Many plants, including alien plants, are introduced to cover embankments or to plant in riverbed park zones during the course of river management. However, many alien plants have escaped and established in riparian environments. Some alien plants have negative effects on nature conservation and river management. Therefore, estimations of the risk of escape and establishment are needed for the ecologically safe use of useful alien plants. In order to clarify regional differences in the escape risk of alien plants, we conducted a questionnaire investigation to assess the amount of alien plants introduced by river management, and we compared the occurrence of alien plants in the riparian environment using the dataset of a national survey on river environments. The results showed that many species of alien horticultural plants were used. In contrast, only a small number of alien revegetation plant species were used; however, these were used in huge amounts. On the basis of the regional differences in occurrence and introduction amount, we clarified the risk of 39 alien species as follows: species having a high escape risk nationwide; species having a high escape risk in the Hokkaido and Tohoku areas, but with a low escape risk in warmer regions; and species having low escape risk in riparian environments nationwide.
