The vertical profiles of water current, temperature, chlorophyll-a (Chl-a), and meteorological conditions were determined in the Ogouchi Reservoir in order to investigate the mechanism underlying the prevention of phytoplankton growth by a vertical fence. We observed a wind-driven current directed upstream in the surface layer and a countercurrent simultaneously in the middle layer, and then observed the current that traversed from the upstream region of the vertical fence to the downstream region. The water current continuously flowed in a downstream direction beneath the fence, the temperature was the same as that at the point of river inflow. The results indicate that the water temperature of the mixed layer upstream of the vertical fence decreased as did the supply of nutrients to the surface water throughout the reservoir. These findings suggest that the water current was modified by the vertical fence, and phytoplankton growth was prevented.