Mildura Weir represents a key connectivity link between the lower Murray and Darling rivers and the mid-Murray and its tributaries. Large volumes of water for the environment are delivered annually for a range of outcomes, including supporting connectivity for migratory species like Golden and Silver perch, Murray cod and Bony herring, which move between these two major river systems.
Mildura Weir Denil fishway (Built 2013, a key link in the Sea to Hume fish passage program) facilitates upstream migration of medium and large-bodied native fish. Fish sampling showed that the fishway was operating sub-optimally with poor fish attraction and upstream passage rates.
The Commonwealth Environmental Water Holder (CEWH) teamed with NSW Fisheries, Murray-Darling Basin Authority and Goulburn-Murray Water to investigate the fishway performance issues. The CEWH gathered a group of project stakeholders, industry experts and interested parties nicknamed ‘the Brains Trust’ to inform the goals and outcomes of the investigations. Jacobs and Charles Sturt University were contracted to undertake the investigations of the Denil fishway performance and advise improvements.
Investigations showed that installation and operational issues, such as a gap under a flow shielding wall at the fishway entrance, and missing baffles in the Denil ramps, were creating poor hydraulic conditions which were preventing fish from approaching the fishway entrance and moving upstream through the fishway. Modified parts were designed, fabricated and installed. In 2023, testing and monitoring confirmed that local hydraulic performance improved substantially following the retrofit, but overall downstream fish attraction remained limited by broad-scale hydraulic conditions at the wide weir.
The project demonstrated that for fishways, sometimes seemingly small and almost unobservable issues can have significant effects on performance. The project also demonstrated the positive outcomes that can be achieved when a group of engineers, ecologists, river managers, asset owners and operators collaboratively work together.