The picture above displays thre process through which electricity is produced within a hydroelectric power station.
Two main features take place during two moments of the day – the day itself and at night. The first one happens when the river fills the reservoir up to a high-level; when this is the case, the dam is opened, letting water flow down towards reversible turbines that during the day serve as a power station that has a generator that transfers water to power lines that end up in a national grid.
At the same time, the water used during the day is finally kept in a low-level reservoir that transforms through reversible turbines that, at night, pump the liquid -initially coming from the river- back into the high-level reservoir, where the water repeats the process together with the new one coming from the river.
Remarkably, where the water used during the day generates electricity for its direct use, the same energy is used at night to recycle the water intake during the day.