This study was focused on the investigation of the distribution of trees which is an important factor for the improvement of the river environment, and the experimental examination of resistant force of trees to the external forces. The investigation of plant distribution performed in 11 major rivers in Korea showed that the willow family grows spontaneously over the whole country. The field experiments on the resistant force of trees were carried out on the 78 trees of 8 species in 3 different sites to estimate whether pulled out trees damage downstream hydraulic structure. The experiments were performed by the method that a backhoe drew trees and the forces were measured when the trees were overturned and pulled out. The analysis of the experimental results showed that there is a linear correlation between the resistant force and DHB (diameter at breast height).