I see this come up every now and then, but I still don’t see how it will solve the real problem which is the American West has more people than its environment can naturally support. While the opposite is true of the east of the Mississippi, pumping water thousands of miles to where it doesn’t naturally fall sounds like it could further exacerbate the problem later. Granted, I am no environmental scientist or meteorologist, but it sounds like it could create more problems down the line.
Also:
The Great Lakes are another possible source.
The Great Lakes States and Canada would blow a gasket if this happened. There is also the Great Lakes Compact which is a Treaty with Canada preventing this exact scenario from happening; it stipulates that water from the Great Lakes must stay within the Great Lakes Basin.