PORTLAND, Maine — A federal judge must decide whether the Penobscot Indian Nation’s reservation includes the water in the Penobscot River or only some of the islands in it to settle a dispute between the tribe and the state over who can police the waterway.

U.S. District Judge George Singal on Wednesday heard oral arguments in motions for summary judgment filed by attorneys for the tribe, the federal government, the state and municipalities located along the river that intervened in the case. The lawyers agreed that the central question in the lawsuit is whether the tribe’s boundaries as set out in the 1980 Maine Indian Claims Settlement Act include the river or not. They disagreed over how a possible decision in the tribe’s favor might affect the public.

The lawsuit was sparked when then Attorney General William Schneider sent a letter dated Aug. 8, 2012, to the tribal warden service saying that the state, not the tribe, has the authority to charge people with violating fishing regulations and water safety rules, according to court documents.

The tribe sued the state less than two weeks later in federal court in Bangor alleging that its reservation includes the water in the river because of the tribe’s sustenance fishing rights. A year later, municipalities were granted intervenor status in the case.

Singal on Wednesday told attorneys and the tribal members and supporters who packed the courtroom that he wanted “to cut through the case and the lawyer-speak” to consider the essential legal question.

The tribe’s attorney, Kaighn Smith Jr. of Portland, told Singal that the settlement gave the Penobscots ownership of the river but only the water, not the riverbed or its banks.

“In the settlement act, they retained their ancestral rights within the river,” he said. “They also were promised that they would be protected from interference by the state.”

Smith said that a ruling in the tribe’s favor would not change the public’s access to or use of the river in any way.

Assistant Attorney General Steven Miskinis of the Indian Resources Section of the Environmental and Resources Division in the U.S. Department of Justice went a step further and said the tribe owns the riverbed as well as the water but not the banks.

Miskinis filed his motion in support of the Penobscot’s argument. The tribe has called the federal government’s involvement in the lawsuit “unprecedented.”

“If the court rules for the nation, very little would change for the public,” Miskinis said. “The tribe could not exclude the public from the river because that would be discriminatory.”

But the attorney also said that if the tribe were to prevail, the Maine Indian Tribal State Commission could create rules for tribal members and the public who use the river. Tribal members summoned for breaking those rules would appear in tribal court on Indian Island. Nonmembers would appear in Maine District Court.

The state’s lawyer, Assistant Attorney General Gerald Reid, told the judge that the settlement act gave the tribe only the islands in the Penobscot River from Indian Island northward to the confluence of the East and West Branches, not the water or the riverbed. He said that allowed the tribe to regulate fishing from the islands only.

“If you look at the Legislative history, your honor, the U.S. Department of the Interior represented that the islands in the river, the reservation, were made up of 4,000 acres,” he said. “If the river had been included, it would have been 14,000 acres. When the act went into effect, the tribe posted the islands [to indicate it was tribal land] but not the boat launch.”

Reid said that nothing in the settlement act gave the tribe the right to regulate nontribal members on the river.

Municipalities along the river joined the lawsuit as intervenors in an effort to persuade Singal that a decision in the tribe’s favor would give it control over water quality on the river and allow the tribe to impose stricter rules than the state does on municipal discharges into the river.

Smith said that was not the tribe’s intention.

“If the tribe gave up the right to regulate discharge, we’d be happy,” Catherine Connors, the Portland attorney representing the intervenors, told the judge.

Penobscot Chief Kirk Francis said after the two-hour hearing that he was pleased with how Singal had educated himself on the issues in the case and how the judge asked questions that were to the point.

“There are a lot of things that have been brought into this case around discharge sites and control that have nothing to do with the tribe’s claims,” Francis said outside the courthouse. “I thought he did a very good job of making sure on the record that those things were getting clarified.”

The chief said that Singal seemed to “have a hard time reconciling the state’s theory of fishing from the islands only.”

“How does the tribe’s ancestral right to sustenance fishing exist in that theory when the river is the only place to fish?” Francis said. “We feel really positive about our case because we are on the right side of this.”

The is no timeline under which Singal must issue a decision. He did not indicate Wednesday when that might happen. If none of the parties are satisfied with his decision, the case most likely would be set for trial.

It is unclear if or how a decision in the tribe’s favor might affect a separate case concerning water quality in tribal waters that is being handled in federal court in Portland by U.S. District Judge Jon Levy. The state sued the U.S. Department of Environmental Protection alleging that an agency order issued in February would create a double standard for water quality in Maine — one for tribal waters and another for the rest of the state.