he effect of flood disturbance on the morphology of runners, plant productivity and runner-expansion characteristics of Pharagmites japonica was investigated at midstream of the Arakawa River. The dynamic growth model of P. japonica was developed considering the effect of disturbance, i. e., the difference of runner morphology and shoot productivity in the parent colony. The aboveground biomass distribution along the distance from the parent colony was well represented by the model. For analyzing the effect of flood disturbance interval, maximum water levels at three flood events were calculated and evaluated whether the habitat of P. japonica were inundated or not. In addition, the threshold friction velocity of the average grain diameter was compared with the friction velocity at the flood event. Although the gravel movement possibility has a little different tendency with the damaged situation of P. japonica, the clear relationship between the flood interval and the productivity in each habitat is found. The analysis indicates the possibility to calculate the plant productivity under the effect of flood disturbance.