River regulation can considerably alter flow regimes and impact aquatic ecosystems, yet these effects remain poorly understood in largely unregulated catchments such as the Richmond River, New South Wales. This study investigates the ecological response to flow alteration downstream of Toonumbar Dam, focusing on freshwater turtles, fish, and macroinvertebrates. Surveys were conducted across 10 sites in 5 rivers, comprising regulated and unregulated systems, using baited remote underwater video (BRUV), AUSRIVAS habitat assessments, and macroinvertebrate sampling. Hydrological metrics were derived from gauging data to examine flow–ecology relationships.
The eastern saw-shelled turtle (Myuchelys latisternum) was the most frequently recorded species, while Chelodina longicollis was not detected. Turtle abundance was generally low but comparable between regulated and unregulated sites, although absent at one regulated downstream location. Fish communities exhibited low to moderate diversity, with the highest abundance and diversity recorded immediately downstream of the dam. Macroinvertebrate assemblages indicated moderate to poor ecological condition, with evidence of sedimentation impacts in riffles and nutrient enrichment in edge habitats.
Overall, early results suggest that while aquatic fauna persist in regulated reaches, flow regulation and associated habitat changes may influence community structure and ecological condition. This study provides a baseline for ongoing monitoring and informs future environmental flow management in the Richmond River catchment.