The Kamanashi River is a steep river covered with gravel grains, cobbles and boulders. In 1982, the record breaking largest flood occurred and brought large scale sediment deposition in the river channels due to sediment yielding from the upstream mountainous areas. In addition, the grain size distributions became coarser in the upper reaches and finer in the lower reaches due to sorting during the flood. In this study, we developed and applied a flood flow and bed variation analysis method considering the nonequilibrium sediment motions of boulders, gravel grains and sands with large amount of suspended sediments in the 1982 flood. The developed model could explain that the bed materials in the downstream reaches became finer by fine sediment grains transported from the upstream to the downstream due to longitudinal sorting.
