Abstract Tidal wave may be distorted when entering the river of large topography variations. The traditional tidal harmonic may not be applied well to data collected in such a shallow river. Godin (1998) indicated that the hydrodynamics of rivers is dominated by the damping and the distortion induced by quadratic bottom friction. This research try to analyze the distortion of tidal wave in the Tanshui River (northern Taiwan) based on field measurements. Sea levels data are collected at fours stations along the lower river: Pole (4km offshore), Tanshui harbor(on the coast), Kuan-Do Bridge (7km inland) and Yuan-Shan Bridge (18km inland). The total distance of the four stations is 22 km. The synchronizing sampling period is January 10 to February 28 of 2001 (50 days). The analysis is trying to identify the following issues. (1) The transformation of tidal wave from coastal into the river and upper stream. (2) The errors of tidal prediction based on harmonic analysis. Several different conditions are examined including spring-neap tides, influence of fresh water output and peaks of high-low water. The influence of bottom friction of the inner river stations is emphasized. The results show that distortion of tidal wave is more pronounce inside the river. The main reason is explained due to the strength of bottom friction, which is proportional to the flow velocity. (1) During spring tide, a stronger flow contributes a larger bottom friction. (2) A higher water level (h) may have a faster wave speed ( ) that high water occurred before the forecast. The deviation in high water also is more significant than that of low water. (3) River flow increases the water level and cause the deviation of wave propagation inside the river.