The establishment of ecologically responsive environmental flow regimes is a challenge for river managers. Biological communities in river environments are partly constrained by hydraulic characteristics such as water depth, flow velocity and turbulence, so a key step in understanding the links between discharge and biotic response is to relate discharge and hydraulic character. This paper explores the nature of the relationship observed between discharge and hydraulic character in the Cotter River, Australia. Surface flow types or hydraulic patches, expressing a variety of hydraulic characteristics, were mapped in six reaches in this upland gravel-bed river, during 18 different flow releases ranging from 8 to 260 ML per day. The distributions of these hydraulic patches were variable, both spatially and over time. The patches represent hydraulic habitats (of varying depth, flow velocity and turbulence) the diversity of which may contribute to in-stream biodiversity. The study identified a range of flow releases that produce the greatest in-stream diversity of hydraulic habitats, which, if included in managed release strategies, potentially benefits in-stream biodiversity. Copyright © 2006 John Wiley & Sons, Ltd.