© 2017 Botanical Society of Scotland and Taylor & Francis. Background: Floodplain wetlands feature high plant biodiversity, comprising variability in several organisation levels and at different scales. Aims: In a large river floodplain, we aimed to answer whether taxonomic, phylogenetic and functional $α$-diversity of herbaceous assemblages responded to hydrogeomorphic features conditioning the local flood pulse and/or to soil features; and to determine the relationship between $β$-diversity and geographic and environmental distances. Methods: Taxonomic, phylogenetic and functional $α$-diversity indices were computed in 82 sites in the Lower Paraná River floodplain. Relationships between $α$-diversity and environment were explored by generalised linear models. Spatial structure was assessed by semi-variograms. Responses of partitions of $β$-diversity to geographic or environmental distances were tested through multiple regressions in distance matrices. Results: Taxonomic and phylogenetic $α$-diversity show ed a similar pattern in relation to environment, especially to hydrogeomorphic features. The regression model fitted to species richness had the highest explanatory power. Regarding functional $α$-diversity, only the number of plant functional types responded to environmental features. Partitions of $β$-diversity, particularly the replacement of functional strategies, were better explained by environmental than by geographic distances. Conclusions: The diversity patterns of plant assemblages in a large river floodplain highlight the importance of heterogeneity conditioning the local effect of flood pulse regimes.