In order to clarify the distribution pattern of soil seed bank and dispersion of seeds by river water flow, germination test were made by applying the surface soil collected from four sites (main channel side, bank marsh, remnant channel, bank lowland) in floodplain of River Arakawa. The abundance of seed banks in back marsh site is relatively less than any other sites. The seed banks of main channel side site had high seedling density of the species that might be dispersed by water (hydrochory) such as Veronica anagallis-aquatica, Cyperus glomeratus, which did not have special feature adapt to water dispersal. In contrast, the seed banks of the back marsh site apart from main channel had low seedling density of those species. Therefore, the difference of seedling density of the hydrochorous species can be explained by the distance from main river channel. The similarity between the floristic composition of actual vegetation and seed bank was higher in main channel side site than any other sites. It might be reflected that high disturbance by flood had been occurred in main channel side. The spatial heterogeneity of seed bank composition in lowland riparian habitat can be partially attributed to water dispersal.