보 수문을 열어 수위를 조금이라도 낮추면 취수구가 물 밖으로 드러나 강물을 끌어올릴 수 없기 때문이다.
It is because if the water level is lowered even a little by opening the weir gate, the water intake would be exposed out of the water and cannot pull up the river water.