Riparian systems are prone to invasion by alien plant species, which may be facilitated by hydrochory, the transport of seeds by water. Love et al. study gene flow associated with hydrochoric dispersal of the invasive riparian plant Impatiens glandulifera (Himalayan balsam) in two contrasting river systems and find a significant increase in levels of genetic diversity downstream, consistent with the accumulation of propagules from upstream source populations. There is strong evidence for organisation of this diversity between different tributaries, reflecting the dendritic organisation of the river systems studied. The results indicate that hydrochory, rather than anthropogenic dispersal, is primarily responsible for the spread of I. glandulifera in these river systems.