Abstract
The effect of the enlargement of Phragmites japonica on the threshold of gravel movement was investigated by field observation at midstream of the Arakawa River. The lateral biomass distribution was simulated by the growth dynamic model of P. japonica including their shoot characteristics. The extended width by runner system was about 30 m, almost coincided with the width of the 3 year's plant stand in field. The effect of the plant enlargement on the threshold of gravel movement as evaluated by the lateral distribution of the drag force in relation to the plant biomass. The analysis indicated that the area lower than the threshold of gravel movement becomes exponentially large after 3-5 years with no disturbance even if the initial colonization is scattered.
