The Supreme Court ruled today in a 5-4 split that an interstate water authority and Duke Energy Corp. may intervene in a water-use dispute between North and South Carolina, but the city of Charlotte may not.

While the ruling in South Carolina v. North Carolina is procedural, the case sheds light on a sharp division among the justices on the role of third parties in original lawsuits -- cases between U.S. states, which go straight to the Supreme Court. Those cases often involve environmental and natural resources claims.

South Carolina had filed suit against its neighbor in 2007, claiming that North Carolina would unlawfully divert water from South Carolina by following through on a planned interbasin transfer of water from the Catawba River. The state later asked that all three potential intervenors be excluded from the case, which was the subject of today's ruling.

The Supreme Court, which has not yet considered the water-use dispute itself, ruled today that Duke Energy and the Catawba River Water Supply Project have interests distinct from both North and South Carolina, meaning the states would not adequately represent them. While Charlotte has a strong interest in the case, the city was ruled part of "a class of affected North Carolina water users," and thus adequately represented by the state.

Justice Samuel Alito wrote the opinion for the majority, joined by Justices John Paul Stevens, Antonin Scalia, Anthony Kennedy and Stephen Breyer.

Chief Justice John Roberts filed a tersely worded opinion dissenting with the majority's ruling that Duke Energy and the water authority could intervene. His opinion, joined by the remaining three justices, raised concerns that the ruling could turn a system intended to solve interstate disputes into "a forum for airing private interests."

Duke Energy produces hydroelectric power at reservoirs using water from the Catawba River. The Water Supply Project is a joint venture between the two states that treats and distributes Catawba River water to roughly 200,000 people.

"This Court has never before granted intervention in such a case to an entity other than a State, the United States, or an Indian tribe. Never," Roberts' opinion says. "That is because the apportionment of an interstate waterway is a sovereign dispute, and the key to intervention in such an action is just that -- sovereignty."

The majority opinion also broke from the advice offered by U.S. Solicitor General Elena Kagan in a friend-of-the-court brief. Kagan urged the court to turn away all three potential intervenors, arguing "they have the same interest as everyone in the Catawba River Basin who hopes to draw water from the river."