Abstract: It is proved that the diffusive equation is accurate enough, but relatively simple, to calculate flood propagation both in theoretical and practical perspectives.The-stage discharge curve at the downstream boundary is combined with the cont inuity equat ion of the St Venant equations to create the stage discharge relationship type boundary condition for the downstream condition The Laplace transform method is employed to obtain the analytical solut ion to the diffusive equation with the boundary condit ion A case study indicates that the proposed method is adequate to calculate channel flood propagation.