For the high speed and strongly turbulent open channel flow, it is difficult to determine its free surface. The high speed video camera was used to take the definitional photographs with resolution of 5000 frames per second. The thresholds of those photographs were computed using the Otsu’s method. Those collected images were converted to the binary images based on the calculated thresholds. According to the principle of 8th communicating set, the edges of the flow in the images were examined and found, then the coordinates of the boundary pixels were ascertained and the shapes of the free surfaces were determined. It was found that the free surfaces deform remarkably and such deformations lead to surface roughness, breakup and disintegration as the velocity of the flows is about 7.1 m/s. The splashing, projections and bulks were generated on the surfaces and strongly distorted. The kinematics of these projections and cusps may be the reason of aeration.
