Glen Canyon Dam has drastically altered organic matter budgets of the Colorado River. Autochthonous production now dominates tailwater reaches and tributary-derived allochthonous carbon dominates downstream budgets. We evaluated our hypothesis that invertebrate diets would reflect longitudinal changes in resources by examining gut contents of the dominant taxa in the system--simuliids, Gammarus and New Zealand mudsnails (NZMS)--in 2006 and 2007.  During clear water periods, simuliids and Gammarus consumed autochthonous carbon in the tailwaters and allochthonous carbon downstream; NZMS consumed both resources across all sites. In contrast, during turbid water periods or low light seasons, all species diets’ shifted towards reliance on allochthonous carbon. Stable isotope analysis of basal resources and invertebrates confirm observed shifts in resource consumption correspond to shifts in assimilation.  We also measured diatom species composition in simuliid guts. Over 50% of the diets of simuliids collected from the tailwater reach was composed of a planktonic diatom Fragilaria crotonensis. Forty-eight km downstream, this diatom comprised only 6% of the diets.  Our results suggest that (a) consumer diets are flexible and co-vary with resource availability and (b) detailed gut analysis may prove useful for elucidating spatial patterns of resource use by invertebrates.