Glaciers along the coastal plain of southeastern Alaska are rapidly retreating, spilling meltwater into expanding proglacial lakes. One such lake—fed by the receding Alsek Glacier—has grown so much that it transformed a small mountain into a new island.
The Alsek Glacier once enveloped this rocky mound—known as Prow Knob—near its terminus. Over the past 40 years, both of the glacier’s arms have retreated more than 3 miles (5 kilometers), creating Alsek Lake. Landsat images captured in 1984 and 2025 document this transformation, showing that the glacier finally lost contact with Prow Knob this summer, according to a statement from NASA Earth Observatory. Now, the 2-square-mile (5-square-kilometer) landmass is completely surrounded by the waters of Alsek Lake.
According to Mauri Pelto, a glaciologist at Nichols College, this trend is likely to continue. Indeed, rising global temperatures have accelerated the rate of glacial retreat around the world. Now that the Alsek Glacier has lost contact with Prow Knob, its ice is less stable and more prone to calving, which could further accelerate its retreat.
Alsek Glacier’s unexpected resilience
Glaciologists have been monitoring the Alsek Glacier and its proglacial lake for decades. Photographer and self-educated glaciologist Austin Post (1922-2012) imaged the glacier’s terminus in 1960 and later told Pelto that he named the rocky mound for its resemblance to a ship’s prow, according to the NASA statement.
Based on the rate of the Alsek Glacier’s retreat between 1960 and 1990, Post and Pelto previously predicted it would disconnect from Prow Knob in 2020. Surprisingly, the glacier held on five years longer than expected, even as global temperatures have continued to rise.
NASA’s satellite observations indicate that the Alsek Glacier officially separated from Prow Knob sometime between July 13 and August 6, completing the mound’s transformation into an island. This event offers scientists a rare chance to compare long-term predictions with real-world glacier behavior.
Using eyes in the sky to track glacial melt
NASA’s Landsat satellites have been particularly useful for monitoring glacial retreat since the first launched in 1972. These two images were captured by the Thematic Mapper instrument on Landsat 5 and the Operational Land Imager-2 instrument on Landsat 9. Comparing them reveals how icemelt has reshaped the region over the decades.
Since 1984, Alsek Lake has grown from roughly 17 square miles (45 square kilometers) to about 29 square miles (75 square kilometers) today. Though it’s primarily fed by meltwater from Alsek Glacier, it’s also fed and impounded by a rapidly thinning tongue of the Grand Plateau Glacier, according to the National Park Service.
Glacial retreat is a major driver of global sea level rise, but the changes at Alsek Glacier show how it can also reshape entire landscapes. This can have ecological and sociocultural consequences, according to NPS. Through satellite observations and other forms of monitoring, scientists will continue to track the Alsek Glacier’s retreat to understand how this ever-changing region fares in a warming climate.