Environmental water needs for the Daly River

As one of the few rivers in the Top End that flows all year round, the Daly River supports astounding freshwater biodiversity, including more than 90 species of fish, 8 of the 9 freshwater turtle species found in the Northern Territory, and 3 elasmobranch species of conservation significance. Many of these species, as well as the Daly River and catchment themselves, hold significant value for the Aboriginal Traditional Owners of the area.

The Daly’s water resources are attracting increasing attention from the agricultural, forestry and mining industries. This project aimed to collate existing knowledge and generate new knowledge on critical flow-ecology relationships and risk-based scenario models for key environmental assets in the Daly River, to support the sustainable management of its water resources.

The Daly River is in the Top End of the Northern Territory.

• Modelling and mapping of water flows during both the wet and dry seasons has highlighted areas and habitats that are vulnerable to water extraction at different times of year and under different flow conditions.
• The dry-season food web is supported by productivity from the river itself (e.g. in-stream algae) as well as from the floodplain and marine areas – so maintaining connectivity is critical. In particular, water extraction during the dry season could reduce the productivity of algae and aquatic plants that form the base of the food web.
• Juveniles of key fish species, including barramundi and sooty grunter, are likely to be impacted if water is taken from the river, particularly during the dry season. Juvenile sooty grunters prefer to live in shallow, fast-flowing riffle habitats which shrink in the dry season, and there’s likely to be fewer juvenile barramundi no matter when water is extracted.
• The iconic pig-nosed turtle may also be vulnerable to changes in flows, which could potentially reduce their access to nesting, feeding and basking areas. Pig-nosed turtles especially rely on the thermal groundwater springs that flow into the river, using them to stay warm during the colder dry-season months.
• All hydrological seasons and catchment locations are important for fish breeding and the survival of young fish, with the majority of species breeding in the wet season.
• Barramundi need connectivity between the river and ocean to complete their spawning and dispersal migrations, and also need connectivity between the river and its floodplains for feeding during the wet season.

An extract of the Daly River environmental flow considerations as outcomes from this study. A full PDF of this table is available in this stand-alone document and in the project’s final report.

• This study provided important correlative evidence of the importance of wet-season and dry-season flows for freshwater fish movement, spawning and recruitment in the Daly River catchment. Further research on the mechanisms, such as food and habitat availability, and predator control would be important for future research.
• Fish populations were monitored annually in the Daly River over 2006–2019. These data have provided critical baseline information on the status and variability of populations over varying flow conditions. Future monitoring should be reinstated in the coming years to assess and ensure that current and future water developments in the region do not affect the fish population viability.
• While the study detected movements of turtles during the wet season in the main channel, further research should be conducted on understanding the importance of floodplain habitat use by turtles during the wet season.
• There has been limited research on the general ecology (e.g. habitat use, diet movements) of hatchling and juvenile pig-nosed turtles. Given the lack of evidence of recruitment of turtles, research on drivers of recruitment success and failure should be a particular focus.
• The ecological importance of riverbed thermal springs in the Daly River requires urgent further investigation. This study has suggested these areas are likely to be important for growth and metabolism for fish and pig-nosed turtles. The location and extent of influence of these areas also needs to be determined.
• A multi-sensor approach should be used for future investigations of floodplain inundation in the region. Analysis should integrate both Sentinel-1 (SAR) and Sentinel-2 (multi-spectral) data into a framework with the Landsat archive to provide higher spatial and temporal resolution.
• Satellite image analysis can provide reliable estimates of surface water areal extent, but has limited ability to infer depth and volume without the use of a high-quality digital terrain model. We recommend the prioritisation of high-resolution airborne LiDAR surveying of the river channels and floodplains for future research, to make these linkages explicit.