In southeastern Alaska, an island, once hidden beneath ice, has appeared in the middle of a lake formed by the melting of a glacier. This rapid transformation of the landscape offers a striking illustration of ongoing environmental changes.
Prow Knob, a small mountain of about 5 square kilometers (2 square miles), was surrounded by the Alsek Glacier in Glacier Bay National Park. For decades, this glacier has been retreating, gradually detaching from the landmass and giving way to a freshwater lake that has continued to expand. Recent satellite images, captured by
Landsat 9 in August, confirm that the glacial connection is now broken, revealing an isolated island.
Prow Knob in Alaska, once surrounded by ice, is now an island in the middle of a lake.
Credit: NASA Earth Observatory images by Michala Garrison, using Landsat data from the U.S. Geological Survey. At the beginning of the 20th century, the Alsek Glacier extended well beyond its current position, covering the area where Alsek Lake is now located. It split into two arms to go around Prow Knob, named for its resemblance to the prow of a ship. Glaciologists Austin Post and Mauri Pelto had predicted this separation for 2020, but the process took a bit longer than expected.
The complete separation occurred between July 13 and August 6 of this year, according to NASA observations. This phenomenon is not isolated; it is part of a global trend where glaciers are retreating rapidly due to climate warming. The year 2024 was the hottest on record, and 2025 continues this series of records, accelerating ice melt worldwide.
Lindsey Doermann, science writer at NASA's Earth Observatory, points out that in this region of Alaska, water is quickly replacing ice. Proglacial lakes, formed by meltwater, are expanding and altering landscapes, as evidenced by the emergence of this new island.
Glacier retreat
Glacier retreat is a process where the ice mass decreases and withdraws due to accelerated melting. This occurs when air temperatures rise, reducing snow accumulation in winter and increasing melting in summer. Glaciers act as freshwater reservoirs, and their disappearance affects local and global ecosystems.
In regions like Alaska, retreat is particularly visible due to amplified climate warming at the poles. Scientists use satellites to measure these changes, providing valuable data on landscape evolution. This helps forecast future impacts on water resources and sea level rise.
Proglacial lakes, like the one formed near Prow Knob, are direct indicators of this retreat. They are created when meltwater is held back by moraines or other obstacles, altering geography and sometimes causing sudden floods if natural barriers give way.