Comparison of total phosphorus concentration of Yangtze River within the Three Gorges Reservoir before and after impoundment

In order to study the impact of the Three Gorges Project on concentration of total phosphorus(TP),TP concentration of five sections in Yangtze River within the Three Gorges Reservoir was monitored monthly between 1998 and 2009(impoundment began in 2003).Results showed that there have appeared new characteristics in spatial and temporal distribution of TP concentration after impoundment.TP concentration of water column near the dam decreased significantly comparing with that before impoundment.For example,the average TP concentration of Guandukou section and Taipingxi section in 2009 decreased by 42.8% and 54.9% than that before impoundment,respectively.TP concentration in wet season was still higher than that in even and dry seasons.Nearer the dam,smaller difference was observed between values in different seasons.After impoundment TP concentration declined from the tail to the head of the reservoir,and it decreased by 61%-65% from Qingxichang section(in the downstream of Fuling) to Taipingxi section(near the dam).The decrease was prominent especially in wet season.The reason for changes of TP concentration mentioned above was mainly due to sedimentation of sand in the reservoir,which resulted from water level increased and the velocity decreased.