| Due to reduced sediment input under anthropogenic and natural impacts, most of worldwide river deltas suffered erosions over the past one century. Here we presented a not often-seen case:a large river estuary continued to progradate seaward despite substantially reduced sediment input. The Three Gorges Reservoir (TGR) on the upper Yangtze, the largest dam ever built in the world, launched its operation in 2003 and the TGR sedimentation in 2009 reached about 972 million tons (Mt), with a mean deposition rate of 139 Mt/yr, which is nearly equivalent to the mean sediment load of 148 Mt/yr entering the Yangtze estuary during this period. Together with other human activities (such as in-channel sand mining, soil and water conservation practices, and constructions of dams and reservoirs), which were contributing to reduce river sediment load, the TGR aggravated the tendency of decrease in the Yangtze sediment load and suspended sediment concentration (SSC) entering the estuary. The Yangtze annual sediment load entering the estuary during 2003-2009 after the TGR operation were dramatically decreased by 68% and 56% in comparisons with those during the two time periods (1953-1987 and 1988-2002) before the TGR operation, while the runoff only had a small decrease of 9% and 14%, respectively. Besides, although the Yangtze sediment load and SSC entering the estuary in both wet season and dry season tended to decrease after the TGR operation, the decreases in wet season was much larger than those in dry season. The mean high tide line as the coastline delineated from multi-temporal remote sensing data of Landsat during 1974-2010 at intervals of about 8 years were used to examine the progradation and recession of the Yangtze Estuary with GIS tools. During the period 1974 to 2010, the entire estuary generally continued to progradate seaward, leading to a total area of increase of 667 km2 with a progradation rate of 18.5 km2/yr. During the period, the maximum progradation occurred at the eastern parts of Chongming Island and Nanhui district, where the coastline progradated seaward about 8 and 6 km, respectively, with mean net progradation rates of 0.22 and 0.17 km/yr, respectively. The main reason for the progradation of the Yangtze Estuary despite the reduced sediment supply due in part to the TGR heavy sedimentation is that the human activities such as sea reclamation works, filling project, and port constructions.In order to analyze the influence of the flow of the East China Sea on the outflow system of the Yangtze delta, and the Impact on the situation of Yangtze River massive sediment transportation to the East China Sea. We have analyzed the granularity of 209 surface layer deposit sample witch from Bohai Sea, Yellow Sea and East China Sea. And explore its distribution and the relationship between the source and hydrodynamic environment. The results show that there are 7 types of sediments in the research area, sediment types are including sand, silty sand, muddy sand, sandy silt, sandy mud, silt and mud. Of which the most widely distributed are muddy sand, silty sand, followed by sandy silt, sand, silt, mud are scattered. The main factor witch affect the distribution of sediment types are the source material, topography and hydrodynamic conditions. Since nearly 2ka Fujian and Zhejiang's muddy sediment sources are mainly the Yangtze river's sediment; The northern Jiangsu shoal argillaceous area was formed by a large number of long-term accumulation of sediment from the abandoned old Yellow River deltas when they enteres the sea by 1128-1855. The surface layer sediments sources are variety in the area of middle Southern Yellow Sea and the modern sediments dynamic actions are very severe. Several possible sources are including the modern Yellow River sediment material, the sediments from old Yellow River Delta in Northern shoal and the Yangtze River sediments. North the Yellow Sea argillaceous area is a multiple source argillaceous sediment area, and its source sediment, including the Shandong Coastal current sediment (the Yellow River material), the Liaodong Peninsula rivers sediment, and the Yellow Sea warm current sediment (south the Yellow Sea sediment), etc. The Yellow River delta argillaceous area mainly caused by the sediment every year massively inputs Bohai. According to the State Bureau of Oceanic Administration 908 special office compilations " marine sediment survey technical standard" stipulated that the granularity parameter the moment law mathematical computation obtains Bohai Sea, yellow Sea and the East China Sea deposit average grain diameter to be situated between 2.0-7.8 PHI, the sorting coefficient is situated between 1.00-2.90, skewness is situated between (-1.8)-(+2.8), the kurtosis is situated between 1.60-3.50, Shows the distribution of sea sediment thickness of variation in grain size and sorting is generally poor the main grade is not prominent, the grade distribution range is broad, the hydrodynamic force process margin of fluctuation is very positively, the skewness's deposit widely distributes extremely, the deposit granularity frequency curve is smooth. As research and technological development, the need for provenance analysis of sediments to do quantitative analysis, in order to achieve more clearly explain to the deposition of the East China Sea continental shelf evolution and to a better understanding of the impact on the Yangtze River delta caused by Three Gorges'reservoir, and the influence to whole of the East China Sea. One coin has two sides, the Three Gorges Project is no exception, the Three Gorges project's comprehensive benefits is huge, but the influence is profound to the environment. The influence has two aspects, both the positive impacts, and the negative one. We should recognize that explore and take advantage of the positive impact, while make every effort to overcome and solve the negative effects to enable the comprehensive benefits of the Three Gorges Project in the best condition. In summary, study on the Three Gorges Reservoir shoreline erosion changes since the Yangtze River estuary has an important practical and theoretical value. |
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