## 2 Study Area and Data Collection
### Data Collection  
To investigate the incoming flow and sediment regime entering the Xiaobeiganliu reach, we collected hydrological data of LM station in 1960-2014 from the Yellow River Conservancy Commission of China (YRCC), and data included the daily mean discharge and sediment concentration. Figure 2 shows variations in the annual water discharge and sediment discharge at LM during hydrological years and flood seasons. From 1960 to 1973, the average annual water volume was approximately \(310.8\times 10^{8}\) m\({}^{3}\)/yr, with an average annual sediment amount of \(10.6\times 10^{8}\) tons. In 1974-1985, the average annual water volume did not change much, but the average annual sediment amount decreased to \(6.5\times 10^{8}\) tons due to soil and water conservation measures on the Loess Plateau. During the period from 1986 to 2002, the Longyangxia and Liujiaxia reservoirs on the upper Yellow River operated jointly. The average annual water volume entering the Xiaolangdi reach decreased to \(197.2\times 10^{8}\) m\({}^{3}\)/yr during this period, with the flood volume accounting for approximately 42%. The average annual sediment amount decreased to \(4.7\times 10^{8}\) tons, with the flood amount accounting for 81%. In 2003-2014, the average annual water volume did not change much, but the average annual sediment amount decreased dramatically to \(2.1\times 10^{8}\) tons because of the implementation of the water and soil conservation and the construction of various sediment trap dams in the Middle Yellow River.  
Figure 2: Temporal variations in the flow and sediment regime entering the Xiaobeiganliu reach: (a) annual water discharge; (b) annual sediment discharge.  
According to the observed data, the channel evolution process in the Xiaobeiganliu reach could be divided into four stages (Figure 3a): rapid deposition period (1960-1973), quasi-equilibrium period (1974-1985), continuous deposition period (1986-2002), and slow scour period (2003-2015), and the process was characterized by deposition in flood seasons and scouring in non-flood seasons. From 1960 to 2015, the total cumulative channel evolution volume in the Xiaobeiganliu reach was approximately \(20.2\times 10^{8}\) m\({}^{3}\), with a cumulative deposition volume of \(39.6\times 10^{8}\) m\({}^{3}\) in the flood season and a cumulative scour volume of \(19.4\times 10^{8}\) m\({}^{3}\) in the non-flood season. The TG elevation is the erosion datum of the Xiaobeiganliu reach, which has an important influence on the fluvial processes of this reach. The post-flood TG elevations from 1960 to 2015 were also collected from the YRCC. Figure 3b shows that the TG elevation increased rapidly after the operation of the Sanmenxia Reservoir, and the elevation increased from 323.4 m in 1960 to 326.6 m in 1973. The TG elevation increased to 327.6 m in 1979 and then decreased to 326.6 m in 1985 during the quasi-equilibrium period. During the continuous deposition period, the TG elevation continued to increase from 327.2 to 328.8 m. From 2003 to 2015, the Sanmenxia Reservoir carried out a prototype experiment, and the maximum operating water level in the non-flood season was controlled below 318 m. During this period, the TG elevation decreased slowly, reaching 327.7 m in 2015.  
Channel aggradation processes shrank the main channel significantly in the Xiaobeiganliu reach, which may bring potential hazards to the safety of flood control. It is more important to investigate the morphodynamic adjustments when it underwent channel aggradation processes than other periods. Therefore, we collected post-flood cross-sectional profiles at 25 sedimentation sections in the Xiaobeiganliu reach in 1986-2001 from the YRCC to investigate channel lateral migration during the continuous deposition period.  
Figure 3: Variations in the cumulative channel evolution volume and the base level: (**a**) cumulative channel evolution volume; (**b**) Tongguan (TG) elevation.