GROWTH OF WOODY PLANTS IN THE DOWNSTREAM OF NAKAGAWA RIVER AND ITS HYDRAULIC INFLUENCE

The purpose of this research is to investigate the colonization process of woody plants, mainly Elaeagnus umbellata, on an alternating bar in a downstream reach of the Nakagawa River, Tokushima, Japan, and also to clarify their influences on hydrogeomorphic features and flood flows in the reach. Based on field vegetation survey and inspection of aerial photographs, it is shown that the colonization started in the late 1970s, and thereafter, the total cover-rate of the woody communities has been increased up to 50% after rather abrupt expansion occurred three times at intervals of about 6 years. The record of flood discharge indicates that seed dispersal of Elaeagnus umbellata due to autumnal floods is closely related to the above-mentioned abrupt expansion. It is found that the woody communities have caused characteristic changes in river-bed configuration of the reach and, through numerical calculations, that they have come to significantly raise the flood stage.