Retreating Alsek Glacier reveals new island in southeast Alaska

This satellite image shows the retreat of Alsek Glacier in Glacier Bay National Park and Preserve in Alaska, as it loses contact with a land mass known as Prow Knob, centre right, revealing an island, Aug. 6, 2025. (NASA Earth Observatory via AP)

JUNEAU, Alaska — A retreating glacier revealed a new island in Alaska this summer, as lake water filled in to surround a land mass once hugged by ice.

Mauri S. Pelto, a glaciologist at Nichols College in Massachusetts, had anticipated for some time that the Alsek Glacier in southeast Alaska’s Glacier Bay National Park and Preserve would detach from the land mass referred to as Prow Knob. As the glacier has retreated, it has eroded a basin now filled by Alsek Lake, which is fed by the nearby Alsek River, glacier melt and icebergs, he said.

Pelto for years has used satellite imagery as part of his work chronicling changes in glaciers, and he had been checking images of the area at least once a month as he watched for the separation to occur, he said. It appears to have happened sometime between late July and early August.

Glacier Bay has over 1,000 glaciers, according to the park. While many glaciers in Alaska are retreating, not many new islands of size are revealed by their retreat, Pelto said. Prow Knob is roughly 2 square miles (5 square kilometres), and its highest point is just over 1,000 feet (304.8 metres), he said.

Imagery from the early 1980s, shared by NASA Earth Observatory, shows the Alsek Glacier largely surrounding Prow Knob, with Alsek Lake on one side. The glacier at that time shared a connection with Grand Plateau Glacier, the images show.

Over time, the lake has expanded as the glaciers have retreated. Alsek Lake is one of three lakes next to glaciers in the region that has seen marked growth since the 1980s, Pelto said.

Becky Bohrer, The Associated Press