Abstract
Understanding the gap between a target ecological state of a river and its current state is the first pivotal step in the process of nature restoration. The target rivers in this study had many native species and few alien species in the past. But feasible target setting posed a challenge due difficulties in obtaining historical information of small and medium sized rivers in rural area. For this reason, river classification using topographical and geological features was done to clarify the characteristics of each river group. But some rivers had many alien species than others despite belonging to same characteristic group. To find out the reason, the water quality and the number of river structures were compared. The results showed that poor water quality and placement of structures in the steep downstream section of the rivers contribute to creating a suitable habitat for alien species.
