The study dam is located in the province of Quebec. According to Quebec law, the Dam has a classification of “Extreme”, and its corresponding Inflow design flood is the Probable Maximum Flood (PMF). To produce flood inundation maps for the emergency response plan, a dam break study was performed. The first part of the study consisted in determining the spring and summer-fall Probable Maximum Precipitations (PMPs) and Floods (PMFs). These were estimated for the dam watershed and other sub-basins, to the outlet of LacSaint-Jean, covering a total of approximately 73,000 km2. The PMP optimum value was computed following the approach proposed in HMR-52, using several combinations of storm sizes and orientations. The PMF was simulated with the SSARR hydrologic model, calibrated and validated with long-term continuous observed daily data. Flood routing was performed with HEC-ResSim software for the six reservoirs in the system.  The hydraulic model used to simulate the dam failure was constructed in HEC-RAS. It covers more than 370 km of river and a 1,100 km2 reservoir. It is mainly 2D with a few reaches combining 1D/2D. A multiscale mesh was used to reduce the computational times and to capture the complex hydraulic processes within the system when simulating exceptionally high flow conditions. The model was calibrated and validated using various observed data sets, and used to simulate the study dam break, including the cascade break of several downstream dams, to compute the flood characteristics, and to map inundated areas.