I remember reading, back in the mid-80's, an article that quoted an engineer from the Bureau of Reclamation. This was only a few years after Powell had reached full pool, which took a remarkable 17 years. His position was that Lake Powell was going to be the death knell for Lake Mead, because Powell would lose such a large quantity of water to evaporation. It's now believed that Powell loses 860,000 acre/ft per year to evaporation, far more than was previously thought.
Now, up here in Canada, we have a band of usual suspects that like to blather on that the Americans want to take our water. The usual tale is that they want to pump it out of the Great Lakes and ship it to the dry Southwest. These people are usually geographically challenged. Most of where the water needs to go is not only a couple thousand miles away from the Great Lakes, it's a couple thousand feet up. Meanwhile, Canada sends about 200,000 cu/ft per second of clean fresh water across the border via the Columbia. If the need was really serious, you could pull 25-50,000 cu/ft per second out of the Columbia, east of Umatilla, and pump it downhill to the Salton Sea. Use that strictly for irrigation for a decade, and the Salton would be cleaned up dramatically. But, we're talking huge dollars.