Meso-Scale Habitat Distribution and Suitability Evaluation in Large River Systems: A Case Study of the Middle Yangtze River

Meso-scale riverine habitat assessment is important for river ecological assessments and comprehensive river management. However, there remains a gap in meso-scale habitat evaluations in large rivers like the Middle Yangtze River. This study focuses on the Middle Yangtze River and selects typical reaches to conduct meso-habitat assessments. Hydraulic unit habitats are delineated based on flow velocity and water depth. Distribution and diversity of hydraulic units are analysed, together with their variation with discharges. Combining physical habitat model, the suitability of different ecological targets in different hydraulic units is analysed considering the factors of water depth, flow velocity and substrate composition. Key findings indicate that under low discharges, the proportions of different hydraulic units are evenly distributed, leading to high habitat diversity. In meandering and straight subreaches, with increasing discharge, the hydraulic unit composition shifts towards a dominance of deep-fast (D-F) units, resulting in reduced diversity. Meandering subreach exhibits higher diversity compared to straight subreach. In bifurcated reaches, the presence of mid–channel bar and floodplains contributes to higher diversity after flooding. The results of physical habitat model reveal differences in suitability across different ecological targets and different hydraulic units and find that hydraulic units of deep-slow (D-S) and shallow-slow (S-S) are highly important habitat types. The absence of these habitats could negatively impact certain ecological targets, such as macroinvertebrates, juvenile fish and Chinese sturgeon. Therefore, it is essential to maintain these habitats during river management. In contrast, other hydraulic units could be replaced reciprocally because of their similarities in habitat suitability. The findings of this research could serve as a reference for future river management and habitat conservation efforts.