Vertical profiles of water temperature in Rapel reservoir (central Chile, 34° 2′ 23″ S, 71° 35′ 23″ W, 110 m.a.s.l.), recorded the hydrodynamic response of the stratified water column of the reservoir produced by the 8.8 magnitude Maule earthquake (epicentre located in 36°15′36″S, 73°14′ 20″W), showing an intense vertical mixing of deep water. The vertical mixing was characterized by changes in the potential energy of the water column and the eddy diffusivity of the mixing, and these values were used to estimate the mixing efficiency of the event. It is hypothesized that the turbulent flow that mixed the water column was mainly induced by the oscillation of the bed, although surface gravitational waves could also have contributed to the vertical mixing. So far, this is the first time that the hydrodynamic response of a reservoir during a mega-earthquake is measured and described.