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Population structure and ecological characteristics of component species of a riparian Ulmus-Quercus forest in central Japan were analyzed with special reference to riparian disturbance regime. Though the dbh distribution of the whole community was L-shaped, those of several component tree populations had several modes, suggesting intermittent regeneration periods in the past. Correlation of spatial distributions among tree populations and subpopulations showed 6 major groups reflecting riparian disturbances in the past and different establishment patterns among species. A cluster of small-sized tree populations (Salix sachalinensis, Alnus hirsuta and Populus maximowiczii