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 and South Australia 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 River Murray to the north.
In their natural state, these waterways and wetlands would regularly flow and fill in response to high water levels in the River Murray. Large floods still occur, but major storages in the upper reaches of the River Murray 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 River Murray channel, for example by lowering pool levels to increase the extent of fast-flowing habitat, which is preferred by large-bodied native fish (such as Murray cod).
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 (FSL), flows to the Lindsay River and Potterwalkagee Creek increase; when weir pools are lowered, flows to both the Lindsay and Potterwalkagee reduce and eventually they stop flowing. Mullaroo Creek is less-affected by weir pool levels, because flows are controlled through the Mullaroo Creek regulator which connects the creek and the River Murray. Moderate lowering of the lock 7 weir pool level has little effect on Mullaroo Creek, but lowering to or beyond 0.5 m below FSL makes it difficult to deliver the recommended minimum flow of 600 ML per day that is required for native fish.
Fluctuation of weir pool levels is a major management consideration for jurisdictions that manage flows in the River Murray and the anabranch waterways of Lindsay, Mulcra and Wallpolla islands. Environmental objectives and associated water regimes for the River Murray sometimes conflict with those for the Lindsay, Mulcra and Wallpolla anabranch systems, so responsible agencies in Victoria and NSW and the Murray-Darling Basin Authority need to collaboratively plan how to manage weir pools and flows effectively.