# Characteristics of Channel Incision Induced by Human Activity in a Wandering Reach in 20 Years
## 4 Results
### Changes in River Bed Downcutting Depth at the Section- and Reach-Scales  
To investigate the characteristics of channel incision spatially and temporally in the wandering reach, the average bed elevations and the cumulative river bed downcutting depths at 28 sedimentation sections were calculated from 1999 to 2018 (Figure 3). The maximum value of \(Z_{j}\) was about 4.2 m and it occurred in the Xiagujie section, which is located approximately 31.3 km downstream of the XLD dam. The minimum value of \(Z_{j}\) was approximately 1.1 m and it occurred in the JHT section. Therefore, the cumulative downcutting depth at the section-scale varied rearkably along the wandering reach. The magnitude of channel incision was affected by the flow and sediment regime and channel boundary conditions at different sections. Figure 4 shows variations in the annual downcutting depths in three sub-reaches of the wandering reach from 2000 to 2018. It shows that channel incision began at the initial operation of the reservoir in Reach 1 (above HYK) and Reach 2 (HYK-JHT), but it began from 2001 in Reach 3 (JHT-GC), which indicates that there was a delayed response of channel downcutting to the reservoir operation downstream in this reach. In Reach 1, the maximum annual downcutting depth was approximately 0.54 m in 2000 and the channel underwent deposition in 2012 and 2018, with the deposition depths of 0.22 m and 0.36 m. In Reach 2, \(\Delta\overline{Z}\) reached up to the maximum value in 2005, with a magnitude of 0.58 m, and the channel underwent deposition in 2008, 2017, and 2018. In Reach 3, the annual downcutting depths were relatively large in 2003 and 2018, with the values of 0.69 m and 0.53 m, and the channel deposition depths in 2000, 2006, 2012, and 2017 were 0.05 m, 0.02 m, 0.08 m, and 0.05 m, respectively. In terms of the whole wandering reach, the maximum value of \(\Delta\overline{Z}\) was 0.45 m in 2003, because several floods occurred in the wandering reach in this year. The annual downcutting depths in the whole reach in 2000-2005 were greater than those in the later periods, which revealed that more intensive channel incision occurred in the initial years after the reservoir operation. According to the calculated data, the magnitude of the accumulative downcutting depth in the wandering reach was approximately 2.9 m from 1999 to 2018. In three sub-reaches, the magnitudes of \(\overline{Z}\) were 2.6 m, 3.0 m, and 3.2 m, respectively, which revealed that the channel incision in the Reach 3 was more severe in the recent 20 years. Significant river bed downcutting was also reported in other alluvial rivers owing to upstream damming. [PERSON] et al. (2019) found that the last 565 km of the Lowermost Yangtze River scoured about 1.2 m in bed elevation on average from 1998 to 2013. [PERSON] et al. (2018) investigated channel changes in the Saskatchewan River, and it indicates that the mean lowering of bed elevation reached 2 m within ten years after the E.B. Campbell Dam construction.  
Figure 2: Channel scour volumes in the wandering reach from 2000 to 2018.