Seimareh Landslide (SL) is globally recognized as one of the largest rock mass movements in the world. It is located along the border of Ilam and Lorestan provinces in southwest Iran, in the heart of the Zagros Mountain Range. There are controversial findings about the mechanism of the landslide formation. This field work study reviewed the possible mechanisms of failure and analyzed post-failure geomorphic features. Drainage pattern disturbance in the depositional region and consequent dammed lake formation are among the most significant characteristics of these features. Seimareh, Jaidar and Balmak are three large landslide-dammed lakes. The present study analyzed the processes responsible for the formation and erosion of the Jaidar and Seimareh Landslide dams using the available annual sedimentation and field measurements of the sediment deposited in these lakes. The results showed that the SL dam has been formed about 935 years after the landslide event. Detailed field investigations indicated a specific hydro-morphological condition in the landslide area. The results implied that the main causes of the failure were probably the particular hydro-morphological characteristic of the landslide source area together with the enormous eroding energy resulted from merging of two high-flow rivers which eroded the base of the southern flank of Kabir-kuh Mountain. However, the unusual size of the landslide suggests that an external factor, e.g., a huge earthquake, might have triggered the failure.