River regulation across south-eastern Australia has simplified channel morphology and altered seasonal flow regimes, reducing the hydraulic diversity required to support native fish across multiple life stages.
Habitat requirements for spawning, juvenile development and refuge depend on a mosaic of lotic habitat, adjacent slackwater areas, and strong velocity gradients along channel margins (Tonkin et al, 2024). Previous two-dimensional hydraulic modelling demonstrated these habitats respond non-linearly to discharge: spawning habitat generally increases with flow; slackwater habitat declines until higher geomorphic features engage (Streamology, 2023).
In straightened irrigation channels, such as Upper Gunbower Creek, environmental water alone is insufficient to recreate this diversity, with new physical habitat required. Instream restoration typically prioritises large wood, but this project adopted a novel rock-based approach – field observations suggested that native fish congregate around rocky outcrops and that wood–rock structures (e.g. modified fish hotels) often support higher fish diversity. We applied a fit-for-purpose 2D hydraulic model to quantify slackwater, spawning habitat and hydraulic diversity across flow scenarios.
Modelling results informed the location and design of rock blanket and pyramid structures and nesting variants. The simulations showed intermediate discharges maximised hydraulic diversity. The rock structures created localised velocity shear zones adjacent to refuge areas without measurable upstream afflux.
This transferable approach demonstrates how modelling-led, joint ecological – geomorphology disciplinary design can restore hydraulic diversity in regulated rivers. Construction commences later this year, followed by targeted monitoring to quantify fish habitat use and ecological responses to the installed habitat.