On Dec. 8, 2015, Mr. Tompkins was on a kayaking trip on General Carrera Lake, which straddles Chile and Argentina, with a group of friends, including Mr. Chouinard. Ms. Tompkins, who was several hours away by car, had discreetly given a member of the group a satellite phone, a device that Mr. Tompkins and Mr. Chouinard hated. After a while, emergency calls started to come in. Mr. Tompkins’s kayak had capsized in windy conditions, and he had spent about an hour in the frigid water before being taken out.

When she found out, she crawled underneath the parked small plane he would often fly to explore the parks. “I wouldn’t come out,” Ms. Tompkins said, adding, “I didn’t want any part of it.”

Mr. Tompkins died before she reached the hospital.

“I just crawled up in his bed, and I wouldn’t let him go,” she said through tears, adding, “He was lucky to have lived that long, considering how he lived his life.”

In her grief, Ms. Tompkins felt lost and unsure of how to proceed, but she ultimately decided to double down on her conservation efforts.

“Let’s go for broke,” she recalled thinking.

Carolina Morgado, the executive director of Rewilding Chile, which grew out of Tompkins Conservation, described her in that moment as a woman who “transformed her grief in power.”

In 2018, Tompkins Conservation finalized a deal with the Chilean government in which the organization donated over a million acres of conservation land, with the government adding roughly nine million acres to create five new national parks and expand three. In total, the organization has created or expanded 15 national parks, protecting over 14 million acres in Argentina and Chile — an initiative that continues. The organization and its offshoots have also taken up so-called rewilding efforts, reintroducing jaguars, red-and-green macaws, giant anteaters and other species.