The 2016 Kumamoto earthquakes comprised an Mw 6.2 foreshock and an Mw 7.0 mainshock that occurred at 21:26 JST on April 14 and at 01:25 JST on April 16, respectively. Ruptures and vibrations associated with these earthquakes induced effects on streams' discharge in Kumamoto and Oita prefectures; however, few studies have conducted comprehensive investigation of such changes in those areas. In this study, 174 measurements of river water level were carried out as reflecting increase, decrease, or no change after the foreshock or after the mainshock when compared with the level before the foreshock. Trends of increase and decrease after the foreshock (mainshock) were recognized at 6 and 19 (37 and 15) stream gauges, respectively. The ones that showed a trend of increase after the mainshock were located mostly in mountainous areas (elevation >300 m). A coseismic increase in river water level suggested increased groundwater supply from the mountains via enhanced permeability. The difference in river discharge between upstream and downstream gauges relative to the Suizenji fault zone revealed a huge volume (approx. 900,000 m3) of river water was lost from the river during the 12-h period following the mainshock. It is possible that the missing river water flowed into new fault zones formed by the earthquakes.
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