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5.2.6 Lindsay, Mulcra and Wallpolla islands
System overview
Lindsay, Mulcra and Wallpolla islands cover over 26,100 ha of Victorian floodplain in the Murray- Sunset National Park, as Figure 5.2.5 shows. They form part of the Chowilla Floodplain and Lindsay-Wallpolla islands icon site that straddles the Victoria–SA border in the mid-Murray River system.
The Lindsay, Mulcra and Wallpolla islands floodplain is characterised by a network of permanent waterways, small creeks and wetlands. The Lindsay River, Potterwalkagee Creek and Wallpolla Creek form the southern boundaries of the site and create large floodplain islands with the Murray River to the north.
In their natural state, these waterways and wetlands would regularly flow and fill in response to high water levels in the Murray River. Large floods still occur, but major storages in the upper reaches of the Murray River system have reduced the frequency of small- to moderate-sized floods.
Flows in the mid-Murray River system are regulated through a series of weir pools, generally referred to as locks. Water levels in the weir pools are managed primarily to provide safe navigation and adequate water levels for off-stream diversion via pumps. In recent years, the water level of weir pools 7 and 8 has also been managed to achieve ecological benefits in the Murray River channel, for example by lowering pool levels to increase the velocity of flowing water, which can support drift of golden and silver perch larvae when conditions are suitable for breeding.
Weir pool levels have a big effect on flows in Mullaroo Creek, the Lindsay River and Potterwalkagee Creek. When water levels in locks 7 and 8 are raised above the full supply level, flows to Potterwalkagee Creek increase and Lindsay River starts flowing. When weir pools are lowered, flows to both the Lindsay River and Potterwalkagee Creek cease. Mullaroo Creek is less-affected by weir pool levels and
flows are controlled through the Mullaroo Creek regulator which connects the creek and the Murray River. Moderate lowering of the lock 7 weir pool level has little effect on Mullaroo Creek but lowering to or beyond 0.5 m below full supply level makes it difficult to deliver the recommended minimum flow of 600 ML per day that is required to maintain fast-flowing habitat for native fish, especially Murray Cod.
Fluctuation of weir pool levels is a major consideration for jurisdictions managing flows in the Murray River and the anabranch waterways of Lindsay, Mulcra and Wallpolla islands. Environmental objectives and associated water regimes for the Murray River sometimes conflict with those for the Lindsay, Mulcra and Wallpolla anabranch systems. Responsible agencies in Victoria and NSW and the Murray- Darling Basin Authority collaboratively plan how to manage weir pools and flows effectively.
Environmental values
The Lindsay, Mulcra and Wallpolla islands represent three separate anabranch systems including streams, billabongs, large wetlands and swamps. When flooded, waterways and wetlands within these systems provide habitat for native fish, frogs, turtles and waterbirds. Terrestrial animals (such as woodland birds) also benefit from improved productivity and food resources when the system floods. Large floodplain wetlands (such as Lake Wallawalla) can retain water for several years after receiving inflows; they provide important refuge for wetland-dependent species and support terrestrial animals (such as small mammals
and reptiles).
Mullaroo Creek and the Lindsay River support one of the most-significant populations of Murray cod in the lower Murray River. These waterways provide fast-flowing habitat that Murray cod favour, and contrast with the mostly slow- flowing and still habitats created by the nearby Murray River weir pools. Fish in Mullaroo Creek and Lindsay River breed and produce juveniles that colonise other parts of the Murray system. Waterways and wetlands throughout the icon site support several other fish species including freshwater catfish, golden perch, silver perch, Murray- Darling rainbowfish and unspecked hardyhead.
The reduced frequency and duration of floods in the Murray River has degraded the water-dependent vegetation communities throughout the Lindsay, Mulcra and Wallpolla island system, which has in turn reduced the diversity
and abundance of animals that rely on healthy vegetation for habitat.
5.2 Victorian Murray system
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