The US Supreme Court issued a pair of opinions this term dealing with the role of the federal government in ongoing interstate water disputes: (1) Texas v. New Mexico and Colorado; and (2) Florida v. Georgia. The cases are before the Court under “original jurisdiction” - they are being heard on first impression rather than on appeal (although they have been reviewed and reported on by appointed Special Masters).
Substantively, both cases involve disputes between neighboring states regarding obligations and rights to shared waters, whether by agreement (interstate compact) or common law (equitable apportionment). Texas argues that New Mexico is violating the terms of their 1938 Rio Grande Compact with a self-serving interpretation of that agreement’s obligation on New Mexico to deliver water to a reservoir (in New Mexico) for use in Texas. Florida argues that Georgia is taking more than its equitable share of water for municipal and agricultural use from the Apalachicola-Chattahoochee-Flint basin and harming the downstream ecosystem in Florida. But before reaching the merits of these interstate disputes, the Court first had before it questions balancing federal and state control of shared interstate waters.
In the Rio Grande Compact dispute, the United States weighed in to protect downstream flows that it must deliver to Mexico pursuant to an international treaty. Justice Neil Gorsuch, writing for a unanimous Supreme Court, allowed the United States to bring its claims to defend "distinctly federal interests" - namely treaty obligations and the federal government's role in the Rio Grande compact operations. The case now moves forward with the United States as a key player.
In the Apalachicola-Chattahoochee-Flint basin dispute, Georgia argued that the Supreme Court could not decide the case without the U.S. Army Corps of Engineers at the table. The Corps controls the upstream water in Georgia that Florida wants conserved, and the United States sided with Georgia, stating it would not alter its control of the upstream water (ostensibly for navigation and flood control). The Special Master had sided with Georgia. The Supreme Court, by a 5-4 majority, instead sided with Florida and held that further proceedings were necessary to determine if the Court can provide redress for Florida's alleged harms.
For more detailed analysis of the Florida v. Georgia case and the Supreme Court's closely divided 2018 opinion, see Reed Benson's article: "Can a State's Water Rights Be Damned? Environmental Flows and Federal Dams in the Supreme Court." And check out the “infographic argument explainers” for Texas v. New Mexico and Florida v. Georgia at Subscript Law.