# Characteristics of Channel Incision Induced by Human Activity in a Wandering Reach in 20 Years
## 5 Discussion  
Generally, the incoming flow and sediment regime were regarded as the major factors controlling channel evolution in alluvial rivers [37]. The temporal variation in channel adjustments is usually represented by functions of discharge, sediment concentration, and fluvial erosion intensity when significant channel scour occurred in alluvial rivers [34; 38; 39]. In this study, the fluvial erosion intensity (\(\eta\)) was used to represent the flow and sediment regime, and it can be expressed by \(\eta=\overline{Q}^{2}/\overline{S}/10^{4}\), where \(\overline{Q}\) represents the annual mean flow discharge and \(\overline{S}\) represents the annual mean sediment concentration. In order to investigate the influence of altered flow and sediment regime, the relationship between cumulative downcutting depth in the wandering reach and the annual fluvial erosion intensity at the HYK station was established during the study period, as shown in Figure 6a. It indicates that \(\overline{Z}\) increased with an increase in \(\eta\), and the relationship can be fitted for a logarithmic model, with a high square of correlation coefficient (\(R^{2}\)) of 0.87. Previous studies also indicate that channel adjustments in alluvial rivers are a result of the cumulative effect of several consecutive years' flow discharge and sediment load conditions [3; 39; 40]. Considering the cumulative effect, the relationships between \(\overline{Z}\) and the previous \(n\)-year average fluvial erosion intensity were established, and it is found that the corresponding correlation degree attained the maximum value at \(n=6\). The relationship between \(\overline{Z}\) in the wandering reach and the previous 6-year fluvial erosion intensity (\(\overline{\eta}_{6}\)) at the HYK station is shown in Figure 6b. The relationship was fitted for a logarithmic function: \(\overline{Z}=0.7485\times\ln(\overline{\eta}_{6})-0.2532\), with a corresponding \(R^{2}\) of approximately 0.98. This reveals  
Figure 5: Variation in the stage–discharge curves and water stages from 1999 to 2018: (**a–c**) stage–discharge curves at the HYK, JHT, and GC stations; (**d–f**) water stages at low and high flows at the HYK, JHT, and GC stations. that the previous 6-year average fluvial erosion intensity was the major parameter controlling channel incision in the wandering reach in the recent 20 years. The annual mean flow discharges and the annual mean sediment concentrations at the HYK, JHT, and GC stations were used to calculate \(\overline{\eta}_{6}\) in Reach 1, Reach 2, and Reach 3, respectively. Figure 7 shows the relationships between \(\overline{Z}\) and \(\overline{\eta}_{6}\) in three sub-reaches, and it indicate that the previous 6-year average fluvial erosion intensities were closely related to the cumulative river bed downcutting depths, with all of the corresponding square of correlation coefficients above 0.92 using logarithmic functions. Therefore, the previous 6-year average fluvial erosion intensities at three hydrometric stations can be used to calculate \(\overline{Z}\) in the wandering reach and these sub-reaches during the study period. Figure 8 shows comparisons between the observed reach-scale cumulative river bed downcutting depths and the calculated data using established functions in the wandering reach and three sub-reaches. It shows that the changing processes of the calculated cumulative downcutting depths agreed well with those of the observed data during the study period. However, there were some differences between the calculated cumulative downcutting depths and the observed data in some years. This indicates that channel incision in the wandering reach can be affected by other factors, although \(\overline{\eta}_{6}\) was the key parameter. For example, continuous channel incision in the wandering reach of the LYR can cause the river bed armoring. According to statistics, the median diameters of bed material at the HYK, JHT, and GC stations increased by 265%, 98%, and 70%, respectively, from 1999 to 2018.