A problem of a plane-parallel flow induced by vertical lifting of a symmetric convex body partly immersed in water filling a rectangular prismatic channel with a horizontal bottom is solved within the framework of the shallow water theory. The body width coincides with the channel width, its flat side surfaces are perpendicular to the channel bottom, and its lower downward-convex surface has sufficiently small curvature and is completely immersed in water at the initial time. The liquid flow is obtained analytically in the region adjacent to the lower surface of the body and by means of the numerical solution of shallow water equations by the second-order CABARET (compact accurately boundary-adjusting high-resolution technique) scheme outside this region. Equations that define the motion of the boundary line between the liquid and the lower surface of the body are derived. It is shown that the form of these equations is determined by the sign of the spatial derivative of pressure on this boundary line. Numerical results demonstrating liquid lifting behind the body leaving the water medium are presented.