Effects of Three Gorges Dam operation on spatial distribution and evolution of channel thalweg in the Yichang-Chenglingji Reach of the Middle Yangtze River, China

Abstract The evolution of the channel thalweg and pool-riffle sequences is a critical feature of morphological adjustments in fluvial systems and might have important effects on aquatic habitat quality, channel stability and navigation. Based on the 220 cross-sectional profiles from 2002 to 2016, residual depths and spatial statistical analyses were used to quantitatively analyse the vertical variations and longitudinal distribution of the pool-riffle sequences and channel thalweg in the Yichang-Chenglingji Reach (YCR) of the Middle Yangtze River, which consists of the Yizhi-Zhijiang, Shashi-Gongan and Shishou-Jianli sub-reaches. The results showed that the dramatic reduction in the sediment loads following the construction of the Three Gorges Dam (TGD) has caused significant adjustments in the size, morphological diversity and spatial distribution of pool-riffle structures in the YCR. The average depth of the pools in the Yizhi-Zhijiang Reach and the Shishou-Jianli Reach overall exhibited a nonlinear growth trend, with the average residual depth increasing by 1.4 m and 1.9 m, respectively. However, the average residual depth in the Shashi-Gongan Reach showed a decreasing trend, with a reduction of approximately 1.2 m. The longitudinal lengths of the riffles in all three reaches contracted to different degrees, and the specific percentage of longitudinal river length occupied by riffles (total riffle length/total sub-reach length) decreased by 8%, 9.6% and 5.9% in the three sub-reaches from upstream to the downstream. In addition, the morphological diversity of the thalweg elevation in the Shashi-Gongan Reach and Shishou-Jianli Reach weakened after the construction of the TGD, while the bed topography in the Yizhi-Zhijiang Reach became more complex and diverse. Moreover, the pool-riffle structures in the YCR were rearranged and distributed regularly following the initiation of the TGD operation, although the variation characteristics of the average pool spacing in the three sub-reaches were quite different. Quantitative relationship between the average residual depth and the previous four-year hydrological condition was proposed for the three sub-reaches based on the Delayed Response Model (DRM) and verified using the measurements from 2013 to 2016.