The working theory of a hyower station built on a river is demonstrated by this flow chart. The operating mode of it varies from the daytime to the night.
By daylight, water stored in a high-level reservoir initially flows downstream through the intake under the dam, and then drives turbines, and finally arrives at a low-level reservoir. These turbines, powered by the water flow, can correspondingly operate a generator in the power station. The electricity produced, in the last step, will be transferred to the National grid through power lines.
By night, water in the low-level reservoir can be pumped upstream to the high-level one through the same riverway by reversible turbines spinning in the opposite direction as the daytime. The intake under the dam is a non-return one, which is closed at night in order to ensure that water can flow into the reservoir, but not out.
To sum up, the power station functions in totally opposite ways in the daytime and at night. In the whole process, water is utilized as the energy carrier, which at first releases its gravitational energy to produce electricity and then helps transform electricity into gravitational energy.