[Jim W N]

- signed treaties with native tribes:
Tribal lands were never considered part of a state. These “reservations” are land “reserved” for Indian Nations supervised by the feds, not the states. Nothing unconstitutional there, because the land belongs to the Indians, not the state.

- water rights for agricultural uses
The feds have power based on the Commerce Clause to regulate commerce between the states where necessary. It is not necessary for the feds to own the land, just to regulate where necessary, so as you say, “to protect watersheds and minimize conflicts.”

[Alberta's Child]

That's exactly right. But in the case of Indian reservations, the tribes do not hold title to the land. The U.S. government holds the titles "in trust" -- for the benefit of the tribes that occupy them.

These issues may seem like a mere formality from the outside, but the sovereignty of the title holder is paramount in any of these dealings between different government jurisdictions.