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.