PURPOSE: To permit the operation of the system with the minimum power by a method wherein the system is operated under adjusting the height of the top of a dam, over which water overflows, in accordance with the blade opening degree of the movable blade pump.

CONSTITUTION: The adjusting of the height h0 of the top of the dam 6 is effected by controlling a motor 8, for example, through the command of a computer 7. The opening degree of the blade of the movable blade pump 2 is inputted into the computer 7 and the height h0 of the top of the dam 6 is controlled through the motor 8 so that the minimum static pressure in the system is slightly higher than a saturated pressure at all times by a prepared program based on the flow amount of the system, under the flow amount of the system at that time. Accordingly, the operation of the system may be effected with the minimum power.