Abstract: The process of stream trenching and slope formation cause a continuous change in the stress field of the river bank slopes. The superficial rockmass is unloaded in the change of stress field. The depth of unloading should be indifferent in the same area. An investigation is carried on the stress field at a hydropower station in southwest China. The station has two dams. The up and downtream dams have a distance of 1.4km. The investigation detected that unloading depths of the rockmasses at the two dam sites have wider differences. At the upstream dam site, the unloadin depth is more than 20m, and its extreme depth can reach to 90 m. At the downstream dam site, the unloading depthis only 5~20m. Field survey and numerical simulation indicate that orientation of the rock formation is the key factor for the difference in the unloading depths at the up and downstream dams. Trends of rock formation and river bank slopes at the upstream dam are paralleled. The rockmass of steep dip angle is easy to be cracked. The weathering is invaded. The depth of the rockmass unloading is deep. At the downstram dam, the trends of the rock formation and river bank slopes are nearly are perpendicular. The strength of the rockmass is greater than the bedding planes. The structural surfaces that parallel to the river bank slopes are less. Therefore, the unloading depth of rockmass is within 20m.