Abstract: Through physical model and one-dimensional mathematical model, we analysed the water level variation under different characteristic discharges and the causes of the variation in the downstream river course near Xiaonanhai Hydropower Station in its initial operation stage and at the end of 20-year operation. The modeling results show that after the operation of Xiaonanhai station, water level decreased in the range 4km downstream of the dam compared with the natural status, the maximum decrease (1.47m) happened at 0.4km downstream from the dam during dry season in the initial operation stage. In the initial stage, no obvious water level drawdown happened in the downstream of the 4km range, but with the accumulated sedimentation in Three Gorges Reservoir (TGR), water level will rise higher than that in the initial stage. After 20 years of operation, the maximum increase (0.6m) of water level (compared with that in initial stage) happened at the position 8.2km downstream of the Xiaonanhai dam. The downstream river bed excavation in construction period and the sedimentation with TGR operation are both main causes of water level changing in the river course near Xiaonanhai Dam

Key words: Xiaonanhai Hydropower Station, river course near dam, water level, physical model, mathematic model