I don't quite understand how the law of conservation of energy applies to a hydroelectric dam, such as the one pictured below. My understanding is that the gravitational potential energy of the water in the reservoir will be converted to kinetic energy as it moves through the penstock. Where I'm confused is what happens to the energy of the water as it passes through the blades of the turbine? I see two possibilities. 1. Some of the kinetic energy of the water is transferred to the turbine causing it to spin. But if this is the case the water must slow down, meaning it would get backed up in the pipe? How can water have two different velocities when flowing through a pipe? or 2. The water has the same speed both before and after it passes through the blades of the turbine. But if this is the case the water must have the same kinetic energy on both sides of the turbine, meaning no energy was transferred and thus no power can be generated. Is there something in play here that I'm not seeing? Your thoughts would be much appreciated. Thanks!