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Patterns of plant species richness and community composition were studied in diverse riparian zones. Eight rivers in south-western France, one river in north-western USA and four rivers in south-eastern Spain were compared. A brief review is presented of factors that have been singled out as controlling diversity along rivers. The proportions of plant group classified according to life-span, morphology, ruderality and location along the rivers were analysed. Both proportions of plant traits and total species richness differed considerably between rivers. Semi-arid rivers were more influenced by local patterns than the other rivers. The patterns exhibited by true riparian species and invading, essentially non-riparian species, showed the importance of the relationships between the river and its close surroundings. The mean proportion of ruderals remained roughly the same in all rivers, suggesting some regulatory mechanisms independent of the general environmental settings. The longitudinal patterns of species richness in riparian plant communities of the oceanic rivers could be related to the intermediate disturbance hypothesis, whereas semi-arid rivers showed very irregular patterns of species richness along their courses, as a consequence of the irregular water availability. Exotic and ruderal species were more sensitive than other groups to the longitudinal zonation of the rivers and to environmental factors influenced by humans. Also, the permanency of superficial water appeared as a major factor in the control of riparian community structure in semi-arid regions.