The objectives of this study were to clarify the defects of the current tidal river monitoring method and to propose a better method to obtain water quality data of high quality for tidal rivers. In Japan, the Water Quality Standards for rivers also apply to tidal rivers. The method indicates that water should be sampled from 20% of the water depth below the water surface at an arbitrary time once a month. Since this method was apparently inappropriate to understand the dynamics and water profiles in tidal rivers, field surveys were conducted at different times in the River Rokkaku, which is well mixed. The results showed that the turbidity maximum moved up- and down-stream more than 10 km due to the tide. Based on this fact, a new monitoring method was proposed, which required water samples to be taken with a certain time lag from a high tide at a station. This newly proposed method improved the quality of information on water quality and made data available to ascertain long term trends. Modifications to the new method are suggested to further improve the quality of water quality data for tidal rivers.

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