The Living Murray
Foundation Report on the significant ecological assets targeted in the First Step Decision

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7 Information base for the River Murray Channel

7.1 Introduction

The River Murray Channel SEA connects the riverine ecosystem and many floodplain and wetland features including the other five Significant Ecological Assets (the Barmah-Millewa Forest; the Gunbower and Koondook-Perricoota forests; the Hattah Lakes; the Chowilla Floodplain and Lindsay-Wallpolla Islands system; and the Murray Mouth, Coorong and Lower Lakes). Several threatened species and ecological communities rely on the River Murray Channel.

The River Murray Channel is over 2,000 km in length. It is the bed and banks of the river, the water within it, and the surrounding dependent riverine ecosystem. It connects headwaters, lowlands, the estuary and the ocean, delivering the water, sediment and nutrients required to maintain the integrity of these areas (Young, 2001). With respect to natural resources management, it is unhelpful to consider the Channel in isolation from its floodplain, wetland and estuarine systems, because the integrity of these systems depends on vital connections and exchanges of water, nutrients, organic material and organisms with the river Channel.

The First Step Decision Interim Ecological Objectives for the River Murray Channel are to: increase the frequency of higher flows in spring that are ecologically significant; overcome barriers to migration of native fish species between the sea and Hume Dam; and maintain current levels of channel stability (Table 1.2). The expected outcomes are: expanded ranges of many species of migratory fishes; and, similar levels of channel erosion to those currently (Table 1.2). These outcomes will be achieved through various management opportunities, described later in this chapter. The following sections describe the characteristics of the Channel, exploring the links between the biophysical condition of the Channel and hydrological and other factors.

7.2 Value and condition of the River Murray Channel

7.2.1 Physical description of the River Murray Channel

Geomorphic regions of the River Murray

In its 2,225 km course from the Hume Dam to the Southern Ocean, the River Murray traverses five distinct geomorphic regions (Eastburn, 1990):

The headwaters: from the source of the Murray above Hume Dam to Corowa, a distance of about 660 river kilometres. The headwaters in totality comprise less than 2% of the Murray-Darling Basin, but contribute nearly 40% of the inflow to the River.

The Riverine plains: a vast, flat tract of river and lake deposits where the River flows in shallow, branching Channels, from Corowa, 800 river kilometres downstream, to the Wakool River junction, just west of Swan Hill.

The Mallee Trench: a wide plain of marine origin crossed by the River Murray in a single, well-defined Channel which cuts deeper into the surrounding plain, as it moves downstream. The Mallee Trench extends from the Wakool junction for 850 river kilometres to Overland Corner, in South Australia.

The Mallee Gorge: the River Murray Channel has cut down through hard limestone rock. The river bed intersects the regional watertable, and salty groundwater enters the River through aquifers exposed in the cliff face. The Mallee Gorge covers a river distance of about 280 km, from Overland Corner to Mannum.

The Lower Lakes and Coorong: the terminal lakes, Lakes Alexandrina and Albert, together with the Coorong once formed a huge estuarine system. Barrages now separate the lakes from the Coorong and retain fresh water in the lakes. The distance from Wellington to the Mouth is 73 km.

Figure 7.1 is indicative of the longitudinal profile of the river channel, including the distance from Hume Dam to the Goolwa Barrages. It also illustrates the change in elevation across the course of the river, which influences the stream power, and hence the geomorphic processes in each river zone.

Within the context of these geomorphic regions, geomorphic processes of sediment erosion, transport and deposition are important to river health because the shape of the Channel largely determines the hydraulic conditions and habitat. `Hydraulic condition' refers to the patterns of depth and velocity (to which organisms are highly sensitive), which are an important determinant of sediment transport processes (which in turn are related to the ecologically important processes of bed and bench flushing, and sediment smothering).

Geomorphologic processes are important from a socio-economic perspective, because there are implications for streamside asset holders (e.g., road authorities, farmers) when the river boundary is altered by bank erosion, or through course changes (meander cutoff or anabranch capture), or if changes in the bed level alter the pattern of flooding, limiting navigability.

The most geomorphologically active zones of the River Murray Channel are from Hume Reservoir to Yarrawonga Weir, and from Yarrawonga Weir to the Wakool junction (i.e., in the headwaters and riverine plains). This is explained by the greater concentration of flows in the region of channel capacity (i.e., the flows that are most effective for doing geomorphic work to shape the channel) under the regulated flow regimes in these zones (Maheshwari et al., 1995). The lower River Murray is also highly regulated, but the effect there is to decrease the duration of channel capacity flows, and increase the duration of low flows that are not very effective in modifying the channel shape or transporting sediment (Gippel & Blackham, 2002).

Habitats within the River Murray Channel

Riverine plants and animals need suitable habitat for survival. Habitat features within the River Murray Channel are partially related to geomorphology and occur at a range of spatial scales, from small backwaters near snags, to benches and sand bars, to deep pools at a river bend. This provides a wide range of habitat-types for in-stream biota including algae, macroinvertebrates, vegetation, yabbies and fish (Figure 7.2).

Habitats within the River Murray Channel are also influenced by the flow regime, which in turn is affected by flow management structures and how they are operated. An overview of flow management structures is provided in the following section, followed by a discussion of river operations.

7.2.2 Flow management structures along the River Murray Channel

There are four headworks storages on the River Murray system. Operation of headworks on key tributaries (the Goulburn and Murrumbidgee rivers) and in the Snowy Mountains Scheme also affects flows in the River Murray Channel. There are 14 weirs along the River Murray. Torrumbarry Weir is shown in Figure 7.3. There is also a tidal barrage at the downstream end. The locations of the largest flow management structures are shown in Figure 7.4.

The main functions of the largest flow management structures are indicated in Figure 7.4 and are listed in Table 7.2.

The lower River Murray has ten locks and weirs extending from Wentworth (Lock 10) to Blanchetown (Lock 1) (Table 7.2), built originally (1922-1937) to promote year-round riverboat transport, but now used mainly to preserve stable levels for irrigation (Walker, 2001).

The weirs have little effect on through-flow but exert a major influence on water-level variability within the river channel. In general, the effect of weir operations is to maintain a steady upstream pool level except when flows exceed storage capacity. During high flows the panels and `stop logs' are removed at a certain flow level, and then reinstated at another given level during the flood recession (Table 7.3). At other times the water level is maintained near the target `pool level'. The degree of control increases downstream towards Lock 1, as successive weirs dampen flow variations (Walker, 2001). The ecological implications of the hydrological changes associated with locks and weirs are discussed in later sections of this chapter.

Over the last century, diversion of water from the River Murray Channel has increased, chiefly for agriculture. Today, these diversions, 95% of which are used for irrigation, account for about half of the Basin's annual runoff. Figure 7.5 shows how the government storage capacity and diversions have increased in the Basin since the 1920s. This storage capacity provides the ability to influence the flow regime. In addition, private storage capacity (particularly farm dams) has increased in recent decades. Increased storage provides greater opportunity to modify flow patterns relative to natural conditions, to which the biota has evolved.

7.2.3 River operations

Main operating modes

As a simplification, flows in the River Murray Channel can be classified into three operating `modes', which are:

• supplying mode-when some or all of headwork storages (Dartmouth and Hume reservoirs, Lake Victoria or the Menindee Lakes) are drawn down to meet downstream requirements;

• storing mode-when the large headwork storages are filling and the flows downstream of these storages are confined within the Channel but meet or are in excess of that required to meet downstream requirements; and

• spilling mode-when flow exceeds Channel capacity at a point, typically when at least one of the headwork storages is spilling.

The time of year when each operating mode tends to be implemented is illustrated in Figure 7.6.

This classification into operating modes is a simplification because it is possible at one point of time for one reach of the system to be operated in one mode, while another reach is simultaneously run in a different mode. For example, a major flood in the Darling River in summer may lead to spilling mode being employed for the River Murray Channel downstream of Wentworth, while simultaneously the River Murray upstream of Wentworth may be operated in supplying mode. Nevertheless, this classification into the three operating modes provides a useful framework for understanding current river operations, and, in the future, environmental flow procedures could be tied to a mode of operation on a river reach-by-reach basis, and coordinated between reaches. It could be used in the exploration of environmental flows opportunities into the future.

Operational complexity and flexibility

There are a number of operational issues that increase the complexity of meeting flow targets (eg diversions, minimum flows, or environmental targets) along the River Murray Channel (including those associated with the use of environmental water allocations) during each of the three operating modes. These stem from it being often difficult to predict:

• the location and quantity of rainfall, and subsequent runoff, as Australia has one of the most variable climates in the world (Grayson et al., 1997);

• the magnitude and timing of tributary inflows;

• the timing and quantity of releases from the Snowy Mountains Scheme;

• variations to river transmission `losses' (particularly between Euston and Wentworth, where temperatures can be regularly above 35ºC in summer resulting in high evaporation rates, and the pumped diversions vary in time); and

• the quantity of flows returned from the floodplain or wetlands during and following floods.

Further operational complexities include that:

• the River Murray Channel capacity varies along its length;

• travel time of flow at gauging stations is dependent on flow rate; and

• rating relationships (ie relationships between water level and flow) can `drift' through time, particularly when the gauging cross-section changes through time due to, for example, increase or reduction of coarse woody debris or changes to in-stream sandbars.

Operating procedures to protect specific environmental values

There are many rules for the management of flows along the River Murray Channel that have evolved over the decades to protect specific environmental values of the River Murray system. The formalised operating procedures are listed in Table 7.4. Other practices have been implemented by the operators to protect specific environmental values (Table 7.5). The operating mode during which each is most likely to be implemented is also provided in Tables 7.4 and 7.5. There are also operating rules for the Mitta Mitta River, Edward River and the lower Darling River that are outside the scope of this Foundation Report.

7.2.4 Change to hydrology due to river operations

Introduction

The operation of flow management infrastructure has altered the flow regime over annual, seasonal and daily timeframes, the extent of the impact depending on the location in the river system. Additionally, there have also been changes to the proportion of the flow that is delivered within the River Murray Channel as opposed to via the floodplain.

Annual impacts

Figure 7.7 shows the change in the median annual flow from natural to current conditions and the effect of regulation in the tributaries as they join the River Murray.

Key points to note are that the flow in the Ovens and Kiewa rivers, in north-east Victoria, have been largely unaffected by development, whereas the flow at Albury is 12% higher due to the extra water diverted into the Murray-Darling Basin from the Snowy Mountains hydroelectric scheme. Further downstream, below major diversion points like Yarrawonga Weir, the flow in the river is a lot less than it was naturally. Also, some of the tributaries are delivering far less water to the River Murray than they would have if they were not regulated. The median annual flow to the sea is currently about 27% of that under current conditions.

Seasonal impacts

The seasonal flows within the River Murray Channel have been significantly changed by river regulation. Figure 7.8 illustrates how downstream of Yarrawonga Weir, much of the natural high flows in spring are usually stored in the headworks during the storing mode, and the natural low flows in summer have been replaced by bankfull flows to meet irrigation demands. This is also true downstream of Hume Dam and Yarrawonga Weir. There, the river now runs at near channel capacity (750 GL/month) from January to March, during what was previously the lowest flow period. Figure 7.8 illustrates the significant reduction in seasonal flow variability. Additionally, a greater proportion of the flow is now contained within the river channel. This places increasing pressure on bank stability and reduces connectivity between the river channel and the floodplain.

Figure 7.9 illustrates that the main impact of regulation in other parts of the River Murray Channel, particularly in South Australia, is the reduction in overall flow volume. As illustrated in Figure 7.9, there are eight months (November - June) when the median monthly flow is less than the minimum median monthly flow in any month under natural conditions. The seasonality is similar, although considerably truncated under current operating conditions.

Impacts on day-to-day flow variability

Before river regulation there was a high degree of variability in the flows and/or water level in the River Murray Channel from day to day. Regulation of the river has reduced this variability in different sections of the river. Some of the key impacts are:

• the provision of regulated releases from Hume Dam to meet irrigation demands and South Australia's entitlement may result in flows being maintained at or near channel capacity (25,000 ML/d) for extended periods in some seasons (Figure 7.10); and

• the presence of the locks and weirs in the mid and lower section of the river results in stable water levels for long periods of time, despite variations in the passing flows (Figure 7.11).

As a consequence of flow management, the changes to the flow regime have been considerable at a number of temporal scales, including annual, seasonal and daily. The threats to the environmental values of the River Murray Channel associated with these changes to flow regime, and other factors, are outlined in the following section.

7.2.5 Decline in ecological health

An overview of river health

A recent snapshot Assessment of River Condition in the Murray-Darling Basin evaluated the aggregate impacts of resource use on the condition of rivers in the Murray-Darling Basin (Norris et al., 2001). The results indicated that overall biological and environmental condition was degraded right along the river, with most degradation towards the Mouth. The following summary of condition is from Norris et al. (2001). The results for sections of the River Murray System are shown in Figures 7.12 to 7.15.

Environmental assessment

• Fish populations are in very poor to extremely poor condition throughout the River Murray.

• Macroinvertebrate communities are generally in poor condition and declining towards the river Mouth.

• Riparian vegetation condition along the entire river was assessed as poor with grazing and alterations to the flow regime the major causes.

• Wetland quality is significantly reduced. Most wetland loss is attributed to permanent inundation of areas previously intermittently flooded.

• Hydrological condition in the River Murray Channel is poor for all zones, with the extent, timing and duration of floodplain inundation all significantly impacted.

• Riverine habitat was found to be poor or very poor through all zones with connectivity, riparian vegetation and bed-load all affected by regulation.

• The condition of floodplain inundation has been assessed as very poor in all zones.

• Nutrients and suspended sediments are poor and worsening towards the Mouth.

Salinity concentrations in the Lower Murray are improving as a result of water diversion for irrigation and salt interception schemes. However, this trend is predicted to reverse with increasing salinity over the next 50 to 100 years because of land uses upstream and rises in groundwater levels in the Lower Murray.

Multiple impacts

The degraded condition of the River Murray and the Lower Darling appear to be the consequence of multiple impacts, with the main impacts related to the operation of dams and weirs throughout the system.

Throughout the River Murray and lower Darling River unseasonal flooding of wetlands, loss of connection to the floodplain, habitat simplification, water quality and bank erosion are all significant issues.

 

Some specific examples supporting the decline in native fish numbers

Of the 39 native fish species in the Basin, 16 are listed as threatened under State and Commonwealth legislation. A number of excerpts from the scientific literature supporting that native fish species are in decline in the River Murray System are given in Box 7.1. A number of references are specifically for Silver perch (Figure 7.16). Silver perch were once abundant, but a massive drop in numbers since the early 1900s has been reported. Consequently, Silver perch are on the critically endangered list under Victorian legislation, endangered in the ACT, and vulnerable in NSW.

These reductions in native fish numbers are a response to many changes, including changes to flows. By comparing Figures 7.12 to 7.15, it can be seen that the distribution of fish populations varies along the River Murray Channel. However, on occasion, fishing for native species can be relatively good locally. Recent reports of relatively good Murray cod catches may be due to a combination of successful recruitment of the natural population in the 1990s, cod accumulating in local areas with good habitat, and stocking. Previous chapters of this Foundation Report have indicated healthy populations of native fish at some of the Significant Ecological Assets, especially Lindsay-Wallpolla and Barmah-Millewa Forest. However, while the Murray cod population is variable along the river and through time, there was sufficient evidence of decline that in 2003 the species was listed as `vulnerable' under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999.

7.2.6 Water quality

Salinity

Salinity emerged as a serious issue in the River Murray in the 1960s. In response, governments have invested in dilution flows, in building and operating salt interception schemes, and assisted in the preparation of salinity action plans and land and water management plans.

One of the clear successes of the Murray-Darling Basin Salinity and Drainage Strategy 1988-2001 has been the coordinated efforts of community groups and governments to control and reduce salinity levels in the lower parts of the River Murray, and this success has been widely recognised in recent years (MDBC 1999, MDBMC 1999, 2001). The major salt interception schemes constructed in the Murray-Darling Basin since 1980 and their estimated effect in reducing average river salinity are shown in Table 7.6. The improvement in long-term average salinity levels in the River Murray at Morgan since 1980 is shown in Figure 7.17.

Despite the apparent improvement in average salinity at Morgan, ongoing management is important. Norris et al. (2001) concluded that

Blue-green algae

The large blue-green algae bloom that stretched for over 1,000 km in the Lower Darling River in 1991 (Figure 7.18) caused considerable concern from both human and ecological perspectives. The occurrence of blue green algal blooms depends on a number of factors, including flow. SRP (2003) stated that, as a long term average, there currently are potential blue green algal problems at Mildura Weir and Lock 5 in one year in three, and at locks 1 and 2 in one year in eight.

Turbidity

The turbidity of the Darling River affects water quality in the lower reaches of the River Murray. The Darling River carries a high suspended sediment load, and in comparison to the Murray, its waters are highly turbid (Figure 7.19). Under natural conditions, the Darling River has a bimodal flood hydrograph, with peak discharges occurring in March and September. Under current conditions, higher flows typically occur during the summer months.

Operation of Menindee Lakes has made possible an extended period during which Darling River water can be supplied to the lower River Murray. Naturally, the Darling River would have provided high turbidity water during early spring. However, releases from Menindee Lakes contribute to periods of high turbidity that now extend through to summer and autumn. The unnaturally high proportion of Darling River discharge entering the lower River Murray during summer, contributes to unnaturally high summer turbidity, compared to pre-regulation times. Figure 7.20 shows the elevated turbidity in the Lower Darling River at Burtundy (downstream of Menindee Lakes) and in the River Murray downstream of the junction of the River Murray and the Darling River, during spring and summer. Figure 7.20 shows the difference in turbidity at the junction of the River Murray and the Darling River.

This change in turbidity has ecological consequences. Turbidity reduces light penetration, which can reduce in-stream primary productivity and change food-web dynamics by limiting the growth of submerged plants, restricting the growth of benthic algae biofilms to a narrow subsurface range and reducing the abundance of macroinvertebrates (Boulton & Brock, 1999).

7.2.7 Evidence of geomorphic change

Introduction

The most geomorphologically active reaches of the River Murray Channel are from Hume Dam to Yarrawonga Weir, and from Yarrawonga to the Wakool junction. This is explained by the greater concentration of flows near channel capacity (ie the flows that are most effective for doing geomorphic work) under the regulated flow regimes in these zones (Maheshwari et al., 1995). The lower River Murray is also highly regulated, but the effect there is to decrease the duration of channel capacity flows, and increase the duration of low flows that are not very effective in modifying the channel shape or transporting sediment (Gippel & Blackham, 2002).

Hume Dam to Yarrawonga Weir

In the Hume Dam to Yarrawonga Weir zone, anabranch flows have been altered by river regulation (ID&A, 1993). Anabranch development involves the reduction in the capacity of the main stem, a process that is accelerated by floods. However, river regulation has increased the capacity of the main stem (through widening), and, combined with the reduction in flood frequency that accompanied river regulation, the process of river capture by anabranches has been retarded. Despite this, anabranch development is continuing (Gippel, 2001).

Channels migrate through erosion of the outside of meander bends. The rate of channel migration has increased markedly since settlement, but only for the `active' bends of the river, which make up only 14% of the bends (ID&A, 1993). It is uncertain whether river regulation increased the rate of channel migration, because the resistance of the banks has also been weakened by removal of the natural riparian vegetation (ID&A, 1993).

In the 15-year period between 1977 and 1992 the river between Hume and Yarrawonga widened by an average of 24 m (ID&A, 1993). Unlike bend migration, which was variable, widening was prevalent throughout the zone, occurring in straight reaches as well as on bends. An erosion notch has formed at the level of the long-duration high irrigation flows and a lower bank facet formed at a level associated with long-duration winter low flow releases (ID&A, 1993). Flow river regulation weakens the bank and increases the energy available for erosion. While river regulation is a key contributor to channel widening, clearing and grazing of bank vegetation, snag management, and waves generated by boats are other possible explanations (ID&A, 1993).

Close to Hume Dam, the river has deepened by up to 0.5 m, but this area is now armoured (ie the finer sediments have been flushed away and coarser sediments remain which are less resistant to erosion), so further deepening is unlikely. The deepening process may continue in the future further downstream for up to about 50 km from the dam (ID&A, 1993). Beyond this, the river bed has built up (aggraded), with shallowing of up to 0.6 m since 1977 (ID&A, 1993). However, the shallowing may have begun prior to river regulation, because the bed has shallowed by up to 2 m when surveys conducted in 1876 and 1981 are compared (Rutherfurd, 1990). It is possible that the regulated flow regime lacks the capacity to flush through the zone the sediment eroded from the banks and scoured from the bed close to the dam.

Yarrawonga Weir to the Wakool junction

While regulation has reduced the annual discharge in the River Murray downstream of Yarrawonga Weir by 25%, the discharge through the summer period (due to irrigation demands) has increased by 19% (Gippel & Lucas, 2002).

In this zone, the greatest and most consistent widening over the historical period occurred from Yarrawonga to Bullatale Creek offtake near Tocumwal, where the channel widened by an average of 36 m between 1876 and 1981 (0.34 m/yr), but then stabilised. From the Bullatale Creek offtake through to the Goulburn River junction the channel appears to have widened by about 3-15 m between 1876 and 2002 (0.02-0.12 m/yr). There are only a few cross-sections downstream of the Goulburn River junction, but they indicate widening, with a noticeable increase of 26 m after the 1981 survey. Between 1981 and 2002 the river widened by 1.1-1.2 m/yr from Bullatale Creek offtake to the Barmah Choke and from the Goulburn River junction to Echuca, but the other reaches were relatively stable. The Barmah Choke has been relatively stable since settlement, widening only by about 3 m since 1876 (Gippel & Lucas, 2002).

The cause of the channel widening appears to be related to the maintenance of long duration regulated flows through the irrigation season, worsened by boat wash, and degradation of once dense and extensive common reed beds (Gippel & Lucas, 2002). Bank erosion does not appear to be related to removal of riparian trees. The widening probably progressed through several phases, associated with the phases of river regulation. The widening has affected the riparian zone, reducing levee width in places, and removing trees from the bank (Gippel & Lucas, 2002).

Overall there has been little net aggradation of the bed of the River Murray in this zone since the 1870s (average of 0.35 m from Yarrawonga to Torrumbarry Weir). There has been virtually no bed degradation below Yarrawonga Weir, due to development of a resistant armour layer, which is evident on point bars (Rutherfurd, 1990).

The floodplain sedimentation rate in this zone is 7 mm/10 years. While this rate is low, it still represents a doubling of the floodplain sedimentation rate since European settlement (Kenyon & Rutherfurd, 1999).

7.2.8 Community values of the River Murray Channel

Introduction

It is difficult to separate values for the River Murray Channel as defined in this report to those held for the rivers in the basin as a whole. Below are some of the values people hold for the river Channel as part of the larger River Murray System and Murray-Darling Basin.

Environmental values

The pattern of floods and droughts and the seasonal changes in flows along the River Murray Channel have created a range of channel forms, flows and habitats resulting in the evolution of unique plants and animals. Murray cod and River red gums are two examples. The River Murray Channel is part of a unique landscape that provides many intrinsic values, such as its beauty, wealth of plant and animal life, and existence values.

Indigenous cultural values

As the first peoples of the Murray Darling Basin, Indigenous communities have lived with the river for thousands of years. The river (and its floodplain) has shaped, and is a living part of, the beliefs and lives of Indigenous peoples. It contains sacred and significant places that Indigenous peoples expect to be respected, protected and preserved. Some Indigenous peoples still occupy traditional lands, with the river, the wetlands and the floodplain providing food, medicinal herbs and raw materials.

The language groups and nations along the length of the River Murray Channel include the Wiradjuri, Wergie, Yorta Yorta, Wamba Wamba, Wadi Wadi, Barapa Barapa, Muthi Muthi, Latje Latje, Barkinji and Ngarrendjeri. There is cultural diversity between the Indigenous peoples in relation to traditions, places of importance, creation stories, cultural laws and customs. They do, however, have the same vision for River Murray, which includes treating the River Murray Channel and landscape with due respect. The Indigenous vision for the River Murray is holistic and incorporates spiritual, cultural, economic and social values. Indigenous peoples have a sense of custodianship over country (National Estate Grants Program, 1996-1999).

European heritage values

The River Murray Channel has shaped important elements of post-European history. It was the site of early European settlements, located at crossings with abundant fresh water.

The use of the River Murray Channel for transportation made a major contribution to the development of towns and the pastoral industry, especially from the 1850s to the earlier part of the twentieth century. Paddle-steamers reached as far as Albury in 1855, supplying towns and stations with their needs, carrying wool and other products to markets, and allowing inter-colonial river trade between New South Wales, Victoria and South Australia. Places such as Goolwa, Morgan and Echuca were major river ports, many features of which remain, as well as shipwrecks and other sites along the rivers (Kenderdine, 1993; 1994).

Economic values

The use of the River Murray Channel and the waters it conveys provide numerous economic benefits to Australia. These benefits come from industries including irrigated agriculture, hydro-electricity generation, and tourism and recreation.

Irrigation supports dairy, rice, cotton, beef, wine and horticulture, and has flow-on benefits to the food processing industry. In many cases, families have lived along the river and produced food and fibre using its water for several generations, and industries have been established that rely on an adequate level of reliability of water supply.

Electricity is generated from the river's power at a number of hydro-electricity plants at Hume and Dartmouth dams and at Yarrawonga Weir.

Tourism and recreation include a range of activities on or near the River Murray Channel. Boating on the open river and weir pools is popular, including house boats, paddle-steamers, canoeing, fishing and ski boats. Camping and touring are also popular activities, as are visits to National and State parks and conservation areas. Tourists are also drawn to the wine industry, historic attractions and river-based events and festivals. Further details are provided in Hassall & Associates and Gillespie Economics (2003).

7.2.9 Summary of knowledge of the condition of the River Murray Channel

The high conservation, recreation, economic and heritage value of the River Murray Channel has long been recognized. The area is rich in flora and fauna, including threatened species. Despite its high conservation values, surveys have demonstrated significant environmental changes in the River Murray Channel. Of the various factors impacting the health of the Channel, the altered flow regime due to river regulation is the factor with the greatest potential to affect the flora and fauna.

Over the last century, diversion of water from the River Murray Channel has increased, chiefly for agriculture. Increased storage through time has provided greater opportunity to modify flow patterns relative to natural conditions, to which the biota has evolved. As a result, flow volumes have been reduced in the lower part of the River Murray. In summer the river is used to transfer water from the upper storages to irrigation areas downstream. While bank erosion has been substantial, it appears to have slowed in many places, and much of the river is relatively stable.

Recent reviews of environmental condition data from the River Murray indicated that overall biological and environmental condition was degraded right along the river, with most degradation towards the Mouth.

7.3 Factors impacting on environmental values

Previous scientific panel reports and scientific studies have identified a number of threats to the health of the ecology of the River Murray Channel and associated features (such as wetlands) due to river regulation. However, the decline in ecological condition is not solely due to flow regulation. Further, the threats do not apply equally along the River but vary depending upon the location relative to the main regulating structures and diversion offtakes. Table 7.7 identifies the key processes threatening ecological values from Thoms et al. (2000) and Jensen et al. (2000).

A relatively high level of hydrological variability is a typical characteristic of the natural flow regime in many Australian rivers, including the River Murray (Young, 2001). River regulation inevitably dampens the amplitude of this flow variability with adverse effects on native biota that have become adapted to such variability (Boulton & Brock, 1999).

A direct effect of river regulation is the change in flow regime. The ecological consequences of regulation are evident by an examination of the way that aspects of the `flood pulse' and its sequences (flow history) affect the ecology of plants and animals in rivers in general (Table 7.8, Figure 7.23) (Boulton and Brock, 1999).

Table 7.8 shows a number of characteristics of parts of the hydrograph. Table 7.8 shows that it is not just the volume of water in the river system at any point in time that has an impact on the overall health of fauna and flora species. Other characteristics such as the duration of low and high flows as well as the time for the river height to rise or fall are all factors that affect the mix of species that can successfully recruit and exist.

Perhaps the most common example is that of native and exotic fish species. The duration of zero flow is the time period over which flows are low and constant. Longer zero flow durations may result in loss of flow and stagnant pools of water. Native fish species are not as tolerant as other exotic species where such conditions occur over long periods of time. It is likely that increasing the duration of zero flows encourages recruitment and survival of exotic fish species.

In general, for regulated lowland rivers like the River Murray, species composition of riparian vegetation may change as exotic plant species such as willows invade the river channel, favoured by the constant water level in weir pools (Boulton & Brock, 1999). Lower flows are often warmer in summer, promoting algal growth. Particles may settle out in dams and weirs, improving water clarity and further favouring the proliferation of toxic blue-green algal blooms (Boulton & Brock, 1999). Conversely, the natural water temperature regime may be reversed if cold water is released from dams for irrigation in summer. This change in temperature may inhibit successful reproduction by fishes and other animals that use thermal cues for spawning (Boulton & Brock, 1999).

The presence of weirs in the lower River Murray has extended the area of permanently flooded wetlands, so that about 70% of lower Murray wetlands (backwaters, `side-arms', anabranches, lakes and billabongs) are now connected to the river at weir pool level (Pressey, 1986). Many of these wetlands formerly were subject to larger, more frequent water level changes, and some would have dried periodically. A consequence of prolonged inundation is that the affected wetlands may no longer exhibit the pulse of plant and animal growth associated with a flood following a dry period (Walker, 2001). Similarly, other floodplains and wetlands subjected to drying for too long an interval as a consequence of regulation may not produce a pulse of high productivity when floods occur. Thus, disruption of the natural drying and wetting cycle affects the capacity of the river-floodplain ecosystem to benefit from floods (Walker, 2001).

The same drying-wetting sequence is as significant for habitats within lowland river channels as it for the wider floodplain. Indeed, the term `flood' is usefully applied to all increases in river stage rather than merely overbank flows (Puckridge et al., 1998). Rises and falls in the water level within the River Murray Channel stimulate corresponding responses in the growth of the biofilms (algae, bacteria and fungi growing on sediments, rocks and wood) that provide food for some fish, and for snails and other grazing invertebrates (Walker, 2001).

Not all native fish have breeding cycles directly linked to flooding, but it is likely that recruitment in most, if not all, species is enhanced by high flows that result in widespread flooding (cf. Harris & Gehrke, 1994; Gehrke et al., 1995; Humphries et al., 1999). This may be mediated by access to and from floodplain wetlands, or by increased abundance of food for young fish, exported from the floodplain. Rises in flow that are contained within the river channel may also promote successful recruitment in some fish species (Mallen-Cooper & Stuart, 2003).

Weirs and other regulating structures are significant obstacles to fish movement along the River Murray Channel, and removal of these barriers or installation of fishways will be an important step towards improving habitat access for native fish. Other complementary measures that will enhance the success of recovery of native fish populations include: flow restoration, physical habitat restoration (re-snagging), carp control, improved catchment management and salinity control.

From Table 7.7 not all threats to the health of the ecosystem are to do with flow management. For example, some fish species are known to have strong associations with physical habitat (eg snags) (Crook et al., 2001).

7.4 Opportunities to meet objectives for this site

Introduction

Proposed actions under the First Step Decision are:

• incidental benefits from flows being transferred to other significant ecological assets;

• provision of fish passage from the sea to Hume Dam, through the construction of fishways;

• implementation of priority actions such as re-snagging, weir pool manipulation, and demonstration reaches for native fish (MDBMC, 2004, p. 42).

Through the Environmental Works and Measures Program, an indicative budget of $45.2 million has been allocated for works and measures targeting improved environmental health for the River Murray Channel (MDBMC, 2004, p. 42).

In addition to structural and operational works and measures for each of the six significant ecological assets, the Environmental Works and Measures Program funds a suite of non-asset-specific complementary investigations and actions to support the overall program. Many of these projects will benefit the River Murray Channel. An indicative budget of $27.6 million has been allocated for these complementary investigations and actions (MDBMC, 2004, p. 48).

7.4.1 Increase river flow

The River Murray Channel has been impacted by numerous disturbances, with reduced flows being one of the main factors. The obvious way to address this is to release more water to the Channel. The First Step Decision allocates an average 500 GL per year to the river. This water is principally aimed at watering floodplain wetlands, but the River Murray Channel will benefit, as it will transfer the water, mainly in spring.

7.4.2 Structural and operational opportunities

Fishways

Providing an unimpeded upstream mitigation route from the sea to Hume Dam would serve to improve the health and distribution of native fish populations throughout the River Murray Channel. It is estimated that there are approximately 4,000 barriers to fish movement throughout the Murray-Darling Basin, including a number of the flow management structures along the River Murray Channel.

A Works and Measures Program project to construct fishways at all major barriers to fish migration on the main trunk of the River Murray, termed `Hume Dam - sea fish passage program', is being undertaken by the Murray-Darling Basin Commission. The objective is to improve the biodiversity of native fish and improve access of native fish to habitat within the River Murray by construction of fishways on all the locks and weirs between the sea and Hume Dam to support the recovery of native fish populations (MDBMC, 2004).Works are well underway and the first fishway in this program at Lock 8 is completed

Weir pool manipulation

The Works and Measures Program project `Tristate weir pool manipulation project' is being jointly undertaken by DWLBC, DSE and DIPNR, initially coordinated by the Murray-Darling Basin Commission. The objective is to improve the ecological health of floodplain wetlands through improving the frequency and magnitude of floodplain inundation and re-instating wetting and drying cycles to wetlands permanently inundated by weir pools by weir pool manipulation (MDBMC, 2004). The project will investigate the capacity to raise upstream water levels at weirs based on current load capacities, and undertake economic assessment of impacts, benefits and options associated with increased variation of weir pools and structural adjustments of weirs. The project will also implement a series of river manipulation events each building on the results of preceding trials (MDBMC, 2004).

Resnagging

Resnagging and riparian restoration from Hume Dam to Yarrawonga has been undertaken by the North East Catchment Management Authority, Victoria (MDBMC, 2004). The objective is to improve opportunities for fish recruitment in this reach by investigating the roles and functioning of anabranches and backwaters and opportunities for artificially constructing them. One outcome will be establishment of sufficient quality of habitat to sustain populations of Murray cod and Golden perch (MDBMC, 2004).

River Channel Management Investigations

The Works and Measures Program project `River Murray Channel options investigations' will identify and prioritise a list of potential river and floodplain management works to enhance the environmental health of the River Murray Channel Asset (MDBMC, 2004).

The Works and Measures Program project `The Murray above Hume management plan' is being undertaken by the Department of Infrastructure, Planning and Natural Resources, New South Wales. The objectives are to Development and implementation of a river management plan that will: identify priority reaches for the improvement of instream and riparian habitat on the River Murray above Hume; operate the Snowy scheme regulated flows to minimise damage in the Upper Murray system; and, protect and enhance high values of the Murray in relatively healthy reaches and improve instream and riparian biodiversity for the Upper Murray. The emphasis is on achieving enhanced native ecology-primarily appropriate riparian vegetation (MDBMC, 2004).

Complementary investigations and actions

A range of works and measures already adopted under the Environmental Works and Measures Program will benefit more than one SEA or can be classified as complementary works to enhance the outcomes of other works and measures.

Structural and operational changes

Structural and operational changes to improve downstream flow patterns have been identified for three sites in the River Murray system:

• structural and operational modification of the Menindee Lakes outlet to restore ecologically significant elements of the downstream flow regime;

• modification to the existing water supply arrangements in the Great Darling Anabranch to secure environmental water and improve in-stream health; and

• structural and operational modification to the Lake Victoria outlet to restore ecologically significant elements of the downstream flow regime (MDBMC, 2004).

Each of these projects will provide significant local environmental benefits but will also serve to enhance downstream flow conditions benefiting the Chowilla Floodplain, the Lindsay-Wallpolla system, and the Murray Mouth, Coorong and Lower Lakes. In addition, improving the downstream flow regime will also serve to improve in-stream environmental health for the River Murray Channel itself.

Procurement of mobile pumping infrastructure

Mobile pumping infrastructure could potentially enable target watering of forests and wetlands throughout the River Murray system, providing a short-term solution to reverse local degradation while more long-term options are being investigated (MDBMC, 2004).

Options for target watering through pumping have been identified for the Gunbower and Koondrook-Perricoota Forests, for the Chowilla Floodplain (including the Lindsay-Wallpolla system) and for Hattah Lakes.

Wetland management

A project is being undertaken by the Department of Water, Land and Biodiversity Conservation, South Australia, to develop best practice wetland management. The objective is to develop and demonstrate adaptive management of a degraded wetland system to improve its environmental condition and function in a wetland via initial engineering works and future hydrological regime manipulations (MDBMC, 2004).

7.4.3 Links between ecological objectives and management opportunities

The connection between the ecological condition of the River Murray Channel and flows to the Channel has been firmly established. Other factors are known to affect ecological condition of the Channel, but flow regime is of fundamental importance. The proposed opportunities for managing flows in the Channel aim to reverse or partially reverse the effects of flow regulation, and also to improve in-channel habitat and provide fish passage.

While the additional water allocation provided under the First Step Decision is principally targeted at wetland watering, the River Murray Channel will benefit, as it will transfer the water, mainly in spring. This will partially reinstate one facet of the river's natural flow variability. The Darling Anabranch, Menindee Lakes and Lake Victoria projects will allow for improved downstream flow regimes, and delivery of flows for flood enhancement at wetland assets. The weir pool manipulation projects will aim to increase water level variability in the vicinity of weir pools, which are currently relatively constant. The resnagging projects are complementary-they provide in-channel habitat that fauna can utilise under improved flow regimes. The fishway program will overcome barriers to migration and expand the range of migratory fish species. The projects will be undertaken under the principle that bank erosion and bed degradation/sedimentation will not be altered above current rates. Some parts of the River Murray Channel may still have unacceptable bank erosion rates-a problem that will be addressed locally.

7.4.4 Influence of weirs

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 (eg the major drought of 1967-68) (Wittington et al., 2000). The lengths of the weir pools affecting Lindsay-Wallpolla Islands range from 29 km for Lock 7 to 77 km for Lock 6 (SKM and Roberts, 2003, p. 13).

The weirs alter the nature of the riverine environment, and also impede fish movement. The main hydrological impact is on in-channel flows and floods that are too small for the weir to be removed. Up to the weir removal flow, the water behind the weir is deeper and the level is more stable than if the weir was not present. An elevated freshwater mound develops under the floodplain, referred to as the flushed zone by Jolly and Walker (1995). Wetlands and in-channel habitats that had a seasonally variable water regime are now permanently wet. The effect on trees depends on distance from the weir pool crest, but ranges from dieback due to water-logging to increased canopy growth, productivity and litterfall (SKM & Roberts, 2003, p. 14). The weir pool favours the growth of fringing emergent macrophytes (SKM and Roberts, 2003, p. 14).

Effluent creeks with low sill levels become permanently connected to the river and only flow when a head difference is created, such as may occur on a rising hydrograph. Channels with higher sills become connected to the river at lower than natural flow levels (SKM & Roberts, 2003, p. 14). Weir pools have created favourable conditions for carp and nuisance native plant species.

River levels downstream of weirs can fall rapidly when weirs are reinstated after a flood. The river level upstream of a weir may fall below the intended weir pool level before the weir is reinstated. In order to restore the pool level as quickly as possible, the weirs are operated so as to capture a high proportion of discharge. This sudden detention of river flow causes a rapid drop in river level downstream. This impact becomes greater downstream as the impact of successive weir reinstatements accumulates (Thoms et al., 2000). For example, a study of river levels downstream of Lock 3 found that prior to regulation, flood peaks took 59 days to recede, but now recede in only four days (Thoms & Walker, 1989). Changes in other catchment factors may also have impacted river flood recession rates, but weir operation has a major impact on the lower Murray.

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