Habitat Unit Mapping in Large River: Combining Ecological Opinions and Hydrodynamic Modelling to Inform River Management

ABSTRACT Restoration of complex river ecosystems requires an understanding of the availability of habitat for populations and multi‐scale choices. Habitat unit (HU) mapping classifies river habitats, aiding in establishing the connection between the physical and biological conditions of rivers. Mid‐scale HU classification can effectively predict fish habitat utilization patterns and has been developed and applied in numerous small to medium‐sized river classifications. However, the delimitation of habitat units (HUs) for large rivers remains in its preliminary stages. This study used a two‐dimensional (2D) hydrodynamic numerical model to calculate hydrodynamic data and forms ecological expert opinions based on the swimming abilities of typical fish species and geomorphic characteristics. A HU mapping procedure was constructed to describe the mesohabitat heterogeneity of HUs in the fluctuating backwater areas of the upper Yangtze River. Results indicated: (1) Rich diversity of habitats in the investigated river segment. HUs exhibit evident patterns in their planar, longitudinal, and lateral distribution. (2) Discharge magnitude influences the stability of HUs, leading to transitions between HU types. During the storage and falling periods, no dominant HU was observed, whereas the fast channel unit dominates during flood period, resulting in a reduction of the preferred habitat of fish. (3) Field monitoring of fish communities validates the potential of HU mapping in describing habitat utilization, which HUs such as pools and riffles were favoured by fish, while fast channel and slackwater unit were actively avoided, confirming the efficacy of the procedure. This research holds significant implications for habitat restoration in river management.