In planning of Sabo master plan one of the fundamental themes is to clarify the mechanism of sediment yield and discharge. For this purpose, the authors made a model of sediment yield and discharge processes in mountainous drainage basin, and then compared the computed results with the actual phenomena. Sediment yield model is on a physical basis to take into account the conditions of rainfall, infiltration and slope stability. For sediment discharge model computation of river bed fluctuation is made in order to reflect the hydraulic condition of river bed reach to sediment discharge. The study area is the upperstream of Tenryu river that is one of the devasted rivers in Japan, where a big sediment disaster occurred in 1982. In this result, it can be said that the model reasonably reproduced the 1982 sediment phenomena by expressing both cases of occurrence and non-occurrence of collapse, time of collapse occurrence, place of collapse, condition of river bed fluctuation and volume of sediment discharge.