The water level of ยฎve river stages and seven groundwater wells in the Taipei Basin were analysed by spectral analysis in the frequency domain. The diurnal, semi-diurnal and quarter-diurnal tidal components of the Tanshui River appear to relate closely to astronomical tides as K 1 , M 2 and M 4 , respectively. It is also found that the diurnal component reveals a reversed phase angle in the middle section of the Tanshui River; the phase of the quarter-diurnal component is also found to be reversed at stations upstream in the Tanshui River and Hsintien Stream. It is believed that these phenomena could be caused by local variation in the river channel topography. The autospectrum and cross-spectrum between groundwater elevation and nearby river stage were observed to correlate highly with the frequency of the astronomical tides K 1 , M 2 and M 4 . From the study of the phase shift and time lag of water level ยฏuctuations at river stages and groundwater wells, it was found that the tidal eects of diurnal, semi-diurnal, and quarter-diurnal components were signiยฎcantly dierent. The relationships between phase and the ยฏuctuated range of atmospheric pressure and water level imply that change in atmospheric pressure does not aect water level ยฏuctuation in the river stage and groundwater well.