ABSTRACT Dispersal is a key process affecting the diversity of natural communities. We addressed hydrochory of wetland plant seeds in the Middle Paraná River floodplain. We first studied seed dispersal by drifting macrophytes in the Paraná River main channel (MC), in a high discharge secondary channel (HD) and in two low discharge channels (LD) during an extraordinary flood. We then experimentally analyzed the effect of standing (SW) and moving water (MW) on seed buoyancy of different plant communities. We recorded seeds of 27 taxa distributed in 12 families. Taxa richness ranged from 17 in LD to 25 in MC, and included seeds of terrestrial, palustrine and aquatic plants. River discharge did not affect seed richness and density, which was probably associated with a homogenization process caused by the flood. Seed buoyancy significantly differed between water movement treatments independently of the source community, lasting longer in SW than in MW. Our results suggest that drifting macrophytes contribute to seed dispersal of several communities in the Middle Paraná River, and probably over long distances. Furthermore, seed buoyancy might be more important for surficial dispersal in low-energy systems, where subaqueous dispersal may be difficult due to the lack of current.