In order to clear the functions of the floodway in the delta area, the history of the floodway development and the branch channel closing and the actual condition of the floodway were examined based on the consideration on the topogra phy and the geology. As the results, two conditions as the floodway of the excavated channel and the kept branch channel on the delta were confirmed: 1) For small capacity with the discharge less than 100% both the mainstream and the branch channel. 2) For restricting the channel separation by the existence of the water use forms on the old main channel.