Flood-driven hydraulic-habitat indicators reveal geomorphic associations and functional complementarity across Contrasting River morphologies

Abstract

Riverine topographic diversity is widely associated with spatial and temporal variation in hydraulic habitat under flood disturbance. Yet, most existing habitat assessments remain limited to static or quasi-steady flow conditions, restricting the development of indicators that capture habitat responses across complete flood cycles. This study examined the flood-prone Leshan reach of the Min River, using the round-mouthed copper fish ( Coreius guichenoti ) as the indicator species. A flood-cycle hydraulic-habitat assessment framework was developed by integrating hydrodynamic–habitat coupling with the Regional Integrated Suitability Index ( RII ), Hydraulic-Habitat Stability Index ( ESI ), Short-Term Hydraulic-Habitat Recovery Index ( EI ST ), and geomorphic-complexity indicators. Results show that river morphology is strongly associated with hydraulic-habitat dynamics: mid-channel bar reaches exhibit higher ESI under flood disturbance and a greater capacity to sustain suitable hydraulic habitat over time, whereas single-channel reaches exhibit lower hydraulic-habitat stability during intense flood disturbance but higher EI ST by the common event-end time. These contrasting responses indicate temporal functional complementarity at the hydraulic-habitat level in maintaining suitable hydraulic conditions and achieving short-term recovery. The Mid-Channel Bar Complexity Index ( MBCI ) is positively associated with ESI and captures additional geomorphic information beyond individual metrics under intermediate flood conditions, reflecting the combined contributions of mid-channel bar abundance, local topographic heterogeneity, and effective flow space. Overall, contrasting river morphologies have the potential to provide functionally complementary hydraulic-habitat responses under flood pulses. The framework provides a quantitative basis for identifying reaches with greater potential to maintain or recover spawning-suitable hydraulic conditions in flood-prone mountainous rivers.