Freshwater flows to estuaries shape habitat, transport nutrients to drive productivity and generate a salinity gradient. The aim of this study was to determine how environmental flows export salt from the Murray Darling Basin and assess how freshwater flow arrest seawater ingress which shapes the salinity gradient and determines available habitat for fish in the Coorong. TUFLOW-FV was used to assess the effects of environmental water delivery on salt export and fish habitat suitability.
The Coorong, a large coastal lagoon, and the Murray Mouth lies at the estuary of the Murray Darling Basin in Australia. The North Lagoon is historically estuarine and connected to the Southern Ocean by the Murray Mouth and The South Lagoon is hyper-saline. Salt export from the Murray Darling Basin to the Coorong through the barrages was 761,534 tonnes in 2017-18; 782,742 tonnes in 2018-19; 677,122 tonnes in 2019-20; and 878,525 tonnes in 2020/21. Without environmental water between 2018 and 2020 there would have been no salt exported from the basin. A return of high flows in 2021/22 exported 3.4 million tonnes of salt from the basin. River flow also arrests salt intrusion from the ocean into the Coorong. Without environmental water, the net import of salt into the Coorong would be in the order of 4 million tonnes over a five year period.
Environmental flows led to fresher conditions in the Coorong and an expansion of suitable fish habitat area. Without environmental water the habitat suitable for mulloway would have contracted by 38% over the three years. A similar trend is evident for other estuarine species. The delivery of environmental water plays a key role in exporting salt from the Murray Darling Basin but also arresting the ingress of seawater, which if left unchecked can lead to excessively high salinity in the South Lagoon.