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The Yellow River flows through multiple provinces in China, shaping the North China Plain, the largest alluvial plain in China. As the control node of basin ecological environment, the confluence of Weihe River and Yellow River is deemed as the gateway to North China Plain. In this study, a numerical simulation of the Weihe River–Yellow River confluence is conducted using a 2D hydrodynamic model and a coupled transport model for dissolved oxygen–biochemical oxygen demand. The results show that: (i) The typical flow field with multiple backflow areas is formed at the stagnant area where main stream and tributary converge and abrupt channel change area in different hydrological periods. The spur dike here mainly affects the velocity of the Weihe River outlet. (ii) There is an obvious concentration transition mixing zone downstream of the confluence, and the width of the mixing zone gradually linear increases with the direction of water flow. (iii) The self‐purification ability of the confluence is strongest in dry period, weaker in level period, and weakest in wet period. Water bodies have stronger self‐purification capacity on riverbanks than in the middle, and it is stronger in the upper reaches of Weihe River compared to Yellow River. Lower reaches also have a stronger self‐purification capacity than upper reaches. The study results can serve as a scientific reference for protecting the ecological environment of the Yellow River. [ABSTRACT FROM AUTHOR]

• A model of DO transport applicable to the confluence was constructed. • The mechanism of secondary flow merger and decay at confluence was obtained. • The relationship between DO mixing and hydrodynamics at confluence was found. • The interaction of secondary flow and DO at confluence may affect the habitat there. In the natural river network, the confluences play an important role as control nodes. It is of great scientific value to study the transport and mixing process of water quality at the confluence of the mainstream and tributaries to improve the health of the river environment. In this study, dissolved oxygen (DO, the essential survival conditions for aquatic organisms) was used as a water quality index, and a 3D transport model of DO considering atmospheric reoxygenation and biodegradation oxygen consumption was constructed. The hydrodynamic mechanism of DO transport, mixing and reoxygenation at confluence under different flow ratios, junction angles and water temperature conditions was studied. The results show that the secondary flow is the main driving force of the transversal mixing at confluence, and there is a high coupling relationship between the evolution of the secondary flow and the mixing process of water quality at confluence. With the increase of flow ratio and junction angle, the intensity of secondary flow at confluence shows an exponential increase, which in turn promotes the transversal mixing efficiency of DO at confluence. The water quality of the polluted confluence under different water temperature conditions showed a trend of gradual recovery, but the higher the water temperature, the weaker the recovery rate. This study can provide theoretical support to deal with the threat to aquatic organisms caused by pollution accumulation at confluence, and provide scientific basis for the improvement of the water environment and water ecology protection in the river confluence and downstream. [ABSTRACT FROM AUTHOR]

[Display omitted] • The 3D hydrodynamic and DO transport model at confluence were constructed. • The atmospheric oxygen mass transfer characteristics of confluence were proposed. • The equations of hydrodynamic and DO transport at confluence are found. As the control cell of river networks, the complex hydrodynamic conditions at river junctions pose significant challenges to pollution control and water ecology protection. The purpose of this study is to construct a 3D hydrodynamic model for dissolved oxygen (DO) transport at the confluence, studying its unsteady mechanism under different flow ratios and junction angles. The results of the study show that: i) The atmospheric reoxygenation distribution at the confluence exhibits a typical layered 3D feature, and the strong flow turbulence therein exerts a significant promoting effect on such reoxygenation. ii) The DO transport pattern at the confluence is characterized by the formation of an arc-shaped concentration distribution zone, followed by a contraction towards the opposite bank of the tributary and downstream transportation. A delay in DO concentration increase occurs within the separation area, while lateral oscillations are observed during downstream transport. iii) The flow ratio primarily governs the lateral migration extent of DO, while the junction angle serves as the main driver for downstream transport rate of DO. Increasing both flow ratio and junction angle can expedite mixing efficiency of DO at the confluence. The findings of this study can further enhance the hydrodynamic mechanism for water quality control at the confluent flume, and serve as a valuable reference for improving water environments and protecting water ecology in natural river confluence. [ABSTRACT FROM AUTHOR]