Abstract: The bottom of Poyang Lake, China’s largest fresh water lake, slopes northward toward the Changjiang (Yangzi) River into which the lake drains. The lake’s outlet is located at the lake’s northern end at Hukou, Jiangxi. In earlier times, the river had been further north but it shifted its course southward, eventually reaching Hukou. This caused the river water to flow to the lake basin, forming natural levees and sand bars which obstructed the lake’s discharge into the river and affected the lake’s later development. However, it is not clear when did the river swing southward to reach Hukou, when did the river water begin to flow to the lake and what was the intensity of the lake’s back flow. This preliminary study is concerned with these questions which thus far have escaped the attention of researchers. High resolution data are obtained from Hole ZK 2, which is located at the outlet and which sometimes submerges during the dry season. The maximum water level at the site reaches 14 m during the flood season. The data indicate that lacustrine facies has dominated the modern sedimentary environment. The earliest date when the river reached Hukou was determined by searching for sediments that would provide clues to river lake interactions. At the depth of 7 48 m, the hole’s profile shows a boundary. Dated by 14 C to approximately 2 360 a BP, the sedimentary environment immediately above and below the boundary line was one of open lacustrine, but the clay minerals, heavy minerals, granularity, magnetic susceptibility and colors are all different, indicating different sources. The location of the hole suggests that the old sediment source was the ancient Ganjiang River (or the river system of the Poyang Basin), and sediment source changed only when the Changjiang water reached the site, bring different sediment. The age of the boundary is determined to be about 2 360 a BP when the Changjiang shifted southward and reached modern Hukou. This date is the date when the lake first experienced back flow from the Changjiang. In the fine sediment in the outlet waterway at Hukou, those from the ancient Ganjiang River have low Xlf while those from the Changjiang have high Xlf. When the back flow intensity is strong, the proportion of sediment from the Ganjiang becomes less while that from the Changjiang increases with higher Xlf, and vice versa. Moreover, the 20 year running average of the back flow for 1950～1984 matches perfectly with the Xlf curve, providing further evidence that Xlf may be used as a proxy indicator of the intensity of the back flow. The Xlf curve indicates that the history of the lake’s back flow can be divided into three stages. During 2 360 a BP～1 550 a BP, the intensity was weaker than that of today. From 1 550 a BP to 880 a BP, the intensity increased in a wavy pattern, with the greatest intensity occurring around 880 a BP. From 880 BP to the present, the intensity has fluctuated violently in six complete cycles, each lasting about 115 years.

Key words: Poyang Lake, Changjiang River, reflow, sedimentology