The case concerned state sovereignty over national resources in regards to need, necessity, and contract. The court's decision was unanimous in favor of Hermann, OK, to whose water resources the Texas-based water district sought access.
Under the compact, the Red River was divided into 5 "sub-basins" in order to keep any upstream state from depriving a downstream state from an ample flow of water. The so-called "5th Sub-basin" was intended to be a sort of reserve that the 4 member states could tap into in times of need, provided the flow was above a certain rate.
Tarrant wanted to purchase water from an area near Hermann, OK in order to supply water to it's district. Hermann refused to sell. So, Tarrant attempted to levy the sale based upon the compact. However, this portion of the 5th sub-basin flows within Oklahoma's borders.
Tarrant's case was the the compact allows this cross border sharing. In their view, the compact is a contract that supersedes local sovereignty concerning natural resources.
This would be like California suing Arizona in order to garner free electricity from solar farms in Arizona's border because California has an equal right to the sunshine that produces it.
The court upheld that the compact does not supersede state sovereignty.
The Red River Compact does not pre-empt Oklahoma’s water statutes because the Compact creates no cross-border rights in its signatories for these statutes to infringe. Nor do Oklahoma’s laws run afoul of the Commerce Clause. We affirm the judgment of the Court of Appeals for the Tenth Circuit.
Justice Sotomayor wrote the unanimous opinion.