In order to arrive at good flood simulation results, it is important not only to develop physically-based runoff models but also to understand the runoff mechanisms in a basin. A large number of refined physical models or theories have been proposed to simulate runoff in watersheds during the last several decades, and has offered a wealth of useful information on the mechanism of water flow in the catchments and its contribution to the runoff. The purpose of present study is to clarify the effects of slope and river on runoff according to the spatial scale of catchments. A catchment is composed of sub-catchments with river channel network and hydrological characteristics such as rainfall, soil properties and initial conditions of the soil are distributed in an actual catchment. In order to evaluate the effects of river and spatial distribution of hydrological characteristics on runoff, runoff calculation is carried out for Tone river basin which has largest cahtchment area in Japan. The method based on morphological and geophysical properties proposed by us 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.