Streams are typically conceptualized as well-oxygenated systems with food webs supported by autochthonous and allochthonous, photosynthetic carbon sources. However, streams are also increasingly recognized for the presence of hypoxic or anoxic conditions, and the presence of methane whether it be in food webs or dissolved form. These contradictions highlight the potential for methanotrophy and chemoautotrophy to contribute to food webs, but these contributions are rarely tested simultaneously, if at all. We identified a geologically complex gravel-bed floodplain in Colorado to test these contributions to the food web. We used δ13C and Δ14Cvalues of dissolved gases, periphyton, organic matter, and consumer biomass to parse contributions of ancient methane and carbon dioxide to macroinvertebrate biomass via methane, sulfur, and ammonia oxidation using Markov chain Monte Carlo mixing model simulations. We found that mean δ13C values of consumer biomass was -33.2 ± 0.9 ‰ and Δ14C -based biomass ages ranged to 10,000 years BP, indicating a contribution of CO2 dated to 15,000 years BP. Ammonia and sulfur oxidation contributed between 11 and 33% of macroinvertebrate biomass, and that 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 when consumer δ13C is not obviously depleted, yet they might provide a cryptic carbon source that complicates our assumption of allochthony and autochthony in streams.