Figure 7 Erosion/deposition masses for the main channel and floodplain of the LYR during 2000-2017.
Here, we illustrate the change in the masses of erosion and deposition in the main channel and the floodplain for the different reaches (see Figure 7). During the period from 2000-2017, the mass of eroded sediment in the main channel was approximately 10.33×108 t, with an average annual erosion rate of 0.61×108 t/yr, while the mass of eroded sediment in the floodplain reached approximately 3.46×108 t, with an average annual erosion rate of 0.20×108 t/yr. The main channel erosion has been caused mainly by low sediment flow, and floodplain erosion may have been caused by human activities such as sand mining and land use.

4.1.2 Floodplain erosion and deposition

The total area of floodplains in the braided reach (between HYK and GC) is nearly four times that of the main channel, but the intensity of topographic adjustment of floodplains is lower than that of the main channel. Figure 8 shows the annual deposition and erosion volumes of the main channel and floodplains between 2000 and 2017. The main channel was fully scoured, with a total erosion volume of 8.38×108m3 and an average annual erosion volume of 49×108 m3/yr. However, sedimentation was the main mode in the floodplain before 2002, and the floodplain was dominated by erosion after 2002, with the exception of a few areas where sediment deposition occurred; the other areas of floodplains were scoured.
During the period from 2000 to 2002, the cumulative depositional volume in the floodplain between HYK and GC was approximately 2.65×108 m3, with an average annual deposition of 1.32×108 m3/yr. In addition, the average annual deposition of each reach was approximately 0.09×108 m3/yr; the right floodplain deposition rate (0.12×108m3/yr) was larger than that of the left floodplain (0.07 ×108m3/yr). During the operation of the XLD reservoir and the implementation of the WSRS, the cumulative erosion was 5.73 ×108m3 in the floodplain between 2002 and 2017, with an average annual erosion value of 0.34 ×108m3/yr, of which 0.11 ×108m3/yr occurred in the left floodplain and 0.23 ×108 m3/yr in the right floodplain.