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The Siwanli landslide is located in the middle section of the Dadu River in the eastern part of the Qinghai-Tibet Plateau in a complex regional geological environment. Strong erosion of the river valley has caused numerous rockfalls and slides on both sides of the valley since the Late Pleistocene. A large number of landslide deposits provide the opportunity for examination of the sedimentary and erosional environment along the Dadu River. Identifying the spatial and temporal characteristics of the large, ancient landslides in the Dadu River valley can help improve understanding of the formation mechanism and evolutionary process of ancient and recurrent landslides. In this paper, the formation mechanism of the ancient, complex, and multistage Siwanli landslide is discussed, as well as an important link between river erosion and the formation and evolution of the landslide. In the first stage, the initial landslide blocked the Dadu River, forming a barrier lake, and multistage landslide dam breaches occurred. The entire landslide dam experienced creep deformation, and local minor multistage sliding occurred at the front and middle parts of the initial landslide. Subsequently, a large landslide formed at the scarp of the initial landslide. [ABSTRACT FROM AUTHOR]