Red Gum Forests: a Collection of Work Undertaken from 1984 to 1988, The University of Melbourne, Forestry Section, Creswick, Victoria. Bren, L. J., O’Neill, I.C. and Gibbs, N.L. 1987. Flooding in the Barmah Forest and its relation to flow in the Murray-Edward system. Australian Forestry Research, 17: 127-144. Briggs, S. V. and. Lawler, W.G. 1989. Management of Murray-Darling wetlands for waterbirds. Proceedings Third Fenner Conference on the Environment. Conservation in Management of the River Murray System - Making Conservation Count. Canberra, September. South Australian Department of Environment and Planning, Adelaide for Australian Academy of Science.

Fluvial Systems Pty Ltd 78 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Briggs, S. V. and Maher, M. T. 1985. Limnological studies of waterfowl habitat in south-western New South Wales. II Aquatic macrophyte productivity. Australian Journal of Marine and Freshwater Research, 36: 707-713. Briggs, S. and Townsend, G. 1993. Restoration and Management of Nearie Lake, National Parks and Wildlife Service. Burch, M.D., Baker, P.D., Steffensen, D.A., Bursill, D.B., Bain, D.B., Ganf, G.G. and Brookes, J.D. 1994. Critical flow and blooms of cyanobacterium Anabaena circinalis in the River Murray, South Australia. In Proceedings Environmental Flows Seminar, Canberra, 25-26 August, Australian Water and Wastewater Association, Artarmon, N.S.W., pp. 44-51. Burns, A., Sheldon, F. and Walker, K.F. 1994. Managing rivers from the bottom up – the importance of littoral biofilms. In Proceedings Environmental Flows Seminar, Canberra, 25-26 August, Australian Water and Wastewater Association, Artarmon, N.S.W., pp. 52-58. Cadwallader, P.L. and Gooley, G.L. 1984. Past and present distributions and translocations of Murray cod Maccullochella peeli and trout cod M. macquariensis (Pisces: Percichthyidae) in Victoria. Proceedings of the Royal Society of Victoria 96: 33-43. Cadwallader, P. L. and Lawrence, B. 1990. Fish. In Mackay, N. and Eastburn, D. (eds), The Murray, Murray-Darling Basin Commission, Canberra, ACT, pp. 316-335. Chesterfield, E. A. 1986. Changes in the vegetation of the river red gum forest at Barmah, Victoria. Australian Forestry, 49: 4-15. Close, A. 1990. The impact of man on the natural flow regime. In Mackay, N. and Eastburn, D. (eds), The Murray, Murray-Darling Basin Commission, Canberra, ACT, pp. 61-74. Cooling, M.P., Lloyd, L.N., Rudd, D.J. and Hogan, R.P. 2001. Environmental water requirements and management options in Gunbower Forest, Victoria. Australian Journal of Water Resources 5: 75-87. Crabb, P. 1988. Managing the Murray-Darling Basin, Australian Geographer, 19: 64-88. Craig, A. E., Walker, K.F. and Boulton, A.J. 1991. Effects of edaphic factors and flood frequency on the abundance of lignum on the River Murray floodplain, Australia. Australian Journal of Botany, 39: 431-443. Cunningham, R. B., Heathcote, C.R. and Nicholls, D. F. 1984. A statistical investigation into river flow, water quality and related issues in the River Murray System I. Report to the River Murray Commission, Canberra, ACT. Department of Land and Water Conservation (DLWC) 2002. Caring for our Natural Resources; Wetlands; Wetlands and Activities in your DLWC Region; Murray Region. http://www.dlwc.nsw.gov.au/care/wetlands/activities/murray/#WetlandsInt ernationalImportance (accessed July, 2002). Denton, M. and Ganf, G.G. 1994. Response of juvenile Melaleuca halmaturorum to flooding: management implications for a seasonal wetland, Bool Lagoon, South Australia. Australian Journal of Marine and Freshwater Research, 45: 1395-1408. Dexter, B. D., Rose, H.J. and Davies, N. 1986. River regulation and associated forest management problems in the River Murray red gum forests. Australian Forestry, 49: 16-27. Doeg, T. J. 1984. Response of the macroinvertebrate fauna of the Mitta Mitta River, Victoria, to the construction and operation of Dartmouth Dam. Occasional Papers from the Museum of Victoria, 1: 101-127.

Fluvial Systems Pty Ltd 79 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Ebsary, R. 1990. Effect of Dartmouth Reservoir on stream temperatures in the Mitta Mitta River. Technical Report 90/10, Murray-Darling Basin Commission, Canberra, ACT. Gehrke, P. C., Brown, P., Schiller, C.B., Moffatt, D.B., and Bruce, A.M. 1995. River regulation and fish communities in the Murray-Darling River System, Australia. Regulated Rivers: Research and Management, 11: 363- 375. Gippel, C.J. 1999. Draft Report on Hydraulic Effect of Snags and Management Options for the Edward River. Report by Fluvial Systems Pty Ltd, Melbourne, to Department of Land and Water Conservation, Albury, NSW. Gippel, C.J., Jacobs, T. and McLeod, T. 2001. Determining the environmental flow needs and scenarios for the River Murray System, Australia. Australian Journal of Water Resources 5: 61-74. Gippel, C.J. and Lucas, R. 2002. Review of Fluvial Geomorphology and Recent Environmental Changes. In EarthTech 2002. River Murray - Yarrawonga- Echuca Action Plan, Specialist Review Attachments. Report to Goulburn Broken Catchment Management Authority, Department of Land and Water Conservation ad Murray Darling Basin Comission. Gordon, N.D., McMahon, T.A., and Finlayson, B.L. 1992. Stream Ecology, an Introduction for Ecologists. John Wiley & Sons, Chichester. Green, S.J. 1999. Drawdown and river bank stability. Master of Engineering Science thesis, Department of Civil and Environmental Engineering, The University of Melbourne, Parkville (unpublished). Green, D., Mustak, S., Maini, N., Cross, H. and Slaven, J. 1998. Assessment of Environmental Flow Needs for the lower Darling River. Report to the Murray Darling Basin Commission by the Department of Land and Water Conservation. Centre for Natural Resources, Ecosystem Management Branch. Harriss, D., Shroo, H. and Everson, D. 1991. Issues for management of the water resources of the Great Anabranch of the darling River. In McGlynn, T., Harriss, D. and Everson, D. (eds) Great Darling Anabranch Forum, 2nd-4th July, Coomealla, NSW. ID&A Pty Ltd. (Ian Drummond & Associates Pty Ltd) 1993. River Murray and anabranches between Hume Dam and Lake Mulwala – investigation of river channel changes. Report to Murray-Darling Basin Commission, Canberra, ACT. ID&A Pty Ltd. 1997. Results of Mitta Mitta River Monitoring Program. Murray Darling Basin Commission. Irish, J. 1993. Nearie Lake Nature Reserve: historical frequency of inflows. Report TS 92.017. New South Wales Department of Water Resources, Parramatta, New South Wales. Appendix 2 in Briggs, S. and Townsend, G. 1993. Restoration and Management of Nearie Lake, National Parks and Wildlife Service. Jacobs, T. A. 1990. River regulation. In Mackay, N. and Eastburn, D. (eds), The Murray, Murray-Darling Basin Commission, Canberra, ACT, pp. 39-58. Jenkins, K. 1999. Environmental values of the Great Darling Anabranch of the Darling River. Report to the Steering Committee of the Darling Anabranch Management Plan. Jensen, A. 1996. Identifying and redressing the ecological consequences of river regulation in the lower River Murray. In Rutherfurd, I. and Walker, M. (eds) Proceedings of the First national Conference on Stream Management in Australia, Stream Management ’96, Merrijig. Coorperative Research Centre for Catchment Hydrology, Monash University, Clayton, Vic, pp. 163-169.

Fluvial Systems Pty Ltd 80 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Jensen, A. 1998. Rehabilitation of the River Murray, Australia: identifying causes of degradation and options for bringing the environment into the management equation. In de Waal, L. C., Large, A.R.C. and Wade, P.M. (eds), Rehabilitation of Rivers: Principles and Implementation, Wiley, Chichester, pp. 215-236. Jensen, A., Good, M., Tucker, P. and Long, M. 2000. River Murray Barrages Environmental Flows. Report to Murray-Darling Basin Commission, Canberra, ACT. Wetlands Management Program, Department of Environment and Natural Resources, Adelaide, South Australia. Jolly, I.D., Walker, G.R. and Thorburn, P.J. 1993. Salt accumulation in semi-arid floodplain soils with implications for forest health, Journal of Hydrology, 150: 589-614. Jones, G. J. 1997. Setting target river flows for the prevention of cyanobacterial blooms in weir pools. Final report to Land and Water Research and Development Corporation, Canberra, ACT. Hume and Dartmouth Dams Operations Review Reference Panel (HDDORRP) 1998. Options Paper. Kenyon, C., and Rutherfurd, I. 1999. Preliminary Evidence for Pollen as an Indicator of Recent Floodplain Accumulation Rates and Vegetation Changes: The Barmah-Millewa Forest, SE Australia. Environmental Management, 24:359-367. Kinhill 1988. Techniques for determining environmental water requirements – a review. Technical Report Series. Kinhill Engineers Pty Ltd. Report to the Department of Water Resources, Melbourne, Victoria. Koehn, J. D., Doeg, T. J., Harrington, D. J. and Milledge, G. A. 1995. The effects of Dartmouth Dam on the aquatic fauna of the Mitta Mitta River. Report to Murray-Darling Basin Commission by Arthur Rylah Institute for Environmental Research, Melbourne. Lawson and Treloar Pty Ltd, S.Brizga & Associates Pty Ltd and Neil M Craigie Pty Ltd. 2001. Geomorphic Investigation of the Influence of Changed Flows on the Mitta Mitta River, Volume 1. Report to North East Catchment Management Authority. Leitch, C. 1989. Towards a strategy for managing the flooding of Barmah Forest. Seminar on Barmah Forest organised by the State Working Group on River Murray Wetland and Forest Management, Department of Conservation Forests and Lands, Benalla Region, Victoria, Australia, 46 pp. Lloyd, L. N. and Walker, K. F. 1986. Distribution and conservation status of small freshwater fish in the River Murray, South Australia. Transactions of the Royal Society of South Australia, 110: 49-57. Lloyd, L. N., Puckridge, J.T. and Walker, K.F. 1989. The significance of fish populations in the Murray-Darling system and their requirements for survival. In Dendy, T. and Coombe, M. (eds), Proceedings Third Fenner Conference on the Environment. Conservation in Management of the River Murray System - Making Conservation Count. Canberra, September. South Australian Department of Environment and Planning, Adelaide for Australian Academy of Science, pp. 86-99. Mackay, N., Hillman, T. and Rolls, J. 1988. Water quality of the River Murray: review of monitoring 1978 to 1986. Water Quality Report No. 1, Murray- Darling Basin Commission, Canberra, ACT. Mackay, N. and. Eastburn, D. (eds) 1990. The Murray, Murray-Darling Basin Commission, Canberra, ACT. Maheshwari, B. L., Walker, K.F., and McMahon, T.A. 1993. The Impact of flow regulation on the Hydrology of the River Murray and its Ecological Implications, Centre for Environmental Applied Hydrology, Department

Fluvial Systems Pty Ltd 81 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

of Civil and Agricultural Engineering, The University of Melbourne and River Murray Laboratory, Department of Zoology, University of Adelaide. Maheshwari, B. L., Walker, K.F., and McMahon, T.A. 1995. Effects of flow regulation on the flow regime of the River Murray, Australia. Regulated Rivers: Research and Management, 10: 15-38. Margules and Partners, Smith, P., Smith, J. and Department of Conservation Forests and Lands Victoria 1990. Riparian Vegetation of the River Murray, Murray-Darling Basin Commission, Canberra, ACT. McKinnon, L. 1997. The effects of flooding on fish in the Barmah Forest. In Banens, R.J. and Lehane, R. (eds) 1995 Riverine Environment Research Forum, Proceedings of the Inagural River Environemnt Research Forum of MDBC Natural Resource Management Strategy Funded Projects, Attwood, Victoria. Murray-Darling Basin Commission, Canberra, ACT, pp. 1-7. MDBC (Murray Darling Basin Commission) 2002. Murray-Darling Basin Initiative, http://www.mdbc.gov.au (accessed July 2002). MDBMC (Murray-Darling Basin Ministerial Council) 1995. An Audit of Water Use in the Murray-Darling Basin. Murray-Darling Basin Ministerial Council, Canberra, 40 pp. Nielsen, D. L., and Chick, A.J. 1997. Flood-mediated changes in aquatic macrophyte community structure. Marine and Freshwater Research, 48: 153-157. Nielsen, D. L., Hillman, T. J., Shiel, R. J., Klomp, N. and Smith, F. 1996. Effect of flooding on microfaunal communities in artificial billabongs. In Wetlands for the Future, Proceedings of the Fifth INTECOL International Wetlands Conference, September, Perth, Western Australia. O’Malley, C. and Sheldon, F. (eds) 1990. Chowilla Floodplain Biological Study, Hyde Park Press for Nature Conservation Society of South Australia, Adelaide, 224 pp. Pardo, I., Campbell, I.C. and Brittain, J.E. 1998. Influence of dam operation on mayfly assemblage structure and life histories in two south-eastern Australian streams. Regulated Rivers: Research and Management, 14: 285-295. Pressey, R. L. 1990. Wetlands. In Mackay, N. and Eastburn, D. (eds) The Murray, Murray-Darling Basin Commission, Canberra, ACT, pp. 167-182. Quinn, G.P., Hillman, T.J. and Cook, R., 2000. The response of macroinvertebrates to inundation in floodplain wetlands: a possible effect of river regulation? Regulated Rivers: Research and Management, 16: 469-477. Reynolds, F. 1987. European Carp, Agfact F3.2.3, NSW Fisheries, Cronulla, NSW. Rowland, S.J. 1989. Aspects of the history and fishery of the Murray cod, Maccullochella peeli (Mitchell) (Percichthyidae), Proceedings of the Linnean Society of N.S.W. 111(3): 201-213. Rutherfurd, I. 1990. Ancient River, Young Nation. In Mackay, N. and Eastburn, D. (eds) The Murray, Murray-Darling Basin Commission, Canberra, ACT, pp. 17-36. Rutherfurd, I.D. 1991. Channel form and stability in the River Murray: a large, low energy river system in southeastern Australia. Unpublished Doctoral thesis, Monash University, Department of Geography and Environmental Science, 2 volumes. Shafron, M., Croome, R. and Rolls, J. 1990. Water Quality. In Mackay, N. and Eastburn, D. (eds) The Murray, Murray-Darling Basin Commission, Canberra, ACT, pp. 147-165.

Fluvial Systems Pty Ltd 82 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Sharley, T. 1992. Strategies for revegetating degraded floodplains. In Catchments of Green. Proceedings of a National Conference on Vegetation and Water Management. Adelaide. Greening Australia Ltd. Sheldon, F. and Walker, K.F. 1997. Changes in biofilms induced by flow regulation could explain extinctions of aquatic snails in the lower River Murray, Australia. Hydrobiologia 347: 97-108. Suter, P.J., Goonan, P.M., Beer, 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. Report to Murray-Darling Basin Commission by Australian Centre for Water Quality Research, Adelaide. Swales, S. and Harris, J. 1995. The expert panel assessment method (EPAM): a new tool for determining environmental flows in regulated rivers. Chapter 10 in Harper, D.M. and Ferguson, A.J.D. (eds), The Ecological Basis for River Management, Wiley, Chichester, pp. 125-134. TSTGRMEF (Technical Subcommittee of the Task Group on River Murray Environmental Flows) 2000. Technical Review of Environmental Flow Options for the River Murray, Draft for Project Board review, March (unpublished). Telfer, A. and Overton, I. 1999. The impact of irrigation on floodplain vegetation health adjacent to the River Murray. In Rutherfurd, I. and Bartley, R. (eds), Proceedings of the Second Australian Stream Management Conference, Vol 2, Adelaide, pp. 609-615. Terrazzolo, N. and Erskine, W.D. 1995. Geomorphic assessment of stream conditions in the upper north East Region of Victoria. In Ian Drummond and Associates Pty Ltd. Upper North East River Management Authority. Stream Zone Restoration Plan: Background and Assessment. Appendix A. Thoms, M. C. 1995. The impact of catchment development on a semi-arid wetland complex: the Barmah Forest, Australia, In Petts, G. (ed) Man’s Influence on Freshwater Ecosystems and Water Use. Proceedings of a symposium held during the XXI Assembly of the International Union of Geodesy and Geophysics at Boulder, Colorado, July. IAHS Publication No. 230: 121-130. Thoms, M. C., Suter, P., Roberts, J., Koehn, J., Jones, G., Hillman, T. and Close, A. 2000. Report of the River Murray Scientific Panel on Environmental Flows: River Murray - Dartmouth to Wellington and the Lower Darling River, River Murray Scientific Panel on Environmental Flows, Murray- Darling Basin Commission, Canberra. 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. 1991. Sediment transport in a regulated semi- arid river: the River Murray, Australia. In Robarts, R.D. and Bothwell, M.L. (eds), Aquatic Ecosystems in Semi-Arid Regions: Implications for Resource Management NHRI Symposium Series 7. Environment Canada, Saskatoon, pp. 239-250. Thoms, M. C. and Walker, K.F. 1992a. Preliminary observations of the environmental effects of flow regulation on the River Murray, South Australia. South Australian Geographical Journal, 89: 1-13. Thoms, M. C. and Walker, K.F. 1992b. Channel changes related to low-level weirs on the River Murray, South Australia. In Carling, P. A. and Petts,

Fluvial Systems Pty Ltd 83 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

G.E. (eds), Lowland Floodplain Rivers: Geomorphological Perspectives, Wiley. Thomson, C. 1992. The impact of river regulation on the natural flows of the Murray Darling Basin, Technical Report No. 92/5.1.Murray-Darling Basin Commission, Canberra, ACT. 11 pp. Tilleard, J. W., Erskine, W.D., and Rutherfurd, I.D. 1994. Impacts of River Murray flow regulation on downstream channel morphology. Proceedings Water Down Under '94, Adelaide, South Australia, Vol. 3, Surface Hydrology and Water Resources, Institution of Engineers Australia National Conference Publ. No. 94/10, pp. 409-415. Tunbridge, B. R. 1977. A survey of the fish populations in the Mitta Mitta River and tributaries before the construction of Dartmouth Dam. Arthur Rylah Institute for Environmental Research, Melbourne, Victoria. Walker, K. F. 1985. A review of the ecological effects of river regulation in Australia. Hydrobiologia, 125: 111-129. Walker, K. F. 1992. The River Murray, Australia: a semiarid lowland river. In. Calow, P. and Petts G. E. (eds). The Rivers Handbook: Hydrological and Ecological Principles, Vol 1, Blackwell Scientific, Oxford, pp. 472-492. Walker, K. F. and Thoms, M.C. 1993. Environmental effects of flow regulation on the lower River Murray, Australia. Regulated Rivers: Research and Management, 8: 103-119. Walker, K. F., Thoms, M.C., and Sheldon, F. 1992. Effects of weirs on the littoral environment of the River Murray, South Australia. In. Carling, P.A., Boon, P.J., and Petts, G.E. (eds), River Conservation and Management, John Wiley & Sons Ltd, pp. 271-291. 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, Canberra, ACT. Wittington, J., Cottingham, P., Gawne, B., Hillman, T., Thoms, M. and Walker, K. 2000. Ecological sustainability of rivers of the Murray-Darling Basin. In Murray-Darling Basin Ministerial Council. Review of Operation of the Cap: Overview Report of the Murray-Darling Basin Commission. Technical Report prepared by Cooperative Research Centre for Freshwater Ecology, Canberra, ACT. Young, W. J. (ed.) 2001. Rivers as Ecological Systems - the Murray-Darling Basin, Murray-Darling Basin Commission, Canberra, ACT.

Fluvial Systems Pty Ltd 84 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Appendix. Glossary of Terms Used in the Environmental Flows Field

Abiotic Non-living components of an ecosystem Abstraction Removal of water from a river (see diversion) Abundance Refers to the number, or relative concentration, of individual organisms Aggradation (of a Also known as sedimentation, it involves progressive channel) increase in bed elevation (channel shallowing) Alien species See Introduced Species Allocation The volume of water a licence holder is entitled to extract during a year, subject to licence conditions and availability. Currently, only licence holders on regulated rivers supplied by irrigation dams have an allocation. (See also Off-allocation flows) Allocation reliability The long-term probability (over wet, dry and average years) of irrigators with 'normal security' water allocations being able to get a certain proportion of their nominal allocation by a specified date Allocthonous Material that has originated from a place distant from where it is deposited Alluvial Groundwater (or sub-surface water) contained in the aquifer/groundwater alluvial deposits near a river. It is usually directly connected to the river and therefore its level is closely related to river levels. Alluvial aquifers can be recharged directly from the river under high-flow conditions. Under low-flow conditions, alluvial aquifers can provide base flow in the river channel Anabranch A stream that leaves the main stream and re-joins it further downstream. A feature of anastomosing streams Anoxic Containing low levels of oxygen Anthropogenic Borne of the actions of humankind Aquifer An underground layer of soil, rock or gravel able to hold and transmit water. Bores, spear-points and wells are used to obtain water from aquifers Armoured bed A bed that has had the finer material removed to expose a stable, tightly packed, bed of usually cobble- sized material that is highly resistant to degradation Assemblage The collection of plants and/or animals that are found in a community Avulsion Sudden change of course of a river, such as formation of an anabranch or meander cut-off Autochthonous Material that has originated from a place close to where it is deposited Backwater A body of stagnant water connected to a river, or water held back by an obstruction such as a dam or weir

Fluvial Systems Pty Ltd 85 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Bank slumping The falling or slumping of a riverbank into the river. May occur due to removal of riparian vegetation, erosion or bank destabilisation. The term is often used in connection with slumping resulting from a rapid decrease in river height, in which water drains more quickly from the river than it does from the banks, which then collapse under their own weight Bankfull The carrying capacity of the channel without spilling onto the surrounding floodplain (see channel capacity) Bar Well formed ridge or deposit of sand or gravel in a stream bed Barrage A construction across the mouth of a river that prevents the entry of seawater; freshwater lies upstream of a barrage Baseflow The sustained flow in a river, not the direct result of runoff from a rainfall event Bedload The solid mineral material transported on the bed of a river, usually during flood events, which may come from catchment slopes or channel banks Bench Small floodplain-like feature in-set within the bankfull channel of a stream or river. Often formed as a persistent depositional feature associated with a particular sub-bankfull channel forming flow, but can also be formed during a single event. Can also be an erosional feature formed from overcutting of the bank above the bed level (akin to the formation of a marine shore platform) Benthic living on or near the bottom of a body of water Billabong A backwater channel, often formed by a cut-off river bend, that forms a lagoon or pool when river levels fall Bioaccumulate The process by which chemical substances are taken up by living things and retained and concentrated as they move up through the food chain Biodiversity (biological The variety of all life forms, comprising genetic diversity) diversity (within species), species diversity and ecosystem diversity Biofilm A film of bacterial slime or matter that accumulates on solid grains of a porous medium Biomass The weight of living material in a unit volume or area at a given time Biota All living things, including micro-organisms, plants and animals Blue-green algae Naturally occurring, microscopic, primitive (cyanobacteria) photosynthetic bacteria. Under certain conditions (including high nutrients, warm still water, strong sunlight into the water) they can bloom into a dense and visible growth and may become toxic

Fluvial Systems Pty Ltd 86 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Building Block Method An approach to determining environmental flows, similar to the Holistic Method, but developed in South Africa. Assumes that the hydrograph can be characterised, and that experts can identify various components, or facets, of the hydrograph that are more important than others for maintaining normal river functioning. The environmental flow regime is built up from these “blocks” or components Bottom-up approach An approach (usually part of the Holistic Method) to determining an environmental flow whereby the regime is built up from hydrological facets (usually a pattern of baseflows combined with certain floods) that the panel assumes from experience or research are the minimum combination required to achieve ecological sustainability (see Building Block Method) Braided stream Characteristic of streams with high gradient, high sediment supply, coarse sediment and high stream power. Not common in Australia. A multi-thread low flow channel can form on the flat sandy bed of some trapezoidal channels at low flow, but this is not a braided river (which is typically very wide and shallow) Cap A limit on the amount of water that may be diverted from the river for human uses, e.g. the Murray- Darling Basin Ministerial Council announced a cap on water use in the Murray-Darling Basin in 1995 Capacity sharing A model of water allocation. The volume of water in a reservoir is shared between water users on a percentage basis. The water users' volumetric allocation is under their control, not the water authority. The water user can sell part of all of their percentage, or order the authority to send it downstream at a certain time. Water entitlements become more of a 'property right' in this model. Water entitlements can be traded easily. Catchment The area of land drained by a river and its tributaries Cease-to-flow When the water in a river stops flowing (i.e. mean velocity is zero) due to lack of rainfall or excessive diversions Channel capacity The volume of water that can pass along the river channel at a certain point without spilling over the tops of the banks Channel maintenance Normally understood to mean a medium-sized flood flow event that redistributes bed material, thereby reshaping and maintaining in-channel geomorphic and habitat features (will remove in-channel vegetation if applied regularly) Chemocline The depth in a waterbody where the concentration of dissolved oxygen reduces to zero Confluence The place where two or more streams flow together

Fluvial Systems Pty Ltd 87 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Contingency allowance A volume of water reserved in a supply dam for release if and when needed for ecological and/or water quality reasons. For example, a release may be required to maintain water levels in a wetland to enable waterbirds to complete breeding, or to flush away an algal bloom Controlled streams Streams where flow is usually controlled by upstream dams or diversion works, resulting in major changes to the natural flow patterns. These include regulated streams as defined in the NSW Water Act 1912, where water is released from storage to meet downstream irrigation needs Cover A characteristic of habitat that involves protection, perhaps to enable hiding from predation, to concealment from potential prey, or protection from the effects of radiation (e.g. trees provide cover for streams and reduce maximum water temperatures) Cross-section A survey of the change in elevation along a transect across a river Crustaceans Invertebrate animals that have segmented legs and hard shells, e.g. crabs, yabbies, prawns Cyanobacteria Photosynthetic eubacteria that have chlorophyll and produce oxygen as a byproduct of photosynthesis Deoxygenated With most or all oxygen removed. Water becomes deoxygenated (i.e. loses its dissolved oxygen) for a number of reasons including stagnation, eutrophication and rising temperatures Desiccation To dry out; may be an adaptation to enable survival through dry periods Desk-Top Method Simple method of determining environmental flow regime based on hydrological records only (see Rule of Thumb Method) De-snagging The removal of fallen trees and dead branches from a watercourse Diatom A type of very small algae with siliceous cell walls that can be used as an indicator of water quality Dissolved oxygen Oxygen in the water (which may be used by aquatic animals) Diversion transferal of water from a river (see abstraction) Diversity The distribution and abundance of different plant and animal communities within an area Drawdown Volumes of water released or extracted from a pool or dam, thereby lowering the water level Drought A natural period of protracted low flow (flow does not have to cease) characterised by failure of seasonal rains (small freshes may still occur) Dryland salinity See Salinity Ecologically sustainable As defined in the Protection of the Environment development Administration Act (NSW) 1991, summarised as having the element of: the precautionary principle; inter-generational equity; conservation of biodiversity and ecological processes; the improved valuation and pricing of environmental resources

Fluvial Systems Pty Ltd 88 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Ecology The study of the interrelation between living organisms and their environment Ecosystem Any system in which living organisms and their immediate physical, chemical and biological environment are interactive and interdependent. Examples are river reach, forest and wetland Effective discharge The discharge range that is responsible for transporting the bulk of the sediment in a stream (a medium-sized flow in humid streams). Some studies show that stream morphology is adjusted to this discharge range. Effluent creek A creek that leaves a watercourse and does not return to it (the opposite of a tributary) (not related to pollution) Environment Components of the earth, including land, air and water, any layer of the atmosphere, any organic or inorganic matter and any living organism, human- made or modified structures and areas, and includes interacting natural ecosystems that include components referred to above Environmental flows Flows, or characteristics of the flow pattern, that are either protected or created for supply the needs of the environment. The concept of an environmental flow includes (but is not limited to): volume of water over some time base; velocity of water in channel; duration of flow event; water level; natural and human induced variation flows on an annual and longer time scale; need for pulses of high flows (e.g. to stimulate fish breeding); and the rate of change of flow. The timing, volume and quality of environmental flows are all critical aspects and, like the natural flow of rivers, different combinations will provide a different range of benefits for each ecosystem. Environmental flows ensure that the key chemical, geomorphological, and ecological processes necessary for healthy river ecosystem keep functioning. Environmental value A particular value or use of the environment that is conducive to public welfare, safety or health, and which requires protection Ephemeral stream A stream that flows for only short periods and then dries up Epilimnion The surface layer of a lake formed as a result of stratification (warming of surface layers in summer, but may be reversed in winter) Erosional notch Step in river bank profile corresponding to water surface level of: a single flow event; a long period of flow at constant height, or; concentrated wave or boat wash erosion Estuary The part of a river in which water levels are affected by sea tides, and where fresh water and salt water mix Euphotic depth/zone The region of a body of surface water extending from the surface down to the deepest level at which there is sufficient light for photosynthesis to occur

Fluvial Systems Pty Ltd 89 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Eutrophication Excessive levels of aquatic plant growth (including algae) resulting from raised levels of nutrients and other factors Exceedance (of flow) The percentage of time (in a flow record) that a certain flow is exceeded (or not exceeded) Exotic An organism or species that is not native to the region in which it s found Expert Panel Method An approach to determining environmental flows that involves using the collective expert opinion of an assembled group of qualified people (not necessarily scientists) Fish ladder or fishway A structure designed to enable fish to move over a physical barrier (dam or weir) in a waterway Flood channel A natural channel in a floodplain, which carries flowing water only during a flood Floodplain Flat land beside a river that is inundated when the river overflows its banks during a flood Floodrunner Shallow channels that run with water only during floods and very high flows Flood Flows that are high enough at their peak to overrun river banks or cause flow through high-level anabranches, floodrunners or to wetlands Flood series analysis Techniques for statistically analysing the frequency of flood events Flow regime The pattern of flow in a river which can be described in terms of quantity, frequency, duration and seasonal nature of water flows Flow, natural A flow regime that is unaffected by any form of flow modification (see Natural flow regime) Flow, regulated A flow regime that is subjected to some form of artificial control by dams, weirs, diversions or other structures or practices, but some technical definitions of regulated streams only include those affected by dams, weirs or diversions (see Unregulated river) Flushing flow A fresh that is of sufficient magnitude to remove built-up surface material from the channel bed (e.g. cleans gravels) Fluvial geomorphology The study of the processes that shape the earth's surface by flowing water. Usually associated with the study of materials processes in rivers, but closely connected with other river sciences Freshes Flows that produce a substantial rise in river height for a short period, but which do not overrun the river banks or inundate areas of land General Security Water Where the user's yearly allocation varies according to Supply the amount of water available, after allowing for 'fixed commitments' which are: expected losses during the year, expected riparian usage, environmental provisions, high security supply commitments, and water held in reserve for subsequent year's supply. General security entitlements account for the vast majority of entitlements in regulated streams.

Fluvial Systems Pty Ltd 90 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Geomorphology The study of origin, structure, development, and processes associated with the earth's landforms or surfaces (see fluvial geomorphology) Groundwater Underground water filling the voids in rocks; water in the zone of saturation in the earth's crust Habitat The type of environment in which a given animal or plant lives and grows, including physical, chemical and biological conditions Headwaters The small streams on the higher ground of a catchment, which flow into a river High flows Higher than normal flows, which occupy much of the river channel or which overrun banks. High Security Water The entitlement holder's share of supply is a fixed Supply yearly volume made available in all but the most extreme drought years. This is typically for town water supply and irrigating permanently established crops (orchards, vines etc.) High-security water use Licensed entitlement to a more secure water supply than under normal-security licences, e.g. for horticulture and town water supplies Holistic Method An approach to determining environmental flows grounded in the philosophy that all aspects of the flow are potentially important to the proper functioning of a river Hydraulic Refers to the local dynamic aspects of flow, usually measured in terms of shear stress, water depth, flow velocity etc. Hydraulic habitat Habitat defined in terms of hydraulic variables that are related to the tolerance or preference of organisms, usually depth and velocity, but also shear stress Hydraulic Habitat A method of determining environmental flows that Method combines measured and/or modelled hydraulic data with knowledge of the hydraulic habitat preferences of the biota (usually key fish species) to provide desired habitat conditions throughout the year (see PHABSIM, IFIM) Hydraulic model A numerical engineering model that simulates the hydraulics of a river and predicts water surface profile based on cross-sections, a long-profile and a roughness coefficient (simple model), or characterises three dimensional flow complexities (sophisticated model). HEC-II, HEC-RAS and MIKE-11 are commonly used models Hydrograph shape Describes the flow pattern of water in a stream, for example, after a storm event or runoff from prolonged rain Hydrological System Includes streams, wetlands, billabongs, floodplains, swamps, ground water recharge areas, ephemeral lakes, estuaries and the sea Hydrology The study of the distribution and movement of water Hypolimnion The lower layer of water in a lake, can be low in oxygen, high in nutrients, and cold if stratified

Fluvial Systems Pty Ltd 91 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Hypolimnetic water Water from the hypolimnion. When released to rivers through low-level outlets of large dams it is usually colder than natural Hyporheic zone The zone around a channel margin in which stream and groundwaters mix. Hyporheic activity can account for a high percentage of stream respiration as oxygen demand. May also be a hotspot of biological activity and play a significant role in stream nitrogen dynamics Hypothesis A proposed answer to a study question, based on best available information, formulated for the purposes of scientific testing. It must be possible to test a hypothesis, and it must be falsifiable. Statistics is concerned with the proper methods of testing hypotheses IFIM In-stream Flow Incremental Methodology. A formalised method of determining environmental flows using hydraulic habitat analysis Incision (of a channel) Also known as bed degradation, it involves the progressive reduction of the bed elevation (channel deepening) Indicator (of water Any physical, chemical or biological characteristic quality, geomorphology, used as a measure of environmental quality ecology etc.) Intermittent stream A stream that flows irregularly Introduced species Species of plants or animals that are not native to Australia (also referred to as exotic or alien species) Inundation To cover with water, usually by the process of flooding Invertebrates Animals without backbones, including worms, insects, shrimps, crabs, snails, shellfish and zooplankton. Macroinvertebrates are large enough to be seen without the aid of magnification (> 1 mm); microinvertebrates need to be viewed through a microscope Irrigation salinity See Salinity Large woody debris Large items of tree trunks, branches or limbs that fall into rivers, usually defined as being larger than 0.1 m diameter and 1 m long (see snags) Lentic Of standing water Levee A natural or constructed embankment close to the banktop that helps to confine river flows Lignum A tall, almost leafless shrub, common on low-lying ground in the interior of Australia Long profile Survey of the pattern of elevation change along the length of a river, usually measured at the thalweg Lotic Of flowing water Low flows Flows that occupy only a small portion of the river channel. Low flows would normally occur when there is little contribution to the river from rainfall events. Macroinvertebrate See invertebrate Macrophyte A plant large enough to be seen without the aid of a microscope

Fluvial Systems Pty Ltd 92 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Manning’s n A parameter used to characterise surface roughness in rivers or floodplains for hydraulic calculations Meander A curve in the course of a lowland river, always present in a series Median The median is the middle value in a data set ranked from lowest to highest Megalitre (ML) One million litres (one Olympic swimming pool is approximately 2 ML). For discharge, 1 m3/s is equivalent to 86.4 ML/d Microbial Pertaining to micro-organisms, often a germ, bacteria or pathogen Mimic natural flow A philosophy that embraces the Natural Flow variability Paradigm, which encourages maintenance of flow variability in a regulated regime. In fact, it is not possible to replicate natural variability across all scales in a regulated river Multi-level offtake An offtake structure within a dam, which can take water from various depths, rather than just one. For instance, if the offtake is only at the bottom of the dam, releases of water may be cold, deoxygenated and nutrient-rich. A multi-level offtake allows releases to be made from upper layers where water quality is often closer to the natural of character inflowing water Natural Flow Paradigm Philosophy which states that discharge variability is central to sustaining and conserving biodiversity and ecological integrity, and it should be preserved in regulated regimes Natural flow regime The likely pattern of flow before European settlement in Australia. Natural flow regime refers to the flow patterns without any regulation or extraction of water. Can be modelled on the basis of known modifications (see natural flow) Natural flow Modelled natural flow, i.e. the flow that would occur if regulation was absent NTU Nephelometric turbidity unit (a unit of measurement for turbidity) Nutrients Nutritional substances (nitrogen and phosphorous are most commonly referred to, but they have various chemical forms). Unnaturally high levels of nutrients can encourage abnormally fast and prolific growth of algae in the water Off-allocation flows Water that has not been released from storage, but comes from dam spills and/or inflows from tributaries below the dam. Licence holders are permitted to extract water from these flows but water so extracted is not debited against their allocation Off-allocation period When access to flows (dam spills or tributary inflows downstream of dams) is permitted by licence holders without debit against their allocated volume. Offtake structure A structure or point of diversion for water transfer. For instance, water is released from a dam via an offtake structure (see also multi-level offtake)

Fluvial Systems Pty Ltd 93 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

On-allocation Water ordered by a licence holder and which will be debited against their actual allocation Pathogen Disease-causing organism Percentile Usually refers to flow duration curves. The horizontal scale of the graph is divided from 0 to 100 percentiles (or per cent of time), while the vertical scale is flow rate (often in ML/day). For example, when looking at flow rates, the 90th percentile is the daily rate that is exceeded on 90% of days at a specific location. Perennial stream A stream that flows most of the time PHABSIM Physical habitat simulation. The hydraulic modelling component of the IFIM method for determining environmental flows Phytoplankton Plant plankton (usually means algae) Point bar Deposit of sand or gravel that develops on the inside of a meander loop Pool A body of water in a river that is relatively deep and slow moving Pool-Riffle morphology A type of three-dimensional streambed morphology characterised by fairly regularly spaced alternating pool and riffles. A common stream type, but not all streams have this morphology Precautionary principle The principle that the lack of scientific certainty should not be a reason to postpone preventative measures to avert threats of serious or irreversible environmental damage Pristine Usually refers to an ecological state unaffected by human disturbance Productivity The measure of the efficiency of energy output compared to input Propagules Parts of a plant (such as seeds, roots or stems) from which new plants can germinate Pulsing supply A strategy to reintroduce variability to releases of water from dams; introducing pulses of flow below dams allows a more natural flow pattern Rain Rejection Flow Water that is ordered but not used because rain subsequently falls. Currently, this water is not debited on the water-user's account. Ramsar Wetlands Wetlands of international importance listed under the Ramsar Convention. To be put on the register, a wetland has to fulfil certain criteria-such as being important to the survival of migratory birds or endangered plant and animal species Range of Variability Identifies annual river flow management targets based Approach (RVA) on a range of variation (e.g. ±1 standard deviation from the mean, or 25th to 75th percentile range) in each of 32 parameters. Environmental flow regime characteristics should lie within the targets for the same percentage of time as they did prior to regulation. Devised by Richter and others Raw water Surface or groundwater that has received no treatment to make it suitable for drinking Reach Relatively homogeneous section of river

Fluvial Systems Pty Ltd 94 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Recession The falling limb of a hydrograph, after the flood has peaked and discharge and water level decline back to baseflow levels Recharge Water that infiltrates through the soil surface to the watertable Recurrence interval The average time (average recurrence interval or ARI) between flood peaks of a given magnitude, e.g. the 1 in 10 year ARI event occurs on average once every ten years Refuge A site where organisms can survive harsh environmental conditions, temporarily (e.g. behind debris during floods) or for extended periods (e.g. in pools during droughts) Regime The prevailing pattern of flows Regulator A structure used to control the flow of water, for example, diverting water away from the main channel down an effluent creek Riffle A shallow area of the river in which water flows rapidly over stones or gravel Riparian release A flow released in a regulated river for riparian use (not often used in Australia) Riparian zone The area along the bank of a river or a stream, which often has water-dependent vegetation Rise and Fall (rate of) Refers to the rate at which water levels (or discharge) change during the onset of a flow event (rise) and the recession of the event (fall) River alluvium Material deposited by a flood Rule of Thumb Method Simple approaches to setting environmental flows using hydrological records that use simple rules based on flow duration or mean discharge to scale down the natural flow regime. Ideally the rules are based on empirical research into local flow-geomorphology- biota relationships. Also known as Desk-Top Method

Salinity The concentration of salts in soil or water, including sodium chloride (NaCl). Clearing deep-rooted vegetation on areas of saline watertables causes Dryland salinity. The uptake of water by plants is reduced, allowing the watertable with soluble salts to rise, killing plants and creating bare areas prone to erosion. Irrigation salinity occurs when irrigation raises the watertable, bringing high concentration of salt within root zones of plants, killing and stunting vegetation. It results from applying more water than can be used by the crop and by clearing of deep- rooted vegetation such as trees. Urban salinity is when rising watertables cause damage to infrastructure such as roads, underground pipes, houses and gardens.

Fluvial Systems Pty Ltd 95 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Scientific Panel Method An approach to determination of environmental flows that uses expert opinion, field inspection literature review and possibly a period of measurement and/or experimentation. The panel is composed only of scientists, engineers or modellers, but may have an associated steering committee of non-scientists. Sediment load The mass of sediment transported past a point in a river over a given time period. Usually expressed as tonne/day. Yield is the load per catchment area, expressed as tonne/day/km2 Shear stress Force per unit area acting parallel to the surface. An important measure used to estimate scour of sediment, and describe the hydraulic habitat of benthic dwelling organisms Siltation The process of deposition of material (can be clay, silt, or sand-sized material) on the bad of a river or lake Snag Large woody debris in rivers Species richness The number of species within a community Spell analysis A spell is a period of consecutive days where the discharge remains either entirely above or below a given threshold. Spell analysis characterises the timing, duration and frequency of these events. Stratification Distinct layers of water in a dam or weir pool, formed when there is little movement to cause intermixing- usually in summer when deeper layers of water become cold and deoxygenated. These changes may, in turn, induce other water quality changes Stream power Work done per unit time, usually expressed per unit of streambed area (W/m2). A useful index of the erosive capacity of a stream Stressed river System used in assessment of appropriate management strategies for water allocation and flow management in uncontrolled streams. A classification based on environmental and water-use criteria Substrate Usually refers to the material acting as the foundation for attachment of organisms in a stream Surface water Water on the surface of the land, for example in rivers, creeks, lakes and dams Suspended solids The smaller, lighter material such as clay, silt and fine sand carried in suspension in water Sustainable (As applied to water resource management.) Management that will meet current needs while conserving natural ecosystems so they can also meet future needs Target (water quality) A level of water quality to be achieved in a specified time frame as a step towards the desired long-term objectives. It is derived from comparing available water quality data/information with the water quality objectives, and considers social and economic factors Taxon (pl. taxa) A general term for a taxonomic group (e.g. a species) Thalweg The lowest point along a transect across a river

Fluvial Systems Pty Ltd 96 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Threatened species Threatened species is a generic term for a plant or animal generally considered as vulnerable or endangered under various threatened species conservation laws. It is used to indicate that there is some level of threat as to the species viability in the wild. Technically, threatened species are listed according a special conservation act Top-Down approach An approach (usually used in association with the Holistic Method) to determining an environmental flow whereby the panel assumes that the entire natural flow regime is ecologically important, but that some facets of it can be modified or omitted without threatening ecological sustainability Top-release (of water) Water released from the top layers of a dam Translucent Dam Flows released downstream of a dam are a proportion (percentile, or some other factor) of the incoming flow (may apply to medium-flow component of an environmental flow regime for example) Transparent Dam Flows are released downstream of a dam as if the dam did not exist (may apply to low-flow component of an environmental flow regime for example) Tract A stretch of land or water with some definable regional character Tributary A river or creek that flows into a larger river Turbidity A measure of the light-scattering properties of water. It indicates how much silt, algae and other material is suspended in water. Highly turbid waters look muddy Uncontrolled streams Streams that are largely free of structures that control flow, such as major dams (unregulated stream) Unregulated River In NSW, rivers whose flows are not controlled by releases for storages and weirs. NB: Some streams below water storages, urban water supply, or other specific purpose dams are 'unregulated' in terms of the Water Act 1912, because the rivers are not gazetted and the stored water is not released for the purpose of supplying licensed water users downstream. Around 12,000 unregulated flow licences have been issued in NSW. No metering of water use occurs Variability The likelihood of variation or change, which can be measured by numerous indices over many different time scales. High variability of river flows means that stream height at any one place can change substantially over time. Variability is determined by catchment size, number of tributaries, slope and climate. On a global scale, Australia has variable river flows. River management for consumptive uses has decreased variability Water quality objective Numerical concentration limits or requirements established to support and protect the designated environmental values of water at a specified site. They are the locally established benchmarks for water quality derived from the Australian Water Quality Guidelines for Fresh and Marine Waters (ANZECC 1992)

Fluvial Systems Pty Ltd 97 Environmental Impacts of Flow Regulation on the River Murray Murray-Darling Basin Commission

Watertable The upper surface of a groundwater body Weir A construction across a river that dams the water, but which may be removable during a flood Weir pool The water held back by a weir, forming a still pool. Where the land is very flat, such as in the lower River Murray, a weir can cause very long pools to form. Wetland The Ramsar Convention has adopted the following definition of wetlands: "areas of marsh, fen. peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water at the depth of water at low tide does not exceed six metres. " In addition, Ramsar provides that wetlands: "may incorporate riparian zone and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six metres at low tide lying within the wetlands." Wetted perimeter The part of the river that is wet, usually measured as the length of wet channel across a transect Wetted perimeter A method of determining environmental flows based method on the non-linear relationship between wetted perimeter and discharge. Rapid loss of wetted perimeter (surrogate for habitat) can occur below a threshold discharge (this becomes the critical minimum discharge) Zone Any continuous tract or area, which can be characterises as different in some respect to adjacent areas Zooplankton Animal plankton

Fluvial Systems Pty Ltd 98