In order to obtain information on the hydrological signature of rivers during and after heavy rain events, small catchment areas are selected as experimental sites. Hydrograph separations based on environmental tracers are performed. Natural isotopic tracers such as 18O, 2H and particularly 222Rn may help to distinguish the components dominating the outflow, particularly of 'pre-event waters', 'event waters' and 'post-event waters'. Even with moderate concentrations in groundwater, radon can be a very sensitive indicator of groundwater input into rivers. The selected microbasin under investigation is situated in the western part of Luxembourg and belongs to the Attert River catchment. At chosen points at the basin's outflow radon detectors continuously measure radon activity in water. The radon monitors are installed together with high precision thermometers, conductivity meters, flow meters and automatic water samplers for chemical analysis. Besides the continuous measurements, grab water samples are taken at different locations along the stream, most of them during periods of heavy rain events. Presented are the results of a one year measurement campaign. During the dry season i.e. during more or less continuous discharge conditions, the observed mean values do not show substantial variations and can be used as reference values. Fluctuations of the measured data during rain events are discussed and the interplay between the different parameters analysed.