The natural, groundwater hydrograph of water-table levels reflects the dynamic response of the groundwater flow system to climatic stimuli. Consequently time-series analysis has been employed to examine the frequency structure of long-term climatological and hydrogeological records, and to compare hydrographs from different parts of Northern Ontario, which are known to experience considerably different climatic regimes. The results of the study show that the soil–moisture zone filters out high-frequency climatic cycles before they reach the water table. Furthermore by dividing groundwater hydrographs into a deterministic component, the mean annual hydrograph, and a stochastic component, the residual time series, it is shown that only hydrographs of observation wells within the same hydrogeological subregion have deterministic components with coincidental inflection points and stochastic components that are inphase and coherent. It is also shown that the amplitudes of the deterministic components vary rather unpredictably, and that the variance spectra of the stochastic component of hydrographs from the same hydrogeological region tend to be similar irrespective of climatic differences within the region.