Methanogenesis is an energetically poor way for microbes to make a living, yet under certain thermodynamic conditions, large quantities of methane can be produced within aquatic ecosystems.  To explore the relationship between hydrologic and thermodynamic variables and methane production in an alluvial aquifer, we measured methane concentrations across the Nyack floodplain of the Middle Fork of the Flathead River in northwestern Montana from July 2008 to the present.  Other groundwater constituents and trace gases (e.g., CO₂ and N₂0) indicative of biogeochemical processes were also measured, as were molecular assessments of microbial community function to assess the relationship between other terminal accepting processes and methanogenesis.  Results indicate that methane concentrations range widely from below detection to 38 mmol/L, where methane hotspots are associated with higher dissolved organic carbon concentrations, intermediate groundwater residence times, and where oxygen is below 3 mg/L.  The abundance of methane in the aquifer suggests that methanogenesis can contribute to floodplain productivity and future work will investigate the incorporation of methane-derived carbon into the hyporheic foodweb.