The Menindee Lakes are the only group of large lakes along the entire length of the Darling River. They comprise 19 lakes, ranging in size from 103 to 15,900 hectares. The biggest lakes are Menindee, Pamarmaroo and Cawndilla.
Before the Menindee Lakes Scheme was constructed in the 1960s, the lakes connected directly to the Darling River via short creeks and ranged in flood frequency from one in two years to one in 20 years. The Menindee Lakes are typical of freshwater lakes in the Murray-Darling Basin with low-lying elliptical or kidney-shaped basins, white sandy beaches and a lunette on the eastern margin. A red soil ridge, sometimes steep, eroding and cliff-like, forms the western edges of the lakes.
The Darling River below Menindee flows in a south-easterly arc for 200km before joining the Murray River at Wentworth. The river channel is young, formed 11,000 years ago. It is relatively narrow with small meanders, while downstream of Pooncarie, it is quite straight.
The Darling spills into the Great Darling Anabranch about 55 km south of Menindee. Between Menindee and the Darling Anabranch is an expansive area of floodplain with River red gum woodland and Black box trees A series of 16 lakes occur along the Darling Anabranch. The lakes connect to the Darling River during floods, providing important habitat for water birds and fish. River red gum floodplain dominates the northern part of the Darling Anabranch. Lakes in this section flood one in three years. The southern half of the Anabranch floods less often with lakes in this section flooding one in 10 years. The Darling Anabranch also receives backwater flows where it joins the Murray River west of Wentworth.
The main lakes at Menindee – Lake Cawndilla and Menindee Lake, support hundreds of thousands of waterbirds, including migratory shorebirds. In January 1996, when Menindee and Cawndilla Lakes were almost dry, over 200,000 waterbirds were recorded including 34 species and 40,000 small wading birds.
In the 1960s, when Menindee Lake was covered in lignum, and again in the late 1970s, black swans nested in their thousands. Hundreds of Australian pelican bred on the southern end of Cawndilla Lake in 1952. Records from the National parks and Wildlife Service Wildlife Atlas for the Darling Anabranch and Lower Darling River reveal 74 water bird species of which 34 were small wading species.
Freckled ducks and Blue-billed ducks recorded in lower Darling River system are listed as vulnerable under the NSW Threatened Species and Conservation Act 1995. In total, the wetlands of the lower Darling River system support 24 species of waterbirds considered rare or endangered. Eleven of these rely on temporary rather than permanent wetland habitat (ie. freckled duck and migratory waders).
During 1992 field surveys when western NSW was in drought, waterbirds concentrated on the deeper Anabranch lakes, Nearie and Little Lakes suggesting their value as drought refuges. It is estimated that the Darling Anabranch and lakes support more than 10,000 waterbirds when flooded.
There seems to be no contemporary evidence of major waterbird breeding colonies in other parts of the lower Darling River system, south of Menindee. Though this may partly reflect incomplete surveys, it possibly represents the true situation. The southern habitat may be inadequate to support major colonies like those that exist elsewhere such as in the Macquarie Marshes.
Nests have been observed in Blackbox trees and woodland fringes around Popio Lake. However, surveys are still required during floods and for nesting sites to determine the value of the Darling River and Darling Anabranch for waterbird breeding.
The lower Darling system offers fish a diversity of habitats and a potential waterway to move from the Murray to the Menindee Lakes and Barwon-Darling River system. Habitats include deep waterholes along the Darling River, expansive lakes, lignum swamps and a mix of slower water interspersed with shallow fast flowing runs.
In 1997 and 1998, three sites in each of six Menindee Lakes were surveyed four times and detected 10 fish species, including Fly specked hardyhead, Crimson spotted rainbowfish as well as the more common species from Darling River surveys. Bony herring, carp, Carp gudgeons, Golden perch and smelt were collected on all trips and from all lakes.
Although not detected in the recent surveys, the Menindee Lakes supports Murray cod and also Silver perch and Fly specked hardyhead, the latter two are listed as vulnerable under the Fisheries Management Act 1994 in NSW.
In a 1997–98 survey in the Menindee Lakes, Silver perch, catfish and Murray cod were expected to occur but were not present. The absence of the first two species reflects their decline in the Murray-Darling Basin in recent decades, whereas anecdotal information suggests that Murray cod remain viable in the Darling system. Recent surveys in the southern Murray-Darling Basin have revealed that the Purple spotted gudgeon and Olive perchlet have become rare, while there are no records of tench or Flathead gudgeons in the Menindee area . The absence of the latter species is surprising given its occurrence in the northern tributaries of the Darling River and in the Murray River.
Interviews with landholders on the southern Darling Anabranch revealed redfin were common in the 1950s, but declined after the regular releases started in 1961. Carp became established and reached high numbers in the 1960s and 1970s. A professional angler reported that carp remained in the Darling Anabranch channel and native species were prevalent in the lakes.
FLOODPLAIN AND AQUATIC VEGETATION
There is little information about the condition of floodplain vegetation in the lower Darling River system. River red gum communities along the channel and billabongs of the lower Darling are experiencing reduced frequency of flooding, reducing floodplain vegetation health and inputs of carbon into the river.
The increased flooding of most of the Menindee Lakes caused by regulation of the Menindee Lake Scheme, destroyed about 13,800 hectares of lignum and 8,700 hectares of Black box growing on the lakebeds.
WHY ENVIRONMENTAL ASSETS HAVE DECLINED IN VALUE
Within the lower Darling River system and the Menindee Lakes, three weirs and constant low flows fragment the Darling River habitats, isolating fish from upstream. The weir at Menindee has allowed fish passage twice since construction in the 1960s, both times during large floods in the mid-1970s and in the early-1990s. Large numbers of fish were reported to have moved upstream during these times, including Golden perch that were tagged in South Australia and recovered 1000 km away in the Barwon and Balonne rivers.
Reduced frequency of flooding
The Darling River channel, flood runners, floodplain and northern anabranch lakes have all experienced reduced frequency of flooding. These habitats potentially contribute large amounts of organic matter to the river when flooded and are productive sites for micro-invertebrates. Elsewhere, reductions of flood frequency have led to declines in diversity and density of micro-invertebrates and waterbirds.
Constant and unseasonal flows
Persistent unseasonal high flows are eroding the riverine benches in the Darling River and changing the shape of the Darling Anabranch channel. In summer, low minimum flows alternate with high flows whenever the Menindee Lakes are used to supply South Australia’s water entitlement. This results in unnaturally rapid fluctuations in water levels and prolonged periods of high summer flows.
Risk of algal blooms
The weir pools in the lower Darling River and the Darling Anabranch are at risk of algal blooms during summer when temperatures are high and flow in the pools is low.
Permanent inundation of the Menindee Lakes has significantly reduced the frequency of drying in the main lakes. Permanent inundation favours different vegetation communities, leads to sedimentation and promotes favourable conditions for carp. It alters the relationship between surface water and groundwater and can lead to increased salinity in surface waters. Permanent inundation disrupts the pattern of flooding and drying in dryland rivers and associated boom and bust cycles in waterbirds
What can be done to restore the environmental values?
The value of environmental assets within the lower Darling River system can be improved by changes to the volume and timing of flows, structural and operational changes and other catchment management improvements.
Two agricultural practices that potentially affect the lower Darling River system are grazing and lakebed cropping. In the case of grazing, fencing of riparian zones and provision of alternate stock watering points would promote regeneration of vegetation communities and increase organic inputs from the floodplain to the river system. Research indicates that seasonal grazing at appropriate rates will not significantly affect riparian zones.
Most of the lakes along the Darling Anabranch and at Menindee (although less often) are used for cropping as floodwaters recede. Research on the impacts of lakebed cropping found reduced micro-invertebrates and fewer small mammals in cropped sites. Lessening these impacts occurs by leaving parts of lakes uncropped.
The relationship between the Darling River and the River Murray system
The Darling River is the main tributary to the Murray River, but is distinct from the River Murray due to the geological history and character of its expansive semi-arid catchment.
Unlike the Murray, the Darling River flows through a flat landscape with basalt outcrops and sandy soils. The lower Darling River system is characterised by a variable flow regime and unpredictable flooding events that spread onto the vast floodplain and spectacular lakes. The clusters of large lakes in this region are unique compared to other parts of the Murray system. The lower Darling system also provides a significant link between the Murray system and the Barwon-Darling catchment. The loss of connection between the Murray River and upper reaches of the Darling River is likely to affect fish recruitment in the Murray River in South Australia.
Since development of the Menindee Lakes Scheme, the lakes have supplied 39% of the annual entitlement water flows to South Australia leading to a constant flow regime. The region plays an important role in water supply in the Murray system. The challenge in restoring environmental values of the lower Darling River system will be to satisfy these needs while improving connectivity, flow variability and flood frequency.
Boom and bust cycles
The plants and animals of dryland rivers are adapted to flow variability and the dynamic wetting and drying cycles leads to what is called a ‘boom and bust’ ecology. As floods spread over the vast flat landscape, filling lakes and billabongs, a pulse of nutrients triggers a succession of animal and plant communities.
Within a few days, the waters are teeming with micro-invertebrates that feed on the rich supply of bacteria, protozoans and algae. Pulses in fish numbers follow, as all larval fish depend on micro-invertebrates for their first feed. Soon wetlands are dense with larger invertebrates and plants, and numbers of waterbirds boom.
Some ducks feed directly on microinvertebrates, while others graze on plants or forage for larger invertebrates.
Eventually the lakes and wetlands dry and the pulse of aquatic life gives way to a dryland phase. As the sediments dry, deep cracks form in the sediments. Both the plants and soil cracks provide habitat to a diverse array of birds, mammals and reptiles that feed on the high numbers of spiders, beetles and insects. When the system floods again many of the terrestrial colonists drown, breaking down in the water to provide a rich supply of nutrients and carbon.
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