公元９世纪以来长江潮区界的迁移过程重建

The tide can now propagate upstream for more than 600 km from the Changjiang River estuary to hinterland, which is rare in the world. In China, information and records reflecting tidal limit and its movement in the Changjiang River can be found from some ancient books, literary works and local chronicles. At the end of the 3rd and the beginning of the 4th century AD, the tide limit of the Changjiang River reached the upstream of Chaisang (now named Jiujiang in Jiangxi Province). At the early 9th century, the tide limit in the dry season retreated to the Penpukou in Jiujiang. In the last ten years of the 13th century, the tidal limit moved back to the Xiaogushan Mountain near the border between Anhui Province and Jiangxi Province. By the middle of the 20th century, the modern researchers generally agreed that the tidal limit retreated to Datong in Anhui Province. Before the main stream of the Changjiang River was cut off by large-scale water conservancy projects, the tide limit had retreated seaward continuously. From the 3rd decade of the 9th century to the last decade of the 13th century, the tide limit moved downstream 64 km during the 470 years, with an average annual rate of 0. 136 km/a. During the 670 years from the end of the 13th century to the middle of the 20th century, the tide limit had moved downstream 170 km, with an average annual rate of 0. 254 km/a, almost twice as much as that of the previous period. We suggest the climate change accounted for the different retreat rate of the tidal limit between the two periods discussed. In addition, a recent study found that during the extremely dry periods, when the discharge of Jiujiang station was about 8440 m³/s, the tidal limit reached near Jiujiang. In the years when the water conservancy project on the main stream of the Changjiang River began to store water, the tide limit actually returned to the previous position of the middle Tang Dynasty, i.e. 1100 years ago.