Little past progress has been made in the evaluation of the impact of dams on the environments of the rivers downstream from the dams, because the impact manifestation mechanisms are complex and the impacts themselves are not easy to abstract. We have conducted field survey to investigate the change in river bed between upstream and downstream of 15 dams. Riverbed material seems coarser at any immediately downstream sites due to the trap of sediment behind dams; however, downward grain size change pattern was different among rivers. So then we applied the statistical analysis to its patterns and classified 15 dams into six groups. And we evaluated their feature by comparing it with the calculated grain size which is estimated from the shear velocity.
