Alaska’s Expanding Lakes: A Complex Shift for Glaciers and Salmon Habitat

Across southeast Alaska, a striking transformation is underway. Glacial lakes are growing – and growing rapidly. A new study, published March 9 in the Proceedings of the National Academy of Sciences, predicts these lakes will quadruple in size over the next century or two, reshaping landscapes and impacting ecosystems in ways both challenging and potentially beneficial. Whereas glacier retreat is often framed as a straightforward loss, the emergence of these lakes is creating new habitats and altering river systems in unexpected ways.

A Landscape in Flux

The St. Elias Mountains, straddling the Alaska-Canada border, are particularly affected. This region is currently losing approximately 60 cubic kilometers of ice annually. These aren’t small ponds; these are substantial bodies of water forming as glaciers melt and retreat, carving out deep grooves in the underlying terrain. Since 1986, glacial lakes across southeast Alaska have already expanded by 60 percent, now covering a combined area of 1,300 square kilometers. Researchers, led by Daniel McGrath of Colorado State University, combined satellite imagery with estimates of ice thickness to map these changes and predict future growth. The study identified approximately 4,200 square kilometers of glacier-covered grooves adjacent to existing lakes, areas primed for future expansion.

How Lakes Accelerate Glacier Melt

The expansion isn’t simply a consequence of melting; it’s also a driver of it. Glacial hydrologist Eran Hood, of the University of Alaska Southeast in Juneau (who was not involved in the study), explains that lakes absorb more solar heat than ice. Glaciers that terminate in lakes melt faster than those ending on dry land. This creates a feedback loop: as lakes grow, they accelerate the retreat of the glaciers feeding them. The team predicts the lakes will continue to expand until they fill these carved-out grooves, potentially reaching a combined size of around 5,500 square kilometers – an area comparable to the state of Delaware.

From Barren Floodplains to Salmon Habitat

The impact extends beyond the glaciers themselves. Historically, many Alaskan glaciers have terminated on dry land, creating barren, rocky floodplains downstream. These areas are characterized by unstable streams that frequently shift course and deposit sediment, making them inhospitable for many fish species. “Those habitats are fairly inhospitable for a lot of fish,” including some salmon, says Jonathan Moore, an aquatic ecologist with Simon Fraser University in Burnaby, Canada. Eggs are often swept away or buried by the constant sediment movement.

However, as glaciers retreat into lakes, the meltwater has time to deposit sediment and warm slightly before entering rivers. This creates more stable river channels and fosters the growth of vegetation along the banks. A 2025 study by Moore and Diane Whited of the University of Montana demonstrated that expanding glacial lakes in southeast Alaska have already led to stabilization of downstream river channels, allowing willows and bushes to colonize the floodplains. This, in turn, creates more suitable habitat for salmon.

A Boost for Salmon, But at What Cost?

The changes are expected to benefit salmon populations. Moore and Hood’s 2021 research suggests that glacial retreat in southeast Alaska could transform approximately 6,000 kilometers of river channels into viable salmon habitat by 2100. The lakes themselves will also provide spawning grounds for sockeye salmon, a commercially important species. However, this transformation won’t be without disruption. The Alsek River, for example, is likely to change course as retreating glaciers cause two lakes to merge, creating a more direct path to the ocean.

The rapid changes also pose immediate risks to communities. Residents of Juneau, Alaska, experience annual flash floods when the Mendenhall Glacier-dammed lake overflows, necessitating the construction of protective levees. These events highlight the potential for dangerous and unpredictable consequences as glacial lakes continue to grow.

Understanding the Long-Term Implications

The study emphasizes the complex interplay between climate, glaciers, and ecosystems. While glacier retreat is a clear indicator of climate change, the formation of glacial lakes introduces a new dynamic. These lakes aren’t simply passive recipients of meltwater; they actively influence the rate of glacial retreat and reshape the surrounding landscape. The researchers acknowledge that predicting the precise timing and extent of these changes remains challenging, given the inherent uncertainties in climate modeling and glacial dynamics. The study’s findings underscore the need for continued monitoring and research to better understand these evolving systems and mitigate potential risks.

Looking ahead, continued monitoring of glacial lake expansion and its impact on downstream ecosystems is crucial. Further research is needed to refine predictions of river channel changes and assess the long-term effects on salmon populations and other aquatic species. Local communities will need to adapt to the increasing risk of glacial lake outburst floods and develop strategies to protect infrastructure and ensure public safety. The University of Texas at Austin continues to lead research in this area, focusing on the interplay of tectonic forces and climate change in shaping mountain landscapes.