The reservoirs in impounded rivers are blocked by dams, implying that plant seeds, especially waterborne ones, have trouble dispersing between reservoirs. We examined whether the tributaries have an important role in the dispersal of seeds to a specific reservoir. We did this by releasing small, coloured wooden cubes in three tributaries of the Stensele reservoir in northern Sweden, one small, one middle-sized and one large-sized. This work was carried out in spring during major flood. At the day of cube release, we counted the number of cubes that dispersed from the tributaries out into the reservoir. In summer, we collected the cubes that had stranded on the banks of the tributaries to learn how far they had spread within each stream. Our examinations showed that a big stream disperses most seeds the longest distance. The smaller the stream, the shorter the distance the seeds were spread, and the fewer the seeds that reached the reservoir. We concluded that large tributaries have an important role in dispersing plants to reservoirs of impounded rivers.