As a prerequisite and foundation for studying the evolution mechanism of river channels, an in-depth understanding of the cross-sectional morphology adjustment is required. As a starting point, it is crucial to systematically summarize and generalize the research findings on channel morphological adjustment obtained to date, particularly in the context of the significant changes in the water and sediment conditions of large rivers that have occurred worldwide. This paper provides a comprehensive review of the research findings on the three following aspects of the Lower Yellow River: the transverse distribution of overbank flow velocity, the transverse distribution of suspended sediment concentration, and the morphological adjustment of the river cross-section. There are various equations available to predict the lateral depth–average flow velocity distribution. These equations are classified into the two following categories: empirical and theoretical formulas. Theoretical formulas are obtained through consideration of the cross-sectional morphology, accounting for inertial force terms caused by secondary flow, and momentum transfer between the main channel and its floodplain. Similarly, empirical equations and theoretical formulas for sediment concentration transverse distribution are also summarized, given the different influencing factors and assumptions. We also discuss the morphological adjustment of river cross-sections based on the analysis of measured data, mathematical model calculation, and the physical model test. In particular, we propose the idea of revealing channel cross-section morphology evolution mechanisms from the theoretical level of water and sediment movement and distribution. This review aims to enhance understanding of overbank flow, sediment transport, and channel morphology in the Lower Yellow River and may also serve to some extent as a reference for the evolution and management of channels in other rivers.

The Xiaobeiganliu reach is a typical wandering reach of the Middle Yellow River that has rapid and significant channel lateral migration, which may threaten the safety of riparian land and flood control structures. To investigate the characteristics and mechanism of lateral migration in the Xiaobeiganliu reach, the temporal and spatial variations in bankfull width and thalweg migration distance were identified during the continuous deposition period, quantitatively analyzing the effect of different boundary conditions on the lateral migration index. The reach-scale bankfull width decreased by 32% from 1986 to 2001 because hyperconcentrated floods often occurred in this reach. The thalweg migration distance varied dramatically at cross-sections, with the maximum annual thalweg migration distance reaching 4290 m. The lateral migration index of the Xiaobeiganliu reach responded well to the upstream and downstream boundary conditions. The previous 3-year average water discharge and 4-year average sediment concentration at the upstream station were two key fluvial factors influencing lateral migration, with the relation being established between the lateral migration index and the two parameters. The Tongguan (TG) elevation was the key downstream boundary condition affecting thalweg migration, and a power function was proposed between the lateral migration index and the variations in annual TG elevation.

The wandering reach of the lower Yellow River has undergone significant channel degradation since the Xiaolangdi Reservoir operation, with a cumulative channel scour volume of 14.1 × 10⁸ m³ in 1999–2018, and severe channel scour has resulted in rapid channel incision in this reach. The annual and cumulative river bed downcutting depths at section- and reach-scales and water stages at low and high flows were calculated to investigate the characteristics of channel incision quantitatively. The results show that the cumulative river bed downcutting depths at sedimentation sections varied significantly, with the magnitude varying between 1.1 m and 4.2 m. The cumulative reach-scale river bed downcutting depth reached up to 2.9 m and water stages at low flows decreased by more than 3.3 m at hydrometric stations. The previous 6-year average fluvial erosion intensity is the major influencing factor controlling channel incision, and empirical functions were established between cumulative river bed downcutting depths and the previous 6-year average fluvial erosion intensity in the wandering reach and three sub-reaches. The data calculated using the proposed equations agreed well with the observed downcutting depths, so these equations can be used to calculate the processes of channel incision in the recent 20 years in the wandering reach.