A new book, "Resurrection: Glen Canyon and a New Vision for the American West," by NAU journalism instructor Annette McGivney, makes the case for draining Lake Powell, with the water going downstream to Lake Mead. [right, Gunsight Butte at Lake Powell. Credit, Nat'l Park Service]
The book review in the Arizona Daily Sun covers some of the history and debate over building Glen Canyon Dam, and the obstacles to changing the water and power dynamics of today.
In summary: it sounds like there are too many vested interests that want the dam and lake to remain.