Scotus established Reserved Water Rights in 1908.
A tribe with a treaty right to farm has a reserved right to the water needed to farm. Reserved water rights supercede Prior Appropriation Water Rights which the water rights doctrine that Montana uses.
A lot of people will say that the injuns are trying to steal the White man's water. Quick! Somebody call Trump!
Trump will tell the injuns they can't have the water and if they can't grow a crop, they'll just have to eat sh*t.