The effect of the artificial flood to river channel disturbance is clarified quantitatively using the results of field observations and numerical simulations (iRIC Nays2D 4.2). The 1st viewpoint is maintenance of distributary leading to former watercourses which induces river channel disturbance at the time of a large flood. The 2nd viewpoint is a scale of river channel disturbance at the time of a large flood. The difference between the existence and non-existence of the artificial flood has been discussed. It was evaluated quantitatively that artificial flood is effective in maintenance of distributaries. Moreover, it was found quantitatively that the effect of mainstream alternation at bifurcations during a large flood which is expected by the maintenance of distributaries.
