The given picture illustrates the cyclical process of producing electricity in a hydroelectric power station.
Overall, there are 2 phases in this procedure, starting with storing river water and ending with conducting the power to the national grid.
At the beginning, the river water that runs into a high-level reservoir is retained by a dam. During the daytime, the liquid flows through a water tube into the power station. At this point, the pressure of water makes the reversible turbines turn around in order to operate the generator. After that, the current is transmitted to the national grid while the rest runs into a low-level pool.
At night, the water stored in the low-level reservoir is pumped back through the pipe into the high-level storage by turbines. During this period, the intake is closed so as to hold back the water until daytime. This process regularly repeats day and night.