Dryland rivers are characterized by high spatio-temporal variability in water resources, with alternating wet and dry reaches and fluctuating stream flow rates. To investigate response to this dynamic, riparian plant communities from perennial, temporally intermittent, and ephemeral-flow reaches of the Hassayampa River and an ephemeral tributary (southwestern USA) were characterized over a three year period, and soil seed banks (emergence method) were assessed. Variance through time in cover and diversity of wetland (hydric) species was lowest at perennial sites and highest at ephemeral flow sites. Wetland plants established along the active channel of the ephemeral reach following a 10-year recurrence interval flood, and compositional similarity between perennial and ephemeral sites was high during this wet year. About half of the wetland species growing at ephemeral flow sites were present in floodplain soil seed banks. Wetland species emerged from soils of the ephemeral Hassayampa River (but not of the ephemeral tributary) but their density and diversity declined with distance downstream of the perennial reach. Wetland species in the seed banks of ephemeral-flow sites were a nested subset of those at wetter sites, raising the possibility of dispersal from upstream sites. These results indicate that wetlands develop episodically on ephemeral reaches of spatially intermittent rivers of the Sonoran Desert following high winter runoff, and suggest that the wetland plants arise from local seed banks and off-site sources. The study also suggests that riverine restoration success is sensitive to location within a stream network: upon re-watering, sites closer to perennial reaches will have greater capacity to self-assemble wetland plant communities.