The Japanese government conducts an artificial flood by dam discharge for the purpose of preservation of river environment at the Satsunai river from 2012. This research was conducted using numerical simulation (iRIC Nays2D4.2) for the purpose of understanding quantitatively the effect of the artificial flood to river channel disturbance. It was evaluated quantitatively that artificial flood is effective in maintenance of anabranches. 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.