How do tidal barrages energy sources work?

Tidal barrages - big damns are built across river estuaries with turbines in them. As the tide comes in, it fills the estuary, and the water is allowed out through the turbines at a controlled speed. The tides are produced by the gravitational pull of the sun and moon. It produces no pollution, but it prevents free access to boats and spoils the view. It also alters habitats. Its reliable but the height go the tide may be a drawback. The initial costs are moderately high, but it has minimal running costs. It creates a significant amount of energy and is renewable.