The nitrogen (N) cycle of freshwater ecosystems influences watershed scale N budgets through both new reactive N inputs via N fixation, and losses back to the atmosphere via denitrification. The relative importance of these N cycle processes throughout river networks is difficult to determine. We measured the disequilibrium of N2:Ar across a number of streams of different size, N loading, fluvial wetland influence, in beaver ponds, and in reservoirs at various time scales (diel, storms, and seasonal) to evaluate the relative importance of N fixation and denitrification. Across all sites, N2:Ar disequilibrium (here defined as N2:Ar-measured / N2:Ar-equilibrium) ranged from 0.9, indicating N fixation, to 1.05, indicating denitrification. Systems with high oxygen conditions (>100% saturation) exhibited only N2:Ar undersaturation (never oversaturation), suggesting N fixation is coupled with high primary production. Undersaturation occurred primarily in a shallow reservoir where light was plentiful and DIN was low. Fluvial wetland dominated streams, where oxygen was low and flow less turbulent, showed the highest N2:Ar disequilibrium, suggesting high denitrification coupled with low gas exchange rates. Within sites, N2:Ar disequilibrium was highest during summers , likely due to higher biological activity coupled with lower flow. Diel variability was periodically evident in three headwater streams intensively sampled over 24-hour periods, showing generally positive ratios but with daytime declines, suggesting overall denitrification but with either reduced daytime denitrification due to higher oxygen or an increase in N fixation during daylight hours. These findings suggest N2:Ar measurements have the potential to provide a quick index of N cycle processes affecting the N balance in surface waters.