In this study, the generation process of the phreatic surface within a river levee is investigated through soil/water/air coupled simulation. Here, the effects of river level fluctuation and rainfall on changes in soil moisture distribution within a river levee are considered. The constitutive model proposed by Ohno et al.1) is formulated to the initial and boundary value problems in accordance with Borja’s multi phases’ concept and is used for simulation. Consequently, it is first found that high groundwater level is maintained for ground consisting of low permeability and high soil water retentivity. Moreover, it is found that the fluctuation of river level directly influences the phreatic surface, and that, in contrast, the effect of rainfall appears after a time lag.