The trophic base for consumers (invertebrates, fish) in the Colorado River is likely derived from two organic matter sources: 1) autochthonous production of algae, and 2) allochthonous inputs of terrestrial detritus from tributaries. Glen Canyon Dam (GCD) has reduced transport of organic matter and sediment from the upriver watershed resulting in clearer water and therefore higher rates of autochthonous production, particularly in the tailwater.  Tributary-derived allochthonous inputs are restricted to weather-related episodes like monsoonal thunderstorms.  We used stable isotopes of hydrogen (dD) to investigate the relative importance of aquatic and terrestrial sources of OM to consumers in the Colorado River pre- and post-monsoon season.  Previous data showed that autochthonous OM is uniformly more depleted in dD than allochthonous OM. Our dD data indicate that autochthonous OM contributed to the food base near GCD but this importance decreased downstream as inputs of allochthonous OM from tributaries began to dominate the organic matter budget. Following the monsoon season, terrestrial inputs appeared to become even more important at downstream sites as dD values of consumers were enriched relative to those measured pre-monsoon. Results will be discussed in light of recent feeding experiments assessing diet-tissue fractionation and water contribution to fish dD.