The diagram explain how helectricity is generated by hydroelectric dam. In that is an explanation of hydroelectrical principle.
First of all, we observate that the system is between a river, the water goes in the turbine chamber, after goes in penstock then into instake, finally in reservoir. At the start, water goes to meet turbine which is linked to generator in powerhouse. Generator is plugged on power lines at long distance.
On other way, how the system works depend of water. In fact, river goes in front turbine in order to make it roll, thanks to this, turbine help generator to product electricity. Energy is stocked into powerhouse, and send to power lines. Howerver, the reservoir help the system to work by storing water and sending it into penstock and make turbine rolling.
Futhermore, in more technical aspect, turbine product mechanical energy thanks to water then turbine rolling and the generator convert it in electrical energy in order to send it to power lines. Moreover, this cycle is renewable thanks to the water cycle in nature. At the end, the system is a ring of happening in order to have at the end electricty, in fact that is the hydroelectrical principle.