Most floodplains have been drastically altered by vegetation clearance and river regulation for human needs. In the mid-latitudes, these degraded ecosystems now face the formidable challenge of rapid warming and drying of climates. If the plant diversity of floodplain forests is to be maintained under future climates, their management must be informed by an understanding of how anthropogenic stressors and environmental gradients shape these ecosystems. We used a field survey to examine the potential drivers (forest structure, flooding and anthropogenic disturbance e.g. grazing and logging) of species richness and composition of Eucalyptus camaldulensis floodplain forests in southeastern Australia. Ninety–three stands were surveyed over 15,500 ha of forest, covering a representative range of forest structures and landscape positions, on a mesic and a semi-arid floodplain. Forest structure was an important predictor of the richness and composition of the understorey of river red gum forests on both floodplains. In particular, richness of native species was associated negatively with increasing canopy cover. On the semi-arid floodplain, where there was recent flooding and grazing, these disturbances were also associated with changes in understorey composition. Recent flooding was associated positively with native species richness, with flooded stands having twice the number of native species found in unflooded stands. Recent grazing was associated with an increase in species richness, which may reflect the concentration of herbivores around flooded areas. Targeted thinning to increase structural diversity among stands may temporarily enhance habitat heterogeneity and plant diversity of floodplain forests. If a diverse, native floodplain forest community is to be sustained in these landscapes, management must include regular flooding that approximates the diversity of historical flooding regimes.