In the natural hydrologic cycle, surface and subsurface water in a watershed are closely related and interact with each other. However, their relationships are affected by human activities. For instance, as the impervious area of a brain spreads due to urbanization, rainfall recharge into unconfined aquifers decreases and consequently the stream flood hazard increases. By renovating channels for flood control, natural streams are reconstructed into artificial channels. As a result, water exchange between stream and aquifer may be impacted and altered. It is well known that small streams in a city become drainage channels in rainy days and run dry in non-rainy days. In order to improve such a river environment, it is necessary to recover the run dry in non-rainy days. In order to improve such a river environment, it is necessary to recover the natural water cycle in the watershed by recognizing the hydrologic connection between streams and aquifers. Although the relationship between a stream and an aquifer has been investigated from various angles, most of the researches dealt with merely the response of an aquifer to fluctuation in the stream stage. At alluvial plains, however, the relationship between the two is interactive and the water exchange between them depends on their relative hydraulic state. Therefore, it is necessary that the stream-aquifer interaction is evaluated by solving two governing equations of the stream flow and the groundwater simultaneously. The purpose of this paper is to clarify a potential role that the aquifer plays for regulating the stream flow, the so-called 'bank storage effect', by expressing the stream flow in the storage function model.