In this study, the critical submergence for a rectangular intake in a uniform canal flow is investigated. The potential solution for the combination of a line sink and a uniform flow is available and known as Rankine half-body. This study is based on the Rankine half-body. Theoretical and experimental results have indicated that, in general, the critical submergence for a rectangular intake can be predicted by means of a critical sink surface which consists of a critical cylindrical sink surface capped with two critical hemispherical sink surfaces at both ends. The effects of intake-duct pipe blockage and location of impervious boundaries on critical submergence of a rectangular intake are presented. Experiments were conducted on a rectangular intake sited in a dead-end uniform canal flow. The agreement between the theoretical and experimental results is found to be acceptable.