Hydraulic complexity at a laboratory-scale confluence: Experimental distribution of bioenergetics metrics and its relation to Yangtze finless porpoise habitats

Confluences are key features in river networks and serve as critical nodes for ecological patterns, often exhibiting rich biodiversity. However, our current understanding of how hydrodynamics influences these patterns remains limited, as most previous studies have focused on ecological factors alone. This study applied a Particle Image Velocimetry system to investigate the spatial distribution of hydraulic complexity metrics (M1, M2, and M4) in a laboratory-scale confluence under both steady and unsteady flow conditions. These metrics, derived from velocity measurements, have been applied in previous studies to characterize aquatic species abundance, richness, and motion. M1 was found to effectively delineate the different hydrodynamic zones of the confluence, peaking within the shear layer. Additionally, M1 can identify the boundary areas with steep velocity gradients inside the separation zone. Field observations of finless porpoises at various confluences in the Yangtze River suggested that the species tend to prefer areas with low M1, such as flow separation and stagnation zones during the flood season, and more generally the confluence area during the dry season. This preference is likely related to their feeding. The study contributed to a better understanding of the ecological dynamics in the Yangtze River Basin, as well as to the conservation of the endangered finless porpoise.