Globe AZ Besh Ba Gowah (1935)

In related news, as of today Lake Mead's surface level is 1082.29 feet, which is lower than it's been since the reservoir was filled in the 1930s.

So, will the internecine be Calif vs Nevada vs Arizona, or U.S. vs Mexico?

I think the issue goes domestic well before it goes international, mostly because of the established order in which people get cut off. The cuts in Colorado water delivery are triggered when the lake's surface elevation hits 1075 feet, or 6.09 feet below its level measured today (7/24). When that happens, annual delivery to the lower basin states will be reduced 4.44%. The obligation to Mexico guaranteed by treaty gets satisfied first. But thanks to a bad deal Arizona signed in the 1960s to end California opposition to the Central Arizona Project and the fact that Nevada already takes a relatively small percentage of the river's water, Arizona gets slapped with almost the entire reduction. Reductions in Colorado River water don't start affecting California until Arizona has lost its entire allocation. Arizonans, meanwhile, have a very close relationship with the Second Amendment and the idea of militias, so I'd expect the internecine warfare to start there.