Boulder, US, Aug 14 (The Conversation) — Each summer, flow from the vast Mendenhall Glacier, located in the mountains above Juneau, Alaska, makes its way into mountain lakes and then into the Mendenhall River which runs through town.
Since 2011, this flow has warranted special attention from scientists and officials due to the developing situation at Suicide Basin, a depression formerly covered by glacier ice. As the glacier retreated, it revealed a sizable basin that now fills with meltwater each summer.
In 2023 and 2024, overflow and tunneling resulted in water surging from Suicide Basin, causing floods in neighborhoods along the river path. The basin experienced another deluge on August 12-13, 2025, with the surge setting record levels at Mendenhall Lake en route to Juneau, prompting officials to advise some residents to evacuate. Although the rising water posed significant danger, newly erected flood barriers seemed to mitigate the potential damage.
The recurring threat of glacial floods facing Juneau is becoming a more common phenomenon worldwide. As experts in earth science and mountain geography, we are particularly focused on how ice loss destabilizes nearby mountain slopes and glacial lakes, raising several concerning issues.
The Increasing Hazard of Glacial Floods
As global temperatures rise, glaciers, such as those in Europe's Alps and Pyrenees, have lost 40% of their volume between 2000 and 2023. While these icy regions have long supplied freshwater to nearly two billion downstream residents, their accelerated melting now presents significant hazards. Retreating glaciers form lakes that are often precariously contained by ice dams or rock moraines, remnants from historic glacial retreats. Excess water or an external event can rupture these natural barriers, unleashing catastrophic floods.
The Mendenhall Glacier's floods are a classic example of "jokulhlaup" or "glacier leap" floods, a term originating from Iceland now commonly associated with Alaska and other northern regions.
Ice Dams and Landslides
Glacial lakes started forming nearly 150 years ago due to warming since the 1860s, but their frequency and growth rate has surged since the 1960s. Communities in the Himalayas, Andes, Alps, Rocky Mountains, Iceland, and Alaska have all faced deviations of glacial lake outburst floods. This type of flooding became tragically evident in the Sikkim Himalayas in October 2023 when floods destroyed infrastructure and claimed over 50 lives. Thawing permafrost, which undermines mountain stability, is a contributing factor that increases the likelihood of avalanches, rockfalls, and other phenomena that can trigger floods.
Historically, these floods have resulted in devastating consequences. In 1941, an avalanche into Laguna Palcacocha in the Peruvian Andes released a flood that wreaked havoc on Huaraz, a city that has seen its hazard grow with both population increase and lake expansion. Today, more than 35,000 people are at risk if a similar event occurs.
Recognizing these dangers, governments are implementing early warning systems and flood risk management strategies. For example, the US Geological Survey monitors Suicide Basin closely in Juneau, preparing intervention measures like gabions to redirect potential floodwaters.
Inside Flooding and Permafrost Thaw
While glacial lake outburst floods dominate headlines, they are not the sole threat. Englacial conduit flooding and permafrost thawing offer additional risks. Meltwater passages within glaciers can create flood patterns that originate internally, while thawing permafrost destabilizes landscapes, making rock and debris flows more likely.
Risk Reduction Strategies
In 2024, a study identified over 110,000 glacial lakes globally, with an estimated 10 million people under threat from potential floods. To mitigate risks, early warning systems, zoning laws, and various infrastructural strategies are being employed. Realizing the growing threat, the UN has designated 2025 as the International Year of Glaciers’ Preservation and declared 2025-2034 a decade to advance cryospheric sciences globally.
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