We can get electricity from any kind of dam except one that blocks all water forever. We get electricity when water flows through, see?
The best kind of dam from the point of view of the power utility is one that allows the water flow to scale freely from zero to overwhelming to match electricity demand, and never allows water to bypass the turbines. But that's an extreme, not the only kind that makes financial sense. If you both reduce the time flexibility and also allow some water to bypass the turbines, you can usually get most of the power at a small environmental cost... usually, not always.
The best kind of dam from the point of view of the power utility is one that allows the water flow to scale freely from zero to overwhelming to match electricity demand, and never allows water to bypass the turbines. But that's an extreme, not the only kind that makes financial sense. If you both reduce the time flexibility and also allow some water to bypass the turbines, you can usually get most of the power at a small environmental cost... usually, not always.