Recently, it has been apprehended that the water environment in the upper Chikugo River in Hita, Oita, where the river channel forks in three tributaries: the Chikugo (the Mikuma), the Kuma and the Shode, becomes worse due to the stagnancy of water. In order to provide fundamental informations of 3-dimensional flow structures and temporal and spatial variations of water quality field surveys were carried out from 2003 to 2004. The following relations between the flow patterns and the water quality were found out:(1) the residence time is affected by the existence of horizontal eddy around the riffles, (2) lower discharge can generate the eddy and (3) turbidity and chlorophyll-a are regulated by the river discharge in the ponded area by the Shimauchi movable weir.
