PURPOSE: To prevent the generation/propagation of phytoplankton in a storage water area by forming a thermocline so as to move the same upwardly or downwardly from an inherent forming position and holding the lower layer part below the formed thermocline to a stagnation state and holding the upper layer part thereabove to a flow state.

CONSTITUTION: A thermocline 2 is formed to a reservoir 1 in a usual state and the water in the reservoir 1 is separated into an upper layer water area A and a lower layer water area B across the thermocline 2. An intake port 4 is provided in the vicinity of the surface 3 of the water in the reservoir 1 and, by taking in water from the water area A in an amount equal to or more than the inflow amount of water. The pondage of the upper layer water area A of the thermocline 2 is reduced and the thermocline 2 can be set to a state substantially risen to a position 2' and, at the same time, the upper layer water area A flows in an intake direction and a stagnation time is also shortened. In this case, the lower layer water area B is isolated by the formed thermocline 2' and almost held to a stagnation state regardless of the flow state of the upper layer water area A. By this constitution, the growth mechanism in the generation/propagation of phytoplankton in the storage water area is destructed.