I'm by no means an expert or a lawyer or someone you should listen to. But this may hint at the complexity. A lot of water rights come from Spanish land grants ~330 years ago. And those were guaranteed by treaty after the Mexican American war. So, the U.S. can do whatever it wants, but treaties are in this weird space below the constitution but above a simple bill through congress to become law.
Water rights are generally old, old law and weird and complicated and special for each little town.
Mexican water rights were considered separate from the underlying land rights unless they were explicitly included in the grant. Moreover, Mexican title was required to be registered with the government shortly after annexation. Most of those titles were in turn siezed by various quasi-legal means, which is where cities like Berkeley come from. There are relatively few water rights remaining from Mexican annexation, most of which are held by municipal institutions like LA and only affect relatively small streams. Those of larger areas, like the Sangre de Cristo grant, have been litigated to death in courts over the past couple centuries and most of the entities involved no longer exist.
Water law is a nightmarishly confusing hellscape, but Guadalupe -Hidalgo isn't an important reason why today.
> things haven't reached that level, it's likely the issue isn't super serious (yet)
This isn't some some weird theoretical aside. Congressional power to modify and break treaties was debated by the founders [1].
Treaties are laws, full stop. Congress breaking them has political consequences. But it's not illegal, and it's no different from amending an act a prior Congress passed.
> if it got really really bad 3/4ths of the states can modify the Constitution and over-ride just about anything
You're describing extreme actions relative to a quotidien one. Armed revolution and Constitutional amendments are rare. Congress amending treaties is mundane.
We try to give the other side the courtesy of consultation, but that's (a) far from even common at this point and (b) sometimes impossible. The water rights in question are no different from any other private water rights. The fact that they originated from a treaty a curiosity at best.
Water rights are generally old, old law and weird and complicated and special for each little town.