The decline of native fishes of the Colorado River in Grand Canyon has coincided with the effects of dam-regulation, which included reduced sediment-load and establishment of nonnative rainbow (Oncorhynchus mykiss) and brown (Salmo trutta) trout.  Downstream of the dam, the Little Colorado River (LCR) periodically contributes sediment, and is also a source of young native fish (e.g., endangered humpback chub, Gila cypha) to the main-stem, creating a potentially important zone of native-nonnative interaction.  As part of an experimental depletion of nonnative fish, diets of >18,000 trout were examined over a two-year period upstream and downstream of the LCR, to assess spatio-temporal variation in diet in response to changes in predator and prey densities, food availability, and turbidity.  Trout were more abundant and consumed more prey upstream vs. downstream of the LCR, though availability of drifting invertebrates was comparable.  Brown trout were most piscivorous, yet the least abundant (<2%) trout.  Rainbow trout consumed mainly Simuliidae (60-80% of diet), but piscivory increased downstream of the LCR where small, vulnerable native fish were more abundant, particularly when high turbidity reduced encounter rates with drifting invertebrates.  Tributary sediment inputs may affect the distribution and mediate interactions of native and nonnative fishes in Grand Canyon.