Two aspects of erosion need to be considered from slope stability point of view. The first is a large scale erosion, such as a river erosion occurring at the base of a slope. The second is a relatively localized erosion caused by groundwater or surface runoff. In the first type, erosion changes the geometry of the potentially unstable rock mass. The removal of material at the toe of a potential slide reduces the confining stress that may be stabilizing the slope. Localized erosion of joint filling material, or zones of weathered rock, can effectively decrease interlocking between adjacent rock blocks. Loss of such interlocking significantly reduces the rock mass shear strength. The resulting decrease in shear strength may allow a previously stable rock mass to move causing slope failure. In addition, localized erosion may also result in increased permeability and ground-water flow thus affecting the stability of rock slope.