Large Scale Particle Image Velocimetry (LS-PIV) is used to measure the surface flow velocities in a mountain stream during high flow conditions due to a reservoir release. A complete installation including video acquisition from a mobile elevated viewpoint and artificial flow seeding has been developed and implemented. The LS-PIV method was adapted in order to take into account the specific constraints of these high flow conditions. Using a usual LS-PIV data processing, significant variations of the water surface elevation were taken into consideration in the image rectification. An intensity threshold was applied to focus on artificial tracers without considering stationary waves and sun reflections on the flow surface. A site-specific float coefficient of 0.79 based on measured vertical velocity profiles was used to convert surface velocities into depth-averaged velocities. Comparison between LS-PIV assessments and 2Dh numerical calculations with the code Rubar20 allows verification and extrapolation of LS-PIV data. LS-PIV velocity measurements permit to assess discharges over the whole high flow event in agreement with leaded current-meter measurements performed at a downstream bridge.