Glen Canyon Dam on the Colorado River has dramatically altered organic matter budgets. Autochthonous production dominates the tailwater reach but allochthonous organic matter from tributaries dominates downstream budgets. We hypothesize that there is a concomitant shift in invertebrate consumption of basal resources. In June 2006, we collected dominant invertebrate taxa (simuliids, Gammarus and New Zealand mud snails (NZMS)) just below the dam and 385km downstream and analyzed gut contents using high-powered microscopy. Simuliid diets shifted from 82% diatoms and 16% amorphous detritus near the dam to 74% amorphous detritus and 22% diatoms downstream. This trend was also apparent, but weaker, for Gammarus (43% diatoms and 55% amorphous detritus upstream, 29% diatoms and 69% amorphous detritus downstream). In contrast, NZMS diets were similar in both reaches, averaging 50% diatoms, 40% amorphous detritus, and 10% filamentous algae. Because amorphous detritus can be autochthonously and/or allochthonously derived, we are conducting stable isotope analysis to determine its origins. Among-species differences in extent of diet shifts may be due to differences in food acquisition (e.g., reliance on fine suspended and deposited material vs. firmly attached benthic epilithon). Therefore, the relative consumption of autochthonous and allochthonous resources by invertebrates depend on availability and consumer identity.