Recognizing the importance of invasive plant species is necessary to preserve, protect, and manage for biodiversity in riparian habitats. Non-metric multidimensional ordination of 28 study sites along a successional gradient on the Willamette River indicated that abundance of reed canarygrass (Phalaris arundinacea L). was strongly correlated with stand plant composition and abundance. High abundance of reed canarygrass was also correlated with lower values of understory species diversity and total species richness. Considering older stands only (n = 19), reed canarygrass presence and abundance was the most highly correlated plant species with overall stand plant species presence and abundance. This was probably due to this invasive species inhibiting establishment of understory tree, shrub, herbaceous species, and late-successional tree species during understory reinitiation. Without intervention to control the establishment and survival of reed canarygrass, and perhaps some other invasive species, such as Himalayan blackberry (Rubus armeniacus L.) and English ivy (Hedera helix L.), it is conceivable that these species will become more influential through time with adverse effects ensuing for overall biodiversity at the riverscape level.