The stability of riverbanks subject to seepage is studied experimentally and theoretically in this paper. By including seepage in a 3-dimensional theoretical analysis, the study first shows how the critical slope or angle of repose of a cohesionless material is related to the ratio of the hydraulic gradient of seepage to its critical value under the fluidization condition. The critical stable slope is shown to be related to not only the hydraulic gradient but also the seepage direction. Measured laboratory data reasonably fit well with the theoretical relationship for the case of injection and suction. The data reveal that the slope is reduced with injection and increased with suction, respectively. Additionally, the study identifies the seepage direction which results in a minimum critical stable slope for a certain hydraulic gradient of seepage.