Summary River floodplains with their constituent biodiversity are among the most endangered ecosystems worldwide. These systems are conceptualised as mosaics of aquatic and terrestrial patches at different successional stages colonised by a wide range of aquatic, semi‐aquatic and terrestrial species. In three reaches in the active corridor of a French Mediterranean braided river, we tested whether community structure and composition of ground‐dwelling terrestrial arthropods varied across the mosaic of terrestrial patches, and identified some of the underlying mechanisms. We predicted that arthropod community structure and composition would vary consistently with the age and position of the patches in the mosaic, due to species habitat preferences and dispersal. We also predicted that between‐patch species turnover would contribute to corridor diversity to a greater extent than within‐patch diversity. The natural flow regime of the Asse River promoted patch diversity in all the reaches considered. Arthropod communities differed according to patch age, indicating that successions had occurred over a decennial timescale. By contrast, the location of patches within the mosaics seemed not to affect communities, suggesting that dispersal was not limited. The contribution of between‐patch diversity to corridor diversity far exceeded that of within‐patch diversity, as predicted by mosaic theory. These results put the onus on river ecosystem managers to preserve natural flow regimes in braided rivers, so as to foster various types of pioneer and mature habitat patches and maintain associated biodiversity.