> The ecological effects of this, though, seem so disastrous. If the Colorado river dries up, what then? The whole of the Southwest just like, dies?

The lakes behind the Hoover dam are are the ecological catastrophe. Releasing the dam water just restores the river canyon to its natural state.

Draining the river is bad, but it's just as bad downstream of the dam. The dam by itself doesn't do anything but hurt the ecosystem it already destroyed.