WASHINGTON — The Supreme Court returned to its majestic mahogany bench on Monday after a pandemic-induced absence of more than 18 months, starting a new term that will include major cases on abortion and gun rights.

Monday’s first case, on water rights, was routine. But the courtroom had changed since the court last heard arguments in person in March 2020.

The seat at the far left was empty, a consequence of a positive Covid-19 test received by Justice Brett M. Kavanaugh on Thursday. He participated remotely from his home, a court spokeswoman said. His questions were piped into the courtroom.

The seat on the far right was occupied by newest member of the court, Justice Amy Coney Barrett, who was making her first appearance at an in-person argument.

Justice Sonia Sotomayor was the only member of the court who wore a mask.

Justice Anthony M. Kennedy, who retired in 2018, attended the argument, seated in the section of the courtroom reserved for visiting dignitaries. He wore a mask.

Justice Clarence Thomas, who very seldom voiced inquiries from the bench before the pandemic, asked the first questions of both of the main lawyers in the case.

The lawyers wore masks except when they were presenting arguments. The lectern at which they made their presentations had been moved back from the bench by several feet.

The public was barred from the courtroom, but the court is providing live audio on its website. Members of the news media were scattered throughout the front rows of the courtroom, a change from their usual spots on benches along its left side. The court required reporters to be tested for the coronavirus and to wear N95 masks.

The justices asked questions in the familiar free-for-all fashion that has long been their practice. But they supplemented such free-form questioning with an opportunity for justices to ask questions in order of seniority one by one after each lawyer argued, replicating the format the court used in the telephone arguments while it was exiled from its courtroom.

Most justices declined the opportunity to ask questions during the one-by-one rounds.

The case, Mississippi v. Tennessee, No. 143, concerned a claim by Mississippi that Tennessee was taking too much water from an aquifer beneath those states and several others.

The justices were skeptical of the argument. “You admit that Tennessee does not enter across the border into Mississippi, isn’t that correct?” Justice Thomas asked John V. Coghlan, a lawyer for Mississippi. “Couldn’t Tennessee make the exact some argument about you?”

Justice Elena Kagan said that “Tennessee is acting entirely within its own borders.”

Justice Barrett expressed skepticism about whether the court should have different rules for water on the earth’s surface and water underneath it.

Some justices asked colorful hypothetical questions. Chief Justice John G. Roberts Jr. wondered whether Tennessee could keep wild horses that had wandered across the state line. Justice Stephen G. Breyer asked about the ownership of fog in San Francisco.

“Suppose somebody came by in an airplane and took some of that beautiful fog and flew it to Colorado, which has its own beautiful air,” he said. “And somebody took it and flew it to Massachusetts or some other place.”

“Do you understand how I’m suddenly seeing this and I’m totally at sea?” he asked. “It’s that the water runs around. And whose water is it? I don’t know.”

Chief Justice Roberts wondered if it made a difference that the water at issue had to be separated from silt. “If somebody showed you, you know, a handful of silt, they wouldn’t say, oh, that’s water,” he said.

David C. Frederick, a lawyer for Tennessee, assured the chief justice that extracting the water was worth the effort.

“I think you would say that it is water,” he said, “because it’s some of the finest water that anyone can drink in the United States. This artesian water is absolutely spectacular water that they have pumped and they have run it over filters that filter out some of the iron and some of the other minerals.”

“It is very pure water,” he said, “and it is delicious.”