Free surface now equations have been derived for flood routing in prismatic channels. In this new formulation, the momentum equations transform to a partial differential equation that has two parameters related to cross-sectional area and discharge of the prismatic channels. These equations have been solved by an explicit finite difference scheme. In computation procedure, after computing the discharge from the new momentum equation, the corresponding cross-sectional area has been obtained from the continuity equation. The results obtained by solution are shown in table and graph.