Coupled Hydrodynamic and Habitat Suitability Models for Reach‐Scale Restoration of Wintering Waterbird Habitats in Rivers: A Case Study on Scaly‐Sided Merganser

ABSTRACT Rivers are pivotal ecosystems for migratory waterbirds, with the habitat preferences of piscivorous species heavily influenced by hydrodynamic factors such as water depth and flow velocity. These variables directly impact food availability and the foraging behaviours of waterbirds. While the integration of hydrodynamic and habitat suitability models has been effective in assessing river conditions and guiding restoration efforts, research specific to instream‐dependent piscivorous waterbirds at the reach scale is scarce. This research concentrates on the scaly‐sided merganser ( Mergus squamatus ), an endangered species on the IUCN Red List, identifying crucial habitat factors—water depth, flow velocity and human disturbance distance—based on their significance in shaping wintering habitat choices for this bird. Through a coupled hydrodynamic habitat suitability model deployed in a segment of the Xitiaoxi River in southeastern China, the study seeks to create a tailored model for the scaly‐sided merganser, assess distribution and landscape features of suitable instream habitats and suggest restoration and management tactics. Results show that despite the dry season revealing suitable areas for water depth (31.1%), flow velocity (16.8%) and human disturbance (80.5%) respectively, the combined suitable habitat only occupies 5.1%, predominantly in shallow regions like riffles and sandbanks. The study proposes strategies to enhance habitat suitability for the scaly‐sided merganser, presenting a pragmatic approach for reach‐scale river restoration, management and proactive conservation efforts beneficial for waterbirds. The findings are relevant not only for southern rivers in China but also for rivers in other countries that support migratory waterbirds and similar piscivorous waterbird habitats.