Impacts of channel complexity and main-to-anabranch width ratio on fish habitat suitability across varying flow regimes

The diversity of river morphology serves as a critical foundation for maintaining riverine biodiversity and ecosystem health. However, current research has predominantly focused on comparisons among different channel types, overlooking the interactive effects of specific channel morphological characteristics and flow regimes on habitat suitability. This study takes the typical anastomosing river—the Haba River—as a case study. By constructing a coupled hydrodynamic and fish habitat model, it systematically analyzes the effects of channel complexity and the main channel-to-anabranch channel width ratio on the habitat quality of Thymallus arcticus under different flow conditions. The results indicate that the influence of channel complexity is flow-dependent: under normal flow conditions, morphologically complex channels exhibit lower overall habitat suitability for Thymallus arcticus. This lower suitability is due to these channels' characteristically lower flow velocity and water depth, which are conditions unsuited for a species that prefers high-flow environments. However, during extreme flood events, complex channels provide more effective refugia, thereby demonstrating higher habitat quality. Regarding the river width ratio, the study reveals that the presence or absence of anabranching channels as a structural feature has a far greater impact on habitat quality than specific variations in the width allocation between the main channel and anabranches. Over the long term, habitat suitability remains relatively stable under the specific channel morphology. Nevertheless, the simpler morphology of non-anabranching channels provides higher flow velocities and greater water depths during sustained flow periods, conditions which are more suitable for Thymallus arcticus. This research emphasizes that river ecological restoration strategies should move beyond static channel classification and fully account for the combined effects of morphological structure and hydrological regimes. For species that prefer high-flow environments, the potential benefits and drawbacks of increasing channel complexity must be carefully evaluated across different flow scenarios to enhance the scientific rigor and foresight of habitat management strategies.