Every summer, people living near the Mendenhall River in Juneau, Alaska, keep a close eye on the water level. When the river level begins to rise rapidly, it’s a sign that Suicide Basin, a small glacier-dammed lake 5 miles up the mountains, has broken through the glacier again and a is underway.

After of ever-larger and more damaging floods in Alaska’s capital city, the U.S. Army Corps of Engineers is discussing an : create a permanent drain from the lake that would prevent it from reaching outburst stage.

The for the project range from US$613 million to $1 billion.

Scientists discuss the glacial outburst floods from Suicide Basin on Mendenhall Glacier above Juneau.

Suicide Basin is just one example of a from glacial lakes that threaten communities around the world, particularly in the Himalayas and Andes, and is transforming Alaska’s landscape as global temperatures rise.

In a new study, colleagues and I documented the in Alaska between 2018 and 2024. We found they are expanding about 120% faster on average today than they were from 1986 to 1999 – more than twice as fast.

Using ice thickness data to reconstruct the shape of the land beneath these glaciers, we found that these glacial lakes could become more than four times larger than they are today as the glaciers melt, increasing the potential for damage to downstream ecosystems and infrastructure from glacial lake outburst floods.

Spencer Glacier in the Kenai Mountains: This lake, and the icebergs in the lake, have become a significant tourist attraction along the Alaska Railroad. Louis Sass

The hazards of glacial lakes

Glacial lakes, often the color of aquamarine gems and sparkling with icebergs, are common around the margins of glaciers around the world. Years of satellite images have documented a – a direct response to .

Tenuously held back by moraines – the jumble of rock and sediment deposited by glaciers at their edges – or dammed by glacier ice, these lakes are anything but stable.

Tam Pokhari glacial lake in Nepal had an outburst flood in 1998 after the basin filled with water and broke through its moraine, leaving a deep gash. The resulting flood was estimated at more than 350,000 cubic feet per second, equivalent to approximately 60% of the Mississippi River’s flow.

Between 1985 and 2020, ice-dammed lakes in Alaska alone broke through their barriers and . Alaska’s vast landscape and low population density means that the impact of these drainages on human infrastructure was fairly minimal, with a few notable exceptions, including Suicide Basin and , on the Kenai Peninsula.

However, the enormous amount of icy water rushing down rivers with each outburst can transform ecosystems, altering river channels through erosion and sediment deposition, tearing out trees and other vegetation, and .

Video shot from a helicopter shows several glacial lakes in Nepal and the aftermath of a glacial lake outburst flood. Nepal Investigative Multimedia Journalism Network

A recent study found that glacial lake outburst floods from moraine-dammed lakes are occurring at an . In the steep, narrow valleys of the Himalayan Mountains, the impact of these events are acute: destroyed hydropower stations, , taking .

More than globally live in areas at risk of glacial lake outburst floods. Mapping where these lakes might form and expand can help people living downstream prepare. That’s what we did in Alaska.

Mapping Alaska’s expanding lakes

Glacial lakes can form in a variety of settings: on the surface of glaciers, in side valleys, and at the terminus, or toe, of the glacier. We found that the fastest-growing lakes are those at the toe, and in our work, we showed that many of these lakes .

We mapped these depressions – known as glacial-bed overdeepenings – by subtracting ice thickness estimates from surface elevations measured by satellites.

As Ellsworth Glacier recedes on Alaska’s Kenai Peninsula, the lake where the ice flow terminates is growing. Haley Boron

We found that more than 80% of the lake growth has occurred in the mapped basins, illustrating how this approach can help locate glacial lakes that are likely to form and expand in the future.

With this information, we found that existing glacial lakes in the region may , growing by as much as 1,640 square miles (4,250 square kilometers). A glacial lake at the terminus of , the largest glacier by area in southeast Alaska, could expand to cover an additional 570 square miles (1,475 square kilometers) alone. That would create what would be the second-largest lake in Alaska.

As glaciers continue to retreat, new basins will be exposed, many of which could fill with water. In total, more than 5,500 square miles (about 14,200 square kilometers) of overdeepened basins exist in Alaska, pointing to a landscape that is going to look very different in the coming decades to centuries.

When a glacier terminates in a lake, the warmth of the water can speed up the ice’s melting, making the glacier flow faster, thin and retreat, thereby expanding the size of the lake. We found that glaciers that terminate in lakes are shrinking .

The future as glaciers retreat

Future climate projections combined with sophisticated glacier models indicate that glaciers will cumulatively retreat by 26% to 41% by 2100, spelling the .

This is concerning for numerous reasons. Glacier mass loss is currently the . Melting glaciers also change the that feed major rivers, particularly Asian rivers such as the Indus and Ganges. And they create hazards, such as the outburst floods that originate from glacial lakes.

The landscapes that we know and love are transforming before our eyes, and with these changes come growing concerns about hazards.

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