A dam opens gates at the top to allow water from the reservoir to flow down large tubes called penstocks. The force of the flowing water spins the blades of a giant turbine. After the water spins the turbine, it flows back into the river on the other side of the dam. When water spins the turbine, it gains kinetic energy. The turbine is connected to a generator that makes electricity as it spins. The electricity goes to the transformer in the powerhouse. The transformer steps up the voltage of the electricity so it may travel through the power lines of the electrical grid to homes and businesses.

The height of the dam is important in determining the speed that the water can spin a turbine. Water can flow much faster with higher dams. The width of the dam does not determine how fast water can flow.