Glen Canyon Dam has altered the habitat and ecology of the Colorado River. Clear water in this tailwater promotes high rates of primary production. Non-native invertebrates and trout dominate animal productivity. We constructed a quantitative food web for the Glen Canyon reach using measured rates of secondary production and a combination of dietary and stable isotope analyses.  Most invertebrate secondary production was derived from diatoms (68%), while 16 and 11% was derived from amorphous detritus and animal material, respectively. 63% of all energy flow was consumption of diatoms and amorphous detritus by non-native New Zealand mudsnails (NZMS) and 34% was consumption by Gammarus lacustris (a non-native amphipod). Based on one season of diets, trout production appears to be derived from 28% filamentous algae and associated diatoms, 23% aquatic invertebrates and 22% terrestrial invertebrates.  NZMS were not present in guts.  A large amount of energy either passes through non-native taxa or is rendered unavailable to higher trophic levels. Using organic matter flows to examine trophic interactions in this tailwater may lead to new insights in food web dynamics and aid in system-wide conservation of native species.