On June 13, 2013, the U.S. Supreme Court decided Tarrant Regional Water Dist. v. Herrmann, holding that the Red River Compact among four south-central states does not preempt the State of Oklahoma's water regulations.

The congressionally sanctioned Red River Compact allocates water rights in the Red River basin among the States of Oklahoma, Texas, Arkansas, and Louisiana, employing a complicated formula dependent on a variety of factors. The Tarrant Regional Water District, a Texas state agency, sought a water permit from the Oklahoma Water Resources Board to take surface water from a point in Oklahoma and, anticipating that the OWRB would deny the permit based on Oklahoma regulations, simultaneously filed suit claiming that the compact preempts the Oklahoma regulations and that the regulations violate the Commerce Clause. The District Court granted summary judgment for the OWRB and the Tenth Circuit affirmed.

The Supreme Court affirmed, holding that the compact does not preempt the state regulations and that the regulations do not violate the Commerce Clause. Finding the compact ambiguous on the question of cross-border rights, the Court concluded that the compact did not grant cross-border rights based on three factors: states' general reluctance to cede their sovereign powers, other interstate water compacts' explicit treatment of cross-border rights, and the parties' course of dealing. The Court also rejected Tarrant's Commerce Clause argument, concluding that the clause was not implicated because the compact did not leave any water "unallocated."

Justice Sotomayor delivered the decision for a unanimous Court.

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