Hopefully not. And it's not that I doubt you, I just wonder whether necessity will trump agreements.
Who's necessity? The Great Lakes would a western version of the Aral Sea if they were diverted to the Great Planes for agriculture. There isn't a single farmer outside of the watershed that has any right to that water. Not one.
You're speaking in absolutes about something that is very new.
It is an absolute. The water belongs to the watershed. These agreements first began to be formalized over a century ago in 1909.
Meanwhile, look at how these agreements fare among US states. Nebraska is always breaking binding agreements with Kansas not to divert too much water. Worst case scenario, they pay a fine.
Between two US states. This is an agreement between two countries, eight states, and two provinces. This isn't a muddy, little smelly stream in a flyover state. This is a significant chunk of the world's surface freshwater. Lake Superior's water only cycles out once every 160 to 180 years. Anything that's taken out above what already naturally leaves the lake will not be replenished for between 160 and 180 years. Lake Michigan and Lake Huron are only marginally better in this regard. Anything taken out of the three larger Great Lakes also has a negative effect on the water level in Lake Erie and Lake Ontario. Your idea could literally (not figuratively) destroy the Great Lakes. We would be repeating all of the same mistakes the Soviet Union made when they decided to divert water out of the Aral Sea watershed so they could grow cotton.
Also, it's important to remember that parts of Alberta, Saskatchewan and Manitoba are all in the Great Plains, too. And in those provinces, once marginal lands are now used widely for food production, too. Canada theoretically might also have an interest in changing its opinion.
No, no they would not. Canada already has ~20% of the world's freshwater supply to itself. If you take a look at a map of Manitoba, Saskatchewan, and the Northwest Territories, you'll see a line of massive lakes. Draining water out of the Great Lakes watershed would not only be unnecessary for Canada, but it would also hurt their economy to a point of crippling Southern Ontario. Taking a significant portion of water out of the system without Canada's approval would be an act of war. You'd also likely see some very violent reactions out here on the US side if you proposed draining the Great Lakes for a bunch of factory farms on the plains and sunbathers in the Southwest.
I'd honestly rather see the West die for lack of water before giving them an ounce. If they want water, they can move out here and pump money into the economy here in the Rust Belt. Half the cities out West shouldn't exist to begin with.