| The Three Gorges Project on the Yangtze River is the world-famous largest hydroelectric project. It will possess comprehensive utilization benefits for flood control, power generation and navigation improvement. But the construction of TGP will produce widespread and profound impact on ecology and environment. Therefore, the water environmental issue after the dam being put into practice has become one of the focuses not only domestically but also overseas. In this paper, a one-dimensional flow model which based on Saint-Venant equation was developed. Then, it was applied to calculate the holistic hydraulic characteristics in the natural condition ( 65.8m water level at the dam) of the Three Gorges reservoir in Chongqing region: that was the 500km-long mainstream beginning at Cuntan and ending at the boundary of Chongqing .The validating result indicated that the model properly reflected the holistic hydraulic characteristics, showing its ability of prediction. Further, the model was applied to predict the hydraulic characteristics of the calculating zone at the time of the second stage of the dam construction (156m water lever at the dam) and the completion of the project (175m water lever at the dam). In addition, the depth-averaged velocity transverse distribution was calculated by means of coupling with water depth, assumpting that the transverse distribution function is power function. Finally, compared the calculation outcome with eutrophication hydraulic critical index, the hydraulic eutrophication sensitive zone was picked out innovatively. The research results showed that: 1. When the reservoir was operated at the water level of 156m, the cross-section average velocity in most of the reaches, varying from 0.15m/s to 1.00m/s, was reduced sharply compared with that in the natural condition. In the up reaches of the calculating area, from Cuntan to Luoji, the difference between the cross-section average velocity of two water level was insignificant. Down the reaches of Luoji, the cross-section average velocity decreased slowly. It fluctuated between 0.25 m/s and 0.50 m/s in the area between Luoji and Fengdu. And, it decreased further from Fengdu to the boundary of Chongqing, varying from 0.15 m/s to 0.20 m/s. 2. When the reservoir was operated at the water level of 175m, the cross-section average velocity, varying from 0.08m/s to 0.45m/s, was reduced by 2/3~3/4 compared with that in the natural condition.Also, the fluctuation extent in 175m water level was smaller than that in the natural condition. In term of river-width, it widened in the water level of 175m. In the calculating area, river-width of most corss-section in the water level of 175m was 800m or more, ever 2000m in some cross-section, increasing 2~3 times compared with the natural condition. 3. In both the water level of 156m and 175m, the gaps between the depth-averaged velocity and cross-section average velocity were smaller than that in the natural condition, indicating that the transverse distribution of depth-averaged velocity trend to become homogeneous within cross-sectional area. Also, the transverse velocity grade reduced dramatically: the transverse velocity grade in 156m was approximately 1/10 of that in natural condition; the transverse velocity grade in 175m was 1/13~1/11 of that in natural condition. 4. In the water level of 175m, the 24.6km lower reaches of Zhongxian, 8.9km reaches near Wanzhuo city, 19.7km reaches near Yunyang and 19.6km reaches near Fengjie city were picked out as the hydraulic eutrophication sensitive zone. The 35~90m width belt in both sides of the hydraulic eutrophication sensitive reaches was identifyed as the sensitive belt. The overall length of the hydraulic eutrophication sensitive reaches was 72.8km, acounting for 14.6% of the length of the calculating area.
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