Work in terrestrial ecosystems has suggested that the breakdown rate of leaf litter may not change predictably with increasing plant species richness. Indeed, it may be that certain combinations of leaf species demonstrate significant non-additive effects on breakdown rates, mediated by the presence of a single key species. Such effects have not been explored in running-water ecosystems despite the strong interest in the conservation and restoration of riparian systems. We documented the magnitude and species composition of leaf litter inputs, the species richness and composition of leaf litter on the streambed, and estimated the breakdown rate of mixed litter in both the summer and autumn in a warmwater stream. We found that leaf species richness of litter packs on the streambed varied from 1 to 11 species, and leaf species composition reflected the composition of litter inputs from May through November. We did not find a general relationship between breakdown rate and leaf litter species richness. However, we did find a strong effect of species composition of leaf packs on the breakdown rate during the summer. Overall, breakdown rates of mixed-species leaf packs were non-additive during the summer, but very predictable in the autumn. In particular, leaf mixtures containing American sycamore always exhibited slower breakdown rates than predicted in summer. One explanation for the discrepancy between summer and autumn results may be decreased temperature in autumn; reduced temperature may have slowed breakdown rates across treatments to the extent that any non-additive effects found in summer were masked by the effect of temperature. Given the importance of detritus to stream food webs, the simplification of plant assemblages along intact or restored streams may have important implications for stream ecosystems.