Human activities in the riparian zone can affect the feeding of stream fish because they alter autochthonous production (periphyton, macrophytes and aquatic insects) and allochthonous inputs (terrestrial insects, leaves, seeds and fruits). In the present study we investigated how the diet and intestinal length of a persistent and generalist fish species (Bryconamericus iheringii, Characidae) responds to riparian modifications in 31 subtropical streams in southern Brazil. We hypothesised that intestinal length would be longer in populations inhabiting streams with converted riparian vegetation as a consequence of greater consumption of an indigestible and low-protein diet. Populations of B. iheringii from streams with a degraded riparian zone and reduced canopy cover had longer intestinal length (after accounting for body size), which was associated with decreased consumption of terrestrial plants and invertebrates and increased ingestion of filamentous algae, macrophytes and detritus. These results indicate that anthropic alteration of riparian zones and increased canopy openness trigger shifts in the diet and intestinal length of B. iheringii. The findings suggest that plasticity in intestinal length is an important characteristic to determine whether fish populations can persist in a variety of habitat conditions and cope with the digestion of a greater proportion of low-quality and low-protein food items in human-altered environments.