New island forms in Alsek Lake as Prow Knob glacier retreats

NASA satellite imagery and field researchers say a retreating glacier in southeastern Alaska created a new island this summer by isolating a small mountain once hugged by ice. The landform, known as Prow Knob, became surrounded by the waters of Alsek Lake sometime between mid July and early August 2025, according to satellite comparisons and glaciologists monitoring the site in Glacier Bay National Park and Preserve.

The transformation was documented in Landsat images taken in 1984 and 2025 that show Alsek Glacier steadily retreating and a proglacial lake filling the basin the ice once occupied. NASA reported that imagery from the Thematic Mapper on Landsat 5 from July 5, 1984 and from the Operational Land Imager 2 on Landsat 9 from Aug. 6, 2025 captures the separation. NASA said the final loss of contact occurred between July 13 and Aug. 6. Mauri S. Pelto, a glaciologist at Nichols College who has been monitoring the glacier for years, said the split appears to have happened in late July or early August, based on his monthly checks of satellite data.

Prow Knob is now roughly 2 square miles, or about 5 square kilometers, with a summit just over 1,000 feet, Pelto said. Alsek Lake...