Mahe colluvium, located in the middle reach of the Yalong River valley, is about 3.5 km upstream of the proposed Lenggu dam site. About 125 vertical meters of the deposit will be immersed in the damming water during the operation of the Lenggu Hydropower Station, which could greatly reduce the stability of the colluvium. The consequent landslide (up to 13 million m3) would be a major threat to the infrastructure of the station. Subsurface investigations indicate that the main colluvium consists of three units: the thicker upper and lower units and a thinner middle deposit. Grain size distribution analysis indicates that the content of coarse materials increases with depth, and the deposit most likely functions as a surficial fine material layer and an underlying coarse material layer. Stability analyses, including the deterministic and probabilistic analyses, show that the entire colluvium deposit is more stable during dry conditions. But the surficial fine material layer has a large possibility of instability (with probability of failure ranges from 22.2 percent to 26.2 percent) even though the deterministic factor of safety is above 1.0. Stability analyses on an impoundment model indicate that pore-water pressure can produce noticeable reduction in slope stability. Under the impoundment scenarios, both deterministic (with factor of safety of <1.0) and probabilistic (with probability of failure ranges from 30.8 percent to 50.8 percent) analyses indicate that the lower section of the main colluvium is unstable.