Executive summary

Flows to Gunbower and Koondrook-Perricoota forests have been modified by regulation of the Murray and Goulburn rivers through water management

infrastructure. Since regulation, spring floods in the Gunbower and Koondrook-Perricoota forests now occur less frequently than under natural conditions, and are of shorter duration.

Reduced frequency and extent of flooding have been significant contributing factors to the decline of overall ecological values of the forests. Wetland areas have reduced and retreated to the lowest lying parts of the forests. Reduced frequency of widespread flooding has enabled River red gums to colonise former wetland areas, which has reduced their value for waterbirds. Permanent water, which is an important habitat requirement for aquatic fauna, such as small fish and colonial nesting waterbirds, has largely been lost from the forests as a result of the reduced frequency of what were once regular flow peaks in late winter and spring. The wetland understorey plants have declined, and terrestrial understorey grasses and shrubs have spread.

The effect of the Barmah Choke in dampening high flows from the upper Murray means that, in general, floods into the Gunbower and Koondrook-Perricoota forests can only be achieved when high flows from the Murray coincide with high flows from the Goulburn River. However, there are also structural and operational opportunities to water these forests. Use of these measures still requires water, but to achieve similar environmental outcomes, the water required is likely to be less than that for enhanced flooding. Enhancement of naturally occurring floods has advantages over other structural and operational means of watering the Gunbower and Koondrook-Perricoota forests. These advantages include temporary restoration of the connection between the river and floodplain, and generation of triggers to biota that would be similar to those under natural flood conditions.

Hattah Lakes

The Hattah Lakes system is a large floodplain wetland system consisting of shallow lakes, streams and temporary swamps and bordered by riverine forest. The Hattah Lakes system supports a range of productive habitat types and biota, although it is most noted for its 18 lakes. Hattah Lakes is a particularly important site due to the size of the system, carrying capacity for fauna, the diversity of vegetation species it supports and its important role in the lifecycles of waterbirds. The main hydro-ecological feature of the Hattah Lakes is the large variation in permanency of the aquatic habitats. The lakes only fill during high flow events in the River Murray. As a result, once flow ceases in the main feeder channel, Chalka Creek, most of the lakes dry within 12 months.

The ecological values of Hattah Lakes have been threatened by river regulation, particularly through a decrease in floods in spring. The frequency of flood events has been reduced by flow regulation, resulting in much less frequent inundation of the lakes, with most flows now failing to reach the inflow threshold to Chalka Creek. The larger flood events that fill most of the lakes have more than halved in their frequency under current conditions. Also, the flood events that do occur are of reduced duration, and there has been an increase in the time between flood events. The median time between major events that flood the entire lake system has doubled under current conditions compared to natural conditions.

Not only has the pattern of floods in the River Murray changed due to regulation, but the passage of these floodwaters to the Lakes has been modified by structural alterations. Chalka Creek has undergone deepening and widening.

Since European settlement the structure and composition of the vegetation around the Hattah-Kulkyne Lakes Ramsar site has been severely modified by domestic stock and native animal grazing, logging, and the introduction of exotic flora and fauna. The impacts of reduced flooding frequency include some dieback and reduced vigour of riparian River red gum, as well as affecting tree distributions. The changed hydrological regime is likely to have altered faunal habitat leading to lower recruitment rates within some fish and macroinvertebrate populations that depend on certain flooding regimes.

Options for improving the ecological condition of the Lakes fall into two categories: increasing the frequency and duration of flooding through allocating additional water; and structural works that more efficiently capture and retain water under the current flow regime.

In considering any future watering options for the Hattah Lakes, it should be noted that only very large floods result in complete inundation of all of the lakes. Smaller floods that have a recurrence interval of every several years affect from 30% to 50% of the Lakes. Utilising these smaller flood events that occur relatively frequently will be just as important as managing the larger flood events that occur less frequently.

An example of a structural and operational option is keeping regulators open until the lakes are filled by a flood event, whereupon they would be closed to retain water in the lakes. The regulators would remain closed until the next flood event where the level of the flood exceeds the water level in the lake. Installing a pump station at Chalka Creek to pump water into Chalka Creek and the Hattah Lakes is another option.

Chowilla Floodplain and Lindsay-Wallpolla Islands system

This Significant Ecological Asset actually comprises three separate locations - Lindsay Island and Wallpolla Island in Victoria, and Chowilla Floodplain, which is largely in South Australia but also extends into New South Wales. The Lindsay Island and Wallpolla Island systems are separated by around 40 river kilometres. Chowilla Floodplain is a separate system located just downstream of Lindsay Island. This area has some characteristics that are distinctly different from the Lindsay-Wallpolla Islands system.

Lindsay-Wallpolla Islands

Lindsay and Wallpolla Islands are formed by a series of anabranches leaving the `islands' situated between the anabranch channels and the main steds into New South Wales. The Lindsay Island and Wallpolla Island systems are separated by around 40 river kilometres. Chowilla Floodplain is a separate system located just downstream of Lindsay Island. This area has some characteristics that are distinctly different from the Lindsay-Wallpolla Islands system.

Lindsay-Wallpolla Islands

Lindsay and Wallpolla Islands are formed by a series of anabranches leaving the `islands' situated between the anabranch channels and the main steds into New South Wales. The Lindsay Island and Wallpolla Island systems are separated by around 40 river kilometres. Chowilla Floodplain is a separate system located just downstream of Lindsay Island. This area has some characteristics that are distinctly different from the Lindsay-Wallpolla Islands system.

Lindsay-Wallpolla Islands

River regulation poses a risk to the environmental health of the system, mainly with respect to the reduced frequency of medium-sized floods. (The seasonal pattern of river flow is largely unchanged.) The effects of regulation in this section of the river can be summarised as causing: long periods of low regulated flows, and, for larger flow events, a reduction in the flood frequency and duration and size of the hydrograph peak.

A number of potential options have been investigated to restore flows to enhance streamflow habitat for the Lindsay-Wallpolla Islands. These range from altering the regional inflows from the River Murray to changing features within the systems (such as structures and levees located along the anabranches of the Lindsay-Wallpolla Islands).

Chowilla Floodplain

The Chowilla Floodplain is on the northern side of the River Murray. Most of the Floodplain lies in South Australia, but it also extends into New South Wales. The Floodplain contains a mosaic of different vegetation communities, and sites of high conservation significance. Although the Chowilla Floodplain retains much of its natural character and attributes, there are large areas of vegetation in poor condition.

The River red gum woodlands of the Chowilla Floodplain area support one of the highest diversities of birds of any terrestrial vegetation association in South Australia. The Black box woodlands provide refuge for many Mallee birds and floodplain and riverine species. The permanent wetlands are important drought refuges for waterbirds and are used for breeding by some species; the ephemeral wetlands are important for waterbird breeding. The Chowilla Floodplain has a high diversity of terrestrial and aquatic habitats, supports populations of rare or endangered species, has fish breeding habitat, supports populations of breeding waterbirds, has habitats not well represented elsewhere, and has a relatively low level of direct human disturbance.

The construction of Lock 6, adjacent to the Chowilla Floodplain, resulted in permanently higher water levels on the adjacent floodplain area and continuous flows of water through the Chowilla Floodplain anabranch system. In parts of the Chowilla Floodplain the water table is now around 2 metres higher than it was under natural conditions. The area is one of natural discharge of saline groundwater, but the higher groundwater levels caused by Lock 6 have exacerbated the salinisation problems. Also, a consequence of the reduction of flood frequency, flushing of salt from the floodplain soils occurs less frequently.

The health of many of the vegetation communities at Chowilla has declined in recent years. Observed episodes of tree stress there can be attributed to a combination of three main factors: prolonged drought (low rainfall), reduced flooding, and soil salinisation. Regular floods are important because they recharge the soil and groundwater, and flush salt that has accumulated through the dry period from the tree root zones. Other factors such as grazing by sheep and the incidence of feral and over-abundant native animals also influence vegetation condition.

River regulation has altered the pattern of flooding on the floodplain areas where River red gum naturally grows. River red gum forest communities that are located close to the Channel or which have access to a constant supply of fresh water are reasonably healthy, but those located in the higher elevation areas have suffered severe tree decline. Die-back of Black box has also occurred in highly saline areas. There is also a lack of regeneration as a result of a lack of flood events.

The River Murray in the Chowilla Floodplain area has prolonged periods of low flow under current levels of water resources development. Flows over 50,000 ML/day used to occur eight years in 10, and now occur only three years in 10. Maximum inundation of the Chowilla Floodplain now occurs less than once every 25 years compared to the natural frequency of once every 8 years. The frequency and magnitude of larger floods have also been reduced.

Floodplain salinisation, reduced flood frequency, rapid flood recession, and stable weir pool levels are the main causes of environmental problems at the Chowilla Floodplain. Actions that could improve the Floodplain's environmental condition include increasing the frequency and duration of floods (either by enhancing natural floods by manipulating releases from Lake Victoria, Menindee Lakes or Lock 6, or pumping onto the floodplain or into wetlands). Other potential actions include structural measures that affect flows and groundwater/salinity management.

Options to reduce floodplain salinity at Chowilla have focused principally on lowering the floodplain water table by pumping, reducing groundwater flow towards the river by pumping, and increasing floodplain inundation. The saline water table beneath the floodplain can be lowered directly by groundwater pumping. There is a need to integrate salinity reduction and floodplain inundation options to maximise the benefits of improved flows.

Murray Mouth, Coorong and Lower Lakes

The Coorong is a long, shallow lagoon, 140 km in length, at the Mouth of the River Murray. The water varies from fresh to marine to hypermarine depending on flow over the Barrages and the degree of Mouth opening. The wetlands surrounding the Murray Mouth are particularly important for bird breeding. The Coorong has been ranked within the top six waterbird sites in Australia, based on the diversity (85 species) and abundance of species found there. In the 1960s, 250,000 small waders were recorded in one section of the estuary, but since the 1990s a range of 3,000-13,000 was recorded over the entire estuarine system. 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.

The Barrages isolate the estuarinrtant for bird breeding. The Coorong has been ranked within the top six waterbird sites in Australia, based on the diversity (85 species) and abundance of species found there. In the 1960s, 250,000 small waders were recorded in one section of the estuary, but since the 1990s a range of 3,000-13,000 was recorded over the entire estuarine system. The numbers of almost all species of waders and waterbirds using the ng the wetlands of the Coorong and Lower Lakes have declined, particularly over the past 20 years.

The Barrages isolate the estuarinrtant for bird breeding. The Coorong has been ranked within the top six waterbird sites in Australia, based on the diversity (85 species) and abundance of species found there. In the 1960s, 250,000 small waders were recorded in one section of the estuary, but since the 1990s a range of 3,000-13,000 was recorded over the entire estuarine system. The numbers of almost all species of waders and waterbirds using the e partic of fish. The fish of the Coorong lagoons include species that move between the sea and the Coorong, and species that are resident in the Coorong. The fish species are favoured where they have unhindered access between the Coorong and the sea, and when salinities are estuarine in the Northern Lagoon. An open Mouth promotes the flux of nutrients, which is favourable for productivity and enhances the survival prospects of larval and juvenile fish.

Regulation of the river system has dramatically reduced the volume of flow delivered to the estuary. Years with annual flows <5,000 GL occurred 7% of the time under natural conditions, but now occur 66% of the time under regulated conditions. Flow to the Murray Mouth now ceases on average once every two years, while before regulation this was experienced once every 20 years. There has been a threefold reduction in the frequency of medium-sized flood events (20,000-80,000 ML/day), and the duration of these events has also decreased.

The only time that the Murray Mouth has closed in recorded history was in 1981, although dredging probably prevented Mouth closure during 2003. Persistent low flows and their effects on promoting marine sand build-up at the Mouth are not new, but since regulation they have increased in frequency and duration.

To maintain the existing Murray Mouth opening, dredging has been undertaken to prevent further accumulation of marine sand deposits within the estuarine areas. To date, this work has been quite successful, but it does not address the cause of the problem.

The tendency for the Murray Mouth to close is caused by sand accumulation associated with coastal processes. River flows can counteract this, but they have been reduced through regulation. Options to maintain an open Mouth through flow manipulation are currently being investigated. In addition to reducing the risk of closure of the Murray Mouth, additional flows over the Barrages can lead to improvements in the habitat for selected biota in the Coorong and Lower Lakes.

River Murray Channel

The River Murray Channel is over 2,000 km in length. It forms the longitudinal and lateral link between the five other SEAs. The River Murray Channel connects headwaters, lowlands, the estuary and the ocean, delivering the water, sediment and nutrients required to maintain the integrity of these areas. 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 within the river channel.

For Indigenous peoples and other Australians, the River Murray plays a large part in identity, history and folklore. The River Murray Channel is part of a unique landscape which provides many intrinsic 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 freshwater. 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, hydroelectricity generation, and tourism and recreation.

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 (ie 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 regulated flows. The lower River Murray is also highly regulated, but the effect of regulation there has been 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.

Habitats within the River Murray Channel are influenced by the flow regime, which in turn is affected by flow management structures and how they are operated. As a simplification, flows in the River Murray Channel can be classified into three operating `modes': supplying mode (storages drawn down to meet downstream water demands), storing mode (storages filling) and spilling mode (storages full and excess water causing flows above channel capacity). 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.

The operation of flow management infrastructure has altered the flow regime over seasonal, annual and daily timeframes, the extent of the impact depending on the location in the river system. Downstream of Yarrawonga Weir, much of the natural high flow in spring is usually stored in the headworks during the storing mode, and the natural low flows in summer have been replaced by bank-full 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 from January to March, during what was previously the lowest flow period.

Flow in the Ovens and Kiewa rivers, in north-east Victoria, has been largely unaffected by water resources 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 less than it was naturally. Also, some of the tributaries are delivering less water to the River Murray than if they were not regulated. The median annual flow to the sea is currently about 27% of that under current conditions. Additionally, a greater proportion of the flow is now contained within the banks. This places increasing pressure on bank stability and reduces connectivity between the River Murray Channel and the floodplain.

A `Snapshot of the Murray-Darling Basin river condition' undertaken in 2001 evaluated the aggregate impacts of resource use on the condition of rivers in the Murray-Darling Basin. The results indicated that overall biological and environmental condition was degraded right along the river, with increasing degradation towards the Additionally, a greater proportion of the flow is now contained within the banks. This places increasing pressure on bank stability and reduces connectivity between the River Murray Channel and the floodplain.

A `Snapshot of the Murray-Darling Basin river condition' undertaken in 2001 evaluated the aggregate impacts of resource use on the conditi condition of rivers in the Murray-Darling Basin. The results indicated that overall biological and environmental condition was degraded right along the river, with increasing degradation towards the Additionally, a greater proportion of the flow is now contained within the banks. This places increasing pressure on bank stability and reduces connectivity between the River Murray Channel and the floodplain.

A `Snapshot of the Murray-Darling Basin river condition' undertaken in 2001 evaluated the aggregate impacts of resource use on the conditirrent ol measures, such as regulators, to construction of fish passages to improve access of native fish to habitat along the River Murray Channel and tributaries.

Linkages between the significant ecological assets

The six SEAs have distinctive characteristics that influence the way the hydrology can best be managed to achieve the expected benefits. Some SEAs need low flows for extended periods (eg. the Murray Mouth). Some need very high flows in spring every few years (eg the Chowilla Floodplain). Flooding at other times of year can be detrimental (as is the case with 'rain rejections' of flows into the Barmah-Millewa Forest).

The environmental demand for increased flows at the three most upstream SEAs (Barmah-Millewa Forest, Gunbower and Koondrook-Perricoota forests and Hattah Lakes) is in relatively wet years, particularly during spring. Despite the needs of these SEAs appearing to be aligned, the reality is more complex due to factors such as changes in flows (including attenuation) in the river between ecological assets and the differences in commence to flow levels. In prolonged periods of low flows, emergency measures (such as dredging of the Murray Mouth, and pumping of water to wetlands or areas of the Chowilla Floodplain and Lindsay-Wallpolla Islands) may be required to maintain the viability of SEAs until wetter climatic sequences recur.

However, some flow events have the potential to achieve multiple ecological objectives at the SEAs. In these cases, identifying links between SEAs (i.e what types of flows have benefits to a number of SEAs simultaneously) will allow future environmental managers to take advantage of such flow events, and in doing so, minimise potential trade-offs in water use between the SEAs.