Abstract: There are noticeable differences in the overburden on river bed and the shape of river valley between the upper dam and the lower dam of a hydropower station on the Dadu river. This paper analyses the forming mechanism of the overburden on river bed and the evolution of river valley on the base of geological structure and global climate. TheUshape glacial valley of the upper dam by glaciations in the last ice age in Epipleistocene remains to this day. A intensive erosion in a later period result in deep cutVshape valley at the lower dam and a profile stair between the upper dam and the lower dam. Since Holocene, strong sedimentation and weak erosion result in deep overburden on the deep cut valley of the lower dam, and a barrier lake layer in the middle of overburden.