The role of Lake Dongting in regulating the sediment budget of the Yangtze River

摘要

Lake Dongting, the second largest freshwater lake in China and located in the middle reaches of the River Yangtze catchment, was formed at the beginning of the Holocene period by sea level rise and has varied in size with changes in local weather patterns. The sedimentation rate in Lake Dongting during the Holocene is about 50 x 10(6) m(3) yr(-1), or 80 x 10(6) t yr(-1) (a sand bulk density of 1.6 x 10(3) kg m(-3)), given the sediment deposition rate as 10 mm yr(-1) and the average lake size as 5000 km(2). By comparing the sediment import and export, it is estimated that the sediment deposition rate of Lake Dongting was 110.6 x 10(6) t yr(-1) from 1956 to 2003. Siltation and raised embankments reduced the size of the lake and its capacity to accommodate floods. The sediment delivery ratio (SDR) of the middle and lower Yangtze is about 0.92 (total sediment output divided by total sediment input) given that the total sediment supply into the middle and lower Yangtze is 455.1 x 10(6) t yr(-1) and the total sediment discharge into the sea is 419 x 10(6) t yr(-1). Therefore, if it were not for Lake Dongting, the sediment flux at Datong would be 73.6 x 10(6) t yr(-1) (80 x 10(6) t yr(-1) x 0.92) more, an increase of 27% during the Holocene and an increase of 26% to 101.75 x 10(6) t yr(-1) from 1956 to 2003. Historically, Lake Dongting had a considerable influence in regulating the sediment budget of the Yangtze. However, afforestation and the construction of large dams, such as the Three Gorges Dam, reduced significantly the sediment deposition in Lake Dongting. In 2003, the completion of the Three Gorges Dam and the subsequent impoundment of water reduced the sediment input from the Yangtze and net deposition in Lake Dongting dropped to 25% and 18% of the mean values of the historic records (1956-2003). During the same period, the amount of sediment deposited in Lake Dongting was only 10% of the sediment discharge at Datong. The influence of the sediment deposited in Lake Dongting on the sediment flux to the sea from the Yangtze has fallen since the completion of the Three Gorges Dam and will be further reduced in future. The evolution of the relationship between Lake Dongting and the Yangtze is a compound result of human impacts coupled with natural self-adjusting processes in the river system.