The purpose of present study is to clarify the effects of slope and river on runoff according to the catchment size. A basin is composed of sub-catchments with river channel network. In order to evaluate the effects of river in runoff, runoff calculation is carried out for Tone river basin. The method based on morphological and geophysical properties is applied to runoff calculation in the sub-catchment areas and unsteady flow calculation is carried out for flood routing in river channel network. In conclusion, flood routing calculation in river is not necessity for the small basin with the catchment areas less than 200km².