In the event the federal government dissolved, what would happen to the Native American reservations

[Skill and Chance]

In a wildly hypothetical situation where the US federal government no longer exists, it's almost surely the case that NATO no longer exists and the idea of interventions in foreign affairs based on universal human rights doctrine no longer exists.  In a world where the US government cannot survive with its nuclear weapons, oceans, mountains and energy reserves, it's quite plausible that no government on the scale of a modern nation state exists anywhere in the world at this point.  Technology has probably regressed.  I would expect a tendency toward feudalism where land = economic and political status.  There would probably be only a handful of places left where non-landowners can still vote or have other political rights. 

The alternative to this is some of the larger states having centralized governments on the scale of European countries population and economy wise.  In this world, the smaller and less economically self-sufficient states from the Rockies west are effectively territories of California.  Texas can effectively dictate policy to everyone between the Rockies and the Mississippi.  A Florida/Georgia coalition and a New York/Massachusetts coalition split the East Coast between them.  There might be several economically powerful states around the Great Lakes, but without any one being able to dominate its neighbors.  Tennessee takes as much of Appalachia as it wants, etc.  The smaller states would probably be allowed to elect their own governments as a formality, but if they go too strongly against their largest neighbor, they get deposed or switch coalitions if it's geographically viable.

In the first scenario, it would depend on each individual tribe's ability to organize and credibly threaten deadly force against outsiders coming into their land.  Various cultural traditions lead me to think Indian tribes would be better equipped to hold onto their land than the average American in this kind of environment.  But it obviously depends on the tribe, the land they have to defend, and their elected leaders at the time.  Native Alaskan villages almost surely get left alone to do their own thing.  A geographically large, high population, and mountainous reservation like the Navajo seems like it would be impregnable against any opponent without an air force.  Small reservations that lie flat on the Plains might not fare as well.   

In the second scenario, it would depend entirely on what the government of the nearest megastate wanted.  Eastern tribes would have multiple alliances to choose from to get the best deal.  For Western tribes, it's California, Texas, or bust. Today, you might expect them to get a better deal under the protection of California than Texas, but with an imperial California that might be natural resource starved, who knows?  Maybe Texas finds common cause with them because they agree to be the mercenaries who depose rebellious governments in the Plains states whenever needed?