Abstract
Spanning the Yangtze River of China, the Three Gorges Dam (TGD) has received considerable concern worldwide with its potential impacts on the downstream side of the dam. This work investigated the spatio-temporal variations of suspended sediment concentration (SSC) at the downstream section of Yichang-to-Chenglingji from 2002 to 2015. A random forest model was developed to estimate SSC using MODIS ground reflectance products, and the spatio-temporal distributions of SSC were retrieved with this model to investigate the characteristics of water-silt variation. Our results revealed that, relatively, SSC before 2003 was evenly distributed in the downstream Yangtze River, while this spatial distribution pattern changed ce 2003 when the dam started storing water. Temporally, the SSC demonstrated a W-shaped curve of seasonal variation as one peak occurred in September and two troughs in March and November, and showed a significantly decreasing trend after three-stage impoundment. After official operation of the TGD in 2009, the SSC was reduced by over 40% than before 2003. Spatially, the most significant changes occurred in the upper Jingjiang section, where the SSC dropped by 45%. During all stages of impoundment, the water impoundment to 135 m in 2003 had the most significant impact on suspended sediment. The decreased SSC has led to emerging risks of bank failure, aggravated erosion of water front and aggressive down-cutting erosion along the downstream of the dam, as well as other ecological and environmental issues that require urgent attention by the government.
Highlights
Suspended matters in river water mainly consist of sediments
After the Three Gorges Dam (TGD) official operation in 2010, Suspended sediment concentration (SSC) remained stable in most sections, except for some curved reaches (Figure 2e)
A W-shaped seasonal variation curve could be roughly identified for the SSC at the downstream of the dam, with the peak occurred in September and two troughs in March and November
Summary
Suspended matters in river water mainly consist of sediments. Suspended sediment concentration (SSC) affects the transmission of light through the water column, transport of pollutants and heavy metals, and production of water-column phytoplankton and other aquatic vegetation [1,2,3]. Suspended sediment in natural waters and their associated processes play a critical role in shaping the physical riverscape and regulating the associated ecological systems [4,5]. In accordance with the operation strategy of “storing clear water and releasing the mud,” the clear water discharged over the spillway of the dam vigorously scours the riverbeds and riverbanks [8]. This process exerts a far-reaching impact on the ecological environment along the downstream from the TGD—bank collapse, soil erosion, and sediment deposition, among others. Understanding suspended sediment distributions and their long-term changes in the downstream of the TGD would be a prerequisite to explain how the suspended-sediment changes over time and space under a changing climate and increased human activity [9,10]
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