Let's leave the Great Lakes where they are. Thank you very much. It's not their fault people decided to live in the desert.
By establishing a national water grid, you reduce the likelihood that the Great Lakes will be damaged by water use. You allow the Great Lakes to be replenished if there is ever a climate driven drought. You allow water to be taken from the mouths of all of the great rivers in North America, reducing the burden on any single one of them.
“Why are the people of the Colorado river so paranoid about their water” might have been a theme 60 or 70 years ago. I guess the answer is: because once that exploitation starts, it will only intensify in the future.