With an ingenious system, the hydroelectric dam transforms the force of the water in electrical energy. 

Simply, the water goes into pipes which go to the turbine. The water pressure makes turn the turbine which is connected to a generator. Then, the generator transforms the rotation mechanical energy into electrical energy. Finally, the electricity goes in the grid. The electrical production depends of the dropping height of the water and of the water flow. 

The advantage of the dam is that it produces electrical energy without polluting the environment.