From the opinion, City of Pocatello v. Idaho et al.:

The City of Pocatello filed a claim in the Snake River Basin Adjudication (SRBA), asserting it had been granted a federal water right under an 1888 Congressional act. A special master in the SRBA determined that no such right existed. The district court affirmed on appeal, as does this Court.

Here is an audio file of the oral argument.