[Melas]

I’m no expert on the subject, but when it comes to the states and water, left unchecked, couldn’t an upstream state cut off the water to a downstream state?

[Theoria]

Sure. Alabama, Georgia, and Florida are fighting over Lake Lanier's water right now.

[tacticalogic]

They might be able to cut off some small tributaries, but they can’t block navigable waterways. That’s going to impede interstate commerce, and that’s a no-no.