The flood flow in rivers possesses characteristics of storage and peak attenuation of discharge. A sufficient understanding of these characteristics is essential for the river design. Our past studies have deepened basic understanding of the flood flow and clarified the influence of unsteadiness of flood flow, levee and main channel alignments and trees on flood channels. The objective of this study was to investigate detailed mechanism of the flood flow storage in channels, such as the effect of main channel width, sinuosity and boundary condition on water storage of flood flows. We conducted some experiments for understanding the above objectives. This study has highlighted the importance of consideration of channel storage in river design projects.