If we all adhered to your notion of constitutionality, we would quickly conclude that our society/culture is un- constitutional. You included.
I'm not arguing about the water rights.
The builder of the project is irrelevant.
Uh, sorry. The builder is QUITE relevant, because the brief raises the issue--it states that anything not permitted to the Federal government is forbidden to it. Therefore, by the farmers' own pleading, the project was constructed illegally. Any lands that are now under Klamath Lake needs to be returned to its rightful owners.
The farmers have the water rights, but the water rights and the water project are not one and the same.
If they're going to argue from a strict reading of the Constitution (as they do here), then they have no case, because the structures from the project should not be there in the first place.
You're trying to have both ends of this argument, and that just does not work.