The temporal dynamics and spatial distribution of the concentrations of dissolved gases (He, Ar, Kr, N2, O2, and CO2) in an infiltrating groundwater system fed by the peri-alpine river Thur (Switzerland) were analyzed before, during and after a single, well-defined flood event. The analysis was based on measurements taken in five different groundwater observation wells that were located approximately 10 m apart and tapped the same groundwater body, but were situated in three different riparian zones. The input of O2 into the groundwater as a result of the formation of excess air was found to be of the same order of magnitude as that resulting from the advection of river water, although the amount of excess air formed and the amount of O2 delivered varied significantly among the riparian zones. The results suggest that the input of O2 into groundwater as a result of excess air formation is controlled not only by the hydraulic conditions prevailing in the river and the groundwater, but also by the thickness of the confining bed at the top of the aquifer. The sandy gravel aquifer itself is too coarse to trap a significant amount of air during the water level rise. The clay layer confining the aquifer, however, acts as a barrier hindering the escape of air from the subsoil to the surface, and hence is likely to be a key factor controlling the trapping and dissolution of air in groundwater.