This study sought to understand community variations in fluvial deposits (FD) in a Brazilian river after the disturbance caused by the immersion of the islands, by the dynamics of water flow due to floods; identify species and forms of establishment and understand the maintenance and restoration of the richness and diversity of ecological groups. A phytosociological analysis of the herbaceous-subshrub component was performed using Relevè in two fluvial deposits in periods for two consecutive years (July to October / 2020 and 2021), post-floods. Fifty 1x1m plots were distributed on each island. Phytosociological parameters were calculated. In the sampling, 42 species were identified, from 35 genera and 24 families in FD1 and 32 species, from 31 genera and 23 families in FD2, totaling 46 species, from 39 genera and 27 botanical families (excluding similarities). Most common growing forms were herbs. Camephytes and phanerophytes were predominant life forms. The most frequent dispersion syndrome was autochory. Variations in phytosociological parameters were observed both among FDs and in the same FD, for analyzed periods. The data were correlated to the Community Assembly Theory. It was concluded that the size of the FD probably influenced richness and that life forms and dispersal mechanisms corroborated the compositions after environmental disturbance (flood).