The supply and availability of dissolved organic carbon (DOC) and its interactions with nutrients affects ecosystem processes, such as respiration, nutrient retention, and nutrient export.  However, little is known about drivers of carbon bioavailability within aquatic ecosystems.  We sampled surface and well water from the main channel and alluvial aquifer of the Nyack Floodplain of the Middle Fork Flathead River in northwestern Montana monthly from May to October, 2008.  We measured DOC and nutrient concentrations, along with dissolved oxygen, specific conductance, pH, temperature, and radon concentration (a surrogate for groundwater residence time (GWRT)).  In the laboratory, we conducted aerobic nutrient-amended incubations of water samples and measured two metrics for DOC bioavailability (bDOC): CO₂ production and DOC depletion.  bDOC increased with GWRT for all months except October, suggesting autochthonous DOC production along groundwater flow paths.  GWRT was also a good predictor of electron acceptor availability (dissolved oxygen, nitrate and sulfate concentrations), but their relationships changed throughout the year.  This research suggests that electron donors and acceptors vary predictably along flow paths within the aquifer, but that the aquifer is a temporally dynamic mosaic of biogeochemical reactions that creates variable patterns of ecosystem respiration, nutrient retention, and export across seasons.