Deep river valleys in Pamir are rich of large-scale rockslides and rock avalanches. Many of them form natural blockages though most of such natural dams have been breached and deeply eroded. Evidence of the outstanding prehistoric outburst floods can be revealed based on several sets of the geomorphic and geological data: 1) vast area covered by debris flow deposits downstream from the breached landslide dams, such as in the upper reaches of the Kokcha River basin in Afghan Badakhshan; 2) convex profiles of the alluvial fans of large rivers where they leave mountain ranges. Such profiles are typical of relatively small fans formed by debris flows, but are abnormal of large rivers with permanent flow. The most impressive one is that of the Pyanj River where it comes out from the Darvaz Range; 3) the anomalously steep longitudinal profiles of river valleys at their sections between two breached natural dams. The upstream one had formed large lake and its lower part had been protected from incision by the material eroded from this dam and stored in the trap between two dams. Such phenomena were found in the Pyanj and Gunt River valleys. Study of these events helps to estimate what could happen after breach of the existing or future rockslide dammed lakes.