A lot of alien plant species invaded the fluvial environment. Up to now, some researchers pointed out that the disturbance by the flood help the invasion of alien plants. But there are a few studies how alien plants invade into the river ecosystem. Because grand-scale experiment was difficult, considering the flood control and water utilization of the river, the alien invasion had not been examined. Fortunately, we have a chance to experiment how the alien species invaded by using a riverbed where the buried seed was removed. At the construction of the riverbed, Sicyos angulatus must be removed from this area. This species make a lot of burial seed population, then the planner of the park washed soils 2m in depth and laid. As a result three of the following became clear, 1) the vegetation of the river was maintained by seeds flow out, 2) the seeds were supplied with rising of the river, 3) the duration of the seed emerging at alien plant species are longer than that at native plant species. The character that the seeds are able to emerge anytime and anywhere like the alien plants species are advantageous at the fluvial environment where rising of the river sometimes occurs. It was shown that the seed emergence occurred in a short term like native plant species missed the occupation chance. This pattern was remarkably admitted in spring in autumn.