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Environmental influences on species' functional traits are important ecological issues to assess biodiversity. Relationships among fish abundance, their functional traits, and environmental conditions across different levels of anthropogenic impacts in a tropical Brazilian river were evaluated. We combined RLQ and fourth-corner methods, utilizing local environmental variables and the Human Footprint Index (HFI) as a human activities indicator. Three HFI levels (High, Intermediate, and Low) and short-term (1993–2009) impact changes at each location were assigned. Fish with internal fertilization and small body sizes were strongly associated with high HFI scores. Grass-dominated riparian areas were common in altered locations, whereas less altered localities exhibited the riparian cover dominated by trees. Highly altered areas showed high HFI, grass-covered riparian zones, and small-sized species with internal fertilization (e.g., Cyprinodontiformes like Poecilia vivipara, Poecilia reticulata, and Phalloceros caudimaculatus). In contrast, less altered areas had tree-lined riparian zones and medium to large-sized species with external fertilization and diverse traits. Underlying deterministic processes shape species distribution, tied to environment and traits. Filtering of traits in the most altered locations may favor small-sized species with internal fertilization. This approach, utilizing HFI and local variables to assess trait–environment relationships of riverine fish, facilitates understanding organisms' responses to environmental constraints. [ABSTRACT FROM AUTHOR]