Disturbance effects of floods in floodplain ecosystems have been widely studied over the last three decades. But few authors have examined evolution of the river channels (channel shifting, channel degradation/aggradation) as the driver of disturbance regime variation for plant communities. In this study, we analysed (i) the impact of channel vertical mobility on the disturbance regime experienced by homogeneous forest stands and (ii) the influence of disturbance frequency and intensity on understory herbaceous and shrub vegetation. We found that patches of heterogeneous floodplain topography are strongly correlated with the spatial position (distance to the channel) and the vertical evolution of the channel that influence flood disturbance regime. In equivalent forest types, small differences in disturbance frequency generate significant differences in understory community such as higher diversity, variability of richness and frequency of annual species and of post-pioneer tree germinants. Our results also show that disturbance frequency is not sufficient alone to explain community modifications: in the range of observed disturbances, water flow and sediment supply/reworking are both necessary for affecting understory vegetation. Sediment reworking maintains some open spaces, decreases dominant species frequency and maintains diversity (for a relatively constant richness). Lastly, we observe difference in responses at event and longer time scales (e.g., one flood versus a few years); therefore effects of disturbance regimes on forest evolution are difficult to evaluate without long term surveys.