This study investigated the characteristics of tsunami propagation into a river by a practical method using the measured water level data. The verification of the proposed method was carried out by a numerical simulation using hypothetical cases, because the availability of measurement data was limited. This method was influenced by the measured data resolution. The sensitivity due to the data resolution was evaluated for the accuracy, and it showed a reasonable range of correlation coefficients compared to the numerical simulation result. Based on the validation result, this method was used to estimate the river discharge and the flow velocity induced by the 2011 Tohoku Tsunami. It was found that the tsunami induced discharge was relatively small compared to the existing design flood discharge. This study will be useful in determining the designing structures, flood prevention, river management concerning future disasters.