In this study, critical submergence for an air-entraining vortex at intakes in a uniform canal flow was investigated. The potential flow solution for the combination of a point sink and a uniform canal flow is available, and is known as Rankine's ovoids or half-bodies. This study was based on Rankine's ovoids, Experiments have shown that the critical submergence occurs when the upper boundary of Rankine's ovoids reaches the water surface directly above the intake center. Theoretical and experimental results have indicated that the critical submergence for an intake in a uniform canal flow is equal to the radius of an imaginary spherical sink surface (assuming no canal flow) where the radial velocity is equal to half of the velocity of the uniform canal flow. The imaginary spherical sink has the same center and discharge as the intake. The agreement between the theoretical critical submergence and experimental results is found to be good.