Streams are typically conceptualized as well-oxygenated systems supported by terrestrial and aquatic carbon fixed by algae and plants. However, streams are also increasingly recognized for the presence of low oxygen conditions, and the presence of methane which is produced in low oxygen conditions. These contradictions highlight the potential for alternative energy sources to support animals in streams. We identified a geologically complex gravel-bed floodplain in Colorado to test these contributions to the food web. We used isotope ratios of dissolved gases, algae, bulk organic matter, and animal biomass to parse contributions of ancient methane and carbon dioxide to animal biomass via alternative energetic pathways. We found that animal biomass ages ranged to 10,000 years BP, indicating a contribution of CO2 dated to 15,000 years BP. Ammonia and sulfur oxidation, two alternative energy pathways, contributed between 11 and 33% of animal biomass, and these pathways facilitated incorporation of geologic carbon into the food web. This work underscored the need for better quantification of redox process contributions to freshwater food webs. Importantly, these pathways are very rarely considered, but they might provide a cryptic carbon source that complicates our assumption that stream energy sources come exclusively from terrestrial plants and algae.