Abstract：Reservoir slope instability is a geological hazard induced by construction of hydroelectric project and is generally believed to be caused mainly by the unfavorable change of hydrodynamic condition in the slope due to reservoir filling and/or operation. Many cases of reservoir slope instability can be found both in China and abroad，the evolution of the hydrodynamic conditions，however，is mainly obtained by seepage analysis，while the systematic and complete field instrumentation data can rarely be found. Taking the opportunity of filling the Three Gorges Reservoir，an automatic hydrologic monitoring system is implemented to monitor the hydrodynamic evolution of a preexisting landslide，Xietan landslide in Zigui County，Hubei Province，China. The system includes the monitoring of reservoir water level，water level in boreholes，seepage pressure and precipitation，etc. The comprehensive analysis of the monitored data shows that the rising speed of water level (or seepage pressure) on the reservoir bank lags behind that of the reservoir filling and the lagging time is closely related to the permeability of materials of the bank. The weaker the permeability of the slip band，sliding-disturbed zone and bed rock are，the longer their lagging time is. If the permeability of the main body is quite good，then only a short lagging time can be observed. Water level data from observation wells are misleading and should be analyzed with the well structure，and standpipe should be used whenever possible. Rainfall-induced water level rising in the main body and its lagging time are greatly influenced by rainfall intensity. Heavy rainfall takes less time to infiltrate into the slope and induces higher water level rising. Therefore，for the analysis of rainfall influence，a basic time unit should be determined according to the rain type. Basic time unit of hour is feasible for Xietan landslide.
