This paper proposes a new approach using the Equivalent Frequency Response Method (EFRM) for analyzing St. Venant model for unsteady one-dimensional flow along with its related models such as kinematic wave, diffusion wave and gravity wave models with two different lower stream boundary conditions. It is possible to calculate the gain, time lag, and impulse response function by the proposed method. Moreover, a new criterion is presented for evaluating the validity of each model for a range of Froude and kinematic wave number based on the obtained impulse response function. Finally, this paper suggests the possibility of applications of EFRM to real runoff analysis.