Hi,
I do not know all the legal details about water rights and (as with any other legal rullings) can change over time.
Example:
Georgia had the the area from the Atlantic Ocean to the Mississippi (and west of the Savannah River). This was the last existing British Colony Charter that was in effect stating that the land between the Atlantic and Pacific Oceans was Georgia. Onviously the Spanish land claimes west of the Mississippi halted the Western end of Georgia. This included most of Alabama and Mississippi.
About 1812, there were a number of issues that the State of Georgia made errors in including land lotteries and the event leading up to the "Trail of Tears" - Cherokee Indians.
The Feds took over settling the conflicting claims for Georgia with the understanding that Georgia give up its claims west of the Chattahoochee River. The Alabama/Georgia boarder was set by Georgia to be the western edge of the water in the Chattahoochee River (not like most river bounderies where it is in the middle of the river). When the Chattahoochee River ruened easterly, the western boundary was laid out in a North-Northwest direction until a short distance from the Tennessee Border when it turned northwesterly.
The western edge of the river was for water rights. The NNW boundary was for coal deposits in the NE Georgia mountains.
In the 1980s (no typo), a lawyer for the state of Alabama used a particular way of presenting the case in Alabama Federal Court and had the water rights set in the middle of the Chattahoochee River. Caused a lot of had feelings between the states. Alabama did not have to pay anything to Georgia for the water rights.
If Georgia had not messed up to start with, it might have worked out that parts of Lousianna, Arkansas, Texas, Oklahoma, New Mexico, Arizona, California, and possibly the southern tip of Nevada would have been in Georgia. A state that is larger than most countries.
Glad it did not happen but interesting to speculate on.
Doug vV