Abstract: Spatiotemporal variations of euphotic depth and its causing factors are investigated for the early impounding stage of Three Gorges Reservoir in the backwater area of Xiaojiang River that is a tributary of the Yangtze River.Water samples were taken at five cross-sections in the backwater area during the period May,2007 through May,2008.The photosynthetically active radiation(PAR) and conventional water quality monitoring indices were derived from the water samples for analysis in this study.The results show that there are no significant spatial variations in the mean euphotic depth at the five sampling cross-sections.However,seasonal variations are strong with the highest value of euphotic depth in the winter and the lowest value in the summer.Regressions between the attenuation coefficient of PAR with the secchi disc transparency and with the concentration of total suspended solids(TSS) show that the two correlations appeared to be strong as revealed by the coefficient of determination R²=0.8619 for the former and 0.764 for the latter,which indicates that the TSS influence on the euphotic depth is significant.The study also finds that there is no significant correlation between the attenuation coefficient of PAR and the chlorophyll-a(Chla) concentration as shown by the low value of R²=0.0113.This result suggests that the euphotic depth,the concentrations of TSS and Chla are all influenced by the river flow and water stage in the backwater area of Xiaojiang River.The spatiotemporal variation of euphotic depth is thus mainly influenced by the sophisticated hydrodynamic and hydrological conditions in the river-reservoir hybrid system.