A federal appeals court overturned the conviction Thursday of a volunteer who left water bottles in the Arizona desert for parched border-crossers and was arrested violating a law that forbids "littering of garbage" in a national wildlife refuge.

The law fails to define garbage, said the Ninth U.S. Circuit Court of Appeals in San Francisco. Quoting a dictionary definition of garbage as "food waste" or "discarded or useless material," the majority in the 2-1 ruling said the law cannot be applied to a defendant who placed containers of purified water in the refuge for human consumption.

In dissent, Judge Jay Bybee said the law prohibits all littering, not just garbage-dumping, on the federally protected lands. Under any definition, he said, "leaving plastic bottles in a wildlife refuge is littering."

A defense lawyer said the ruling was the second legal victory for No More Deaths, one of several self-described humanitarian aid organizations providing supplies and medical help to immigrants entering the United States illegally in a border area with a high mortality rate.

In the earlier case, said attorney William Walker, a judge dismissed felony charges of transporting illegal immigrants several years ago against two volunteers who found three seriously ill immigrants in the desert and drove them to a medical clinic. A third member of the group is appealing a jury conviction in a water-bottle case similar to Thursday's case, Walker said.

"All they want to do is go out and save lives," the attorney said. He said the Obama administration has indicated it is reconsidering the policy of prosecuting the volunteers, which began under President George W. Bush's administration.

The defendant, Daniel Millis of Tucson, was arrested by U.S. Fish and Wildlife Service agents in February 2008 after placing one-gallon water bottles on trails in the Buenos Aires National Wildlife Refuge, near the Mexican border.

A judge convicted Millis of violating the littering law and gave him a suspended sentence. The appeals court majority, led by Judge Sidney Thomas, said Millis might have been charged with another crime, like abandoning property in the area, but could not be convicted of littering garbage.

The U.S. attorney's office was unavailable for comment. The government could ask the full appeals court for a rehearing.

Millis said Thursday that he and other volunteers had found the body of a 14-year-old Salvadoran girl on a nearby trail two days before his arrest, one of hundreds of deaths of immigrants in the Arizona desert each year.

"I hope this ruling is an indication that it's time to restore a little bit of sanity to our border policies," he said.