The river mouth morphology changes in the Samegawa river mouth during its recovery following The Great East Japan Tsunami of 2011 was investigated in this study by analyzing the water level data and aerial photo. The aerial photos provide valuable information on the changes. However, their availability is limited. The correlation coefficient and the linear gradient between the water level data in the river and the tidal level were analyzed. The river mouth narrowing, as well as the river mouth opening has been successfully detected. The assessed parameters correspond well with the condition obtained from the aerial photo analysis when available. In addition, these parameters also provide information on the river mouth condition when there are no aerial photo data. The proposed method in this study will provide an efficient way for analyzing and evaluating the morphology changes at a river mouth.