After the Wenchuan Earthquake, many large-scale debris flows blocked rivers, generated dammed lakes, and produced outburst flood at the seriously hit areas. This paper mainly discussed the formation, outburst, impacts and risks of debris flow dammed lakes. The field investigation showed that the dammed lakes were created by debris flows from gullies and hill-slopes as well as the combination of debris flow and landslides, and also distributed along rivers step-by-step. The height of dams and the length of dammed lakes along river channel varied from 4 m to 18 m and from 400 m to 5000 m, respectively, and the reservoir capacity of dammed lakes were from 1.5 × 105 m3 to 3 × 106 m3. Due to geomorphological impact, dammed lakes commonly partially outburst along their front of debris flow deposition dams (1/4 - /3 outburst) or the suture between debris flow and landslide, and hardly completely outburst. Moreover, the subsequent debris flows continuously increased the magnitude and height of dams, not only increasing the stability of a single dam, but also improving the risks of outburst flood induced by intensive rainstorm. Dammed lakes produced steep rage in the sites of dams with the 4% - 9% of slope and rapidly raised river channel in the upstream due to a mass of alluvial sediment. As a result, the landscapes of step-dams and step-lakes dominate driver channels, significantly increasing the hazards of floods. Then the hazards, impacts and risk of debris flow dammed lakes along Min River from Dujiangyan to Wenchuan were analyzed. In order to mitigate dammed lakes induced by debris flows, the identification model of debris flow blocking rivers, the process of the formation, outburst and evolvement of dammed lakes, and the model of risk assessment for step-dammed lakes were strongly suggested to explore, and be used at the rivers of Min, Yuzi, Caopo, Longxi, Mianyuan, Jian, Shiting, Baishui and Jushui.