Abstract:

Water was turbid within the Yangtze River while clear within Poyang Lake at their confluence for a long time.Recently，such situation however no longer exists，and it has been replaced by clear water within the Yangtze River and turbid water within Poyang Lake.This inversion of clearturbid water in Poyang Lake and the Yangtze River has gained extensive attentions，however few researchers analyzed systemically the causes of this phenomenon.This paper，with the supporting of moderate resolution imaging spectroradiometer (MODIS) and Landsat images，analyzed the starting time and causes of inversion of clearturbid water in Poyang Lake and the Yangtze River.The water clarities derived from MODIS images within the Yangtze River and the northern Poyang Lake revealed that the water clarity of the northern Poyang Lake was higher than that of the Yangtze River before 2004，the water clarity of the Yangtze River，however，was higher than that of the northern Poyang Lake in 2005 and 2006 and the inversion of clearturbid water occurred；The comparison of synchronous water clarities of the Yangtze River and northern Poyang Lake showed that the inversion was influenced doubly by the increased water clarity of the Yangtze River and the declined one of northern Poyang Lake，in which the change of water clarity of northern Poyang Lake played one important role；The number of dredging and shipping vessels derived from Landsat images and its relation with the water clarity of northern Poyang Lake showed that the dredging activities starting in 2001 in northern Poyang Lake resulted in the prominent decline of water clarity in this region.

Key words: Poyang Lake/ the Yangtze River/ inversion of clearturbid water/ remote sense techniques