Fishes are commonly used as environmental flows indicators in regulated rivers; but what happens when a focal catchment has negligible historic scientific survey data or location specific applied research? We provide a case study based on pioneering surveys and conventional acoustic telemetry work in the Peel River, Australia. A platform of unexpected results, including limited movement of large-bodied native species during major river discharge events and the composition of the resident fish assemblage, which is used to identify opportunities and inform environmental flow delivery. More broadly, we use this experience to discuss the continuum spanning data deficiency and incomplete knowledge while attempting to pragmatically inform water delivery.