The M8.0 Wenchuan earthquake of 12 May 2008 triggered over 20,000 landslides in the earthquake affected area of Longmenshan Mountain. It was a common phenomenon that large scale landslides blocked river channels. In total, 257 landslide-dams were identified by field investigation and remote sensing inspection. The dams were distributed in a belt along the rupture zone and in clusters along the river valleys. Emergency risk assessment was carried out due to the rise of water levels behind the dams and catastrophic outburst potential of the landslide-dammed lakes. Under the emergency circumstances, the dam height, dam composition and storage capacity of the landslide-dammed lake were used as indices of the dam’s risk of collapse. Four risk levels were assigned. By analyzing 21 landslide dams in detail, the results of risk assessment were obtained; 1 dam had an extremely high danger risk (Tangjiashan Lake), 7 dams had a high danger, 5 dams medium danger and 8 dams were of low danger. The case of the Tangjiashan rockslide dam that formed the largest landslide-dammed lake, is discussed in detailed. The scenarios analysed were for 20, 25, 33, and 50% of the dam failing, yielding, respectively, flood heights of 4.6, 5.1, 5.7, and 6.2 m, inundation areas of 3.35, 3.84, 4.22, and 4.65 km², and discharges of 6,106, 7,397, 9,062, and 11,260 m³/s, at the Fujiang Bridge in Mianyang, the second largest city in Sichuan Province. The outcome of this analysis was used by emergency managers to plan downstream evacuations. Sluiceways were designed to lower water level of the dammed lakes and to reduce the risk of the outburst floods. Emergency mitigation operations for risk-elimination proved to be effective and successful. On the other hand, debris flows will be the dominating disaster process in future and will likely deposit masses of debris in river channels forming new dams in the coming 5–10 years. It is thus of prime importance to take measures and prevent debris flows from damming main rivers and posing new threats to downstream locations.