The dispute centers on a proposed LNG terminal that energy giant BP wants to build on the Jersey side of the river. New Jersey officials approved the project, which could bring more than 1,300 construction jobs.

Delaware officials, however, have refused to authorize the construction of a 2,000-foot-long pier, which would be built on the part of the river bottom that belongs to Delaware. Without the pier, the project could not go forward.

New Jersey concedes that Delaware owns the land, but says a century-old agreement allows each state to control piers on its side of the river.

A pier on the New Jersey side that can't stretch onto Delaware territory to reach the main shipping channel is worthless, said H. Bartow Farr, who is representing New Jersey. "That's where the ships are," Farr said.

But David Frederick, representing Delaware, told the justices that decisions on what to build on Delaware land belong to Delaware. "Boundaries matter," Frederick said.

On a practical level, he said, Delaware has only twice in 160 years denied permission to build a pier on the Jersey side of the river and both instances involved LNG facilities.

Up to 150 ships a year would dock at the proposed pier, which would be directly across the river from Claymont, Del. Delaware says the proposal raises safety fears because an estimated 22,000 residents living near the river's main shipping channel would be at risk in case of a major accident. BP said the facility would be able to deliver up to 1.2 billion cubic feet of natural gas a day to the Mid-Atlantic region.

The justices puzzled over the states' authority.

What if a murder occurred on a wharf on the Jersey side that sits on Delaware land, Chief Justice John Roberts wondered. "Is it prosecuted in Delaware or New Jersey?" Roberts asked. New Jersey, Farr said.

Justice David Souter asked Frederick whether Delaware could pass a law saying no more piers could be built on the Jersey side of the river. "It depends," Frederick replied, although he later said such a measure probably would not be upheld.

A court-appointed special master concluded earlier this year that Delaware has the authority to block the pier.

Justice Stephen Breyer is not taking part in the case. He owns $15,000 to $50,000 in BP stock, according to his most recent financial disclosure.

His absence could present a complication if his eight colleagues divide 4 to 4. In most Supreme Court cases, a tie means that a lower court ruling is upheld.

But disputes between states are initially decided by the Supreme Court, not lower courts. What would happen in the event of a tie vote is unclear.