Recently riverine trees in the course of gravel bed-river have extremely increased and enlarged their covered-areaover bars and flood plain. Such a situation brings about problems of river management for the environmental aspectsas well as the safety of rivers against flood. In order to clarify such problems, we should get the knowledge abouthow the instream vegetation grows up and expands their covered zones in the gravel-bed river.In this paper, the cause of the excessive growth of Robinia pseudo-acacia was examined in the segment-1 reach ofWatarase River. Especially, from the investigation of the annual rings of tree, it was clear that some forests of Robinia pseudo-acacia in bars were formed right after the main flood. We can consider that the flood caused changeof generation of the trees and the damage which the flood gave to the trees can be classified by the scale of the floodexternal force. Based on the field study and numerical analysis with 2D- flow model, the critical tractive force to bean index of the scale of flood disturbance was evaluated and results showed that rapidly expansion by thegermination from fallen trees makes the excessive growth of riverine trees.