In fact, the "wall" is the dam. During the time leading up It will store water in large enough quantities to be used to rotate the generator. Then release the water through turbines. During low tide.
Actually, "the wall" is the dam at the time of tide, it will prevent access to water in sufficient quantity to be used in the rotation generators and then release the water through a turbine. During the time of low tide
In fact, "The wall." This is the dam at the time the water was up to the water in sufficient quantity to use in the turn the generator and then let the water out through a windmill at the time water.