Unsteady Secondary Flow Structure at a Large River Confluence

River confluences, which are characterized by complex hydrodynamics, are key nodes for flood control and environmental protection. Two field surveys were carried out at the confluence of the Yangtze River and Poyang Lake to investigate the transient character of flow structures, which are often assumed steady. Repeat-transect acoustic Doppler current profile measurements were processed and analyzed, adopting a new method to separate mean flow from turbulence and measurement error based on physics-informed generalized Tikhonov regularization. The two field surveys were characterized by two distinct mixing interface modes: A so-called Kelvin–Helmholtz (KH) mode, and a wake mode. In KH mode, large-scale flow fluctuations were observed. These flow fluctuations exert a substantial influence on the alternation of secondary flows by changing the water surface pressure gradient, affecting the intensity of secondary flows rather than their spatial structure. We infer that temperature-induced stratification is the main cause of this. In the wake mode, multiple vortices in the wake region at the confluence apex also produced flow fluctuations, directly related to the primary velocity gradient. We argue that even for constant incoming flows, secondary flow at river confluences can exhibit channel-scale unsteadiness related to migration of the turbulent mixing interface. Our findings highlight the crucial role of density effects in regulating secondary flow unsteadiness, which is essential for understanding contaminant and sediment dispersal in river systems.