If you build on top of an existing dam, the whole thing needs to be stronger top to bottom, the water pressure at the bottom increases when you add water on top

If you build on top of an existing dam, the whole thing needs to be stronger top to bottom, the water pressure at the bottom increases when you add water on top. Sure there are places where this is possible but not everywhere. Plus the existing reservoirs aren’t full ever, what’s needed is new watersheds to capture new water (or to use less, etc) Making existing dams taller just adds empty space.