A greenish lake sits in front of a snow-topped mountain under a cloud-filled sky.
Imja Tsho is potentially one of Nepal’s most dangerous glacial lakes; a flood would threaten more than 97,000 people. A 2016 mitigation project lowered the lake level by 3.4 meters, reducing the risk. Credit: Daniel Alton Byers, CC BY-SA 3.0

On a July night in the Himalayas, a wet slope gave way and slid into glacial Gongbatongsha Lake. The sudden influx breached the moraine dam, and within minutes the lake had completely drained. A relatively small flow at first, the outburst ballooned as it gushed down a valley in Nepal and merged with two rivers 20 minutes later. Gobbling up rocks and earth, the increasingly powerful wall of water wrecked a highway, ravaged a power station, destroyed homes, and triggered several landslides. The damage toll for the 2016 Gongbatongsha glacial lake outburst flood topped $70 million.

Glacial lake outburst floods (GLOFs) have killed many thousands since the first one was reported in Iceland in the year 850.

Fortunately, no one died in the Gongbatongsha deluge. But glacial lake outburst floods (GLOFs) have killed many thousands since the first one was reported in Iceland in the year 850. Waters unleashed by two outbursts from Nevado Huascarán in Peru, for instance, together caused nearly 7,000 deaths.

Glacial lakes form when meltwater fills depressions left by retreating glaciers; as the planet’s glaciers liquefy, the global inventory of glacial lakes is climbing. The volume and number of meltwater lakes dammed by ice walls or sediment moraines grew by roughly 50% between 1990 and 2018, a 2020 analysis of NASA’s Landsat satellite images found. Meanwhile, populations and infrastructure in mountain communities downstream have also mushroomed. In two recent papers, scientists assessed the global danger of GLOFs as glaciers shrink and lakes and populations swell.

Fifteen Million at Risk

A study published in Nature Communications calculated that up to 15 million people face the risk of a sudden, catastrophic flood. More than half of the threatened population lives in just four nations: China, India, Pakistan, and Peru.

The study censused populations in 1,089 basins containing glacial lakes identified by satellite—a total of 90 million people in 30 countries altogether. Of those, researchers found that one in six live within 50 kilometers of a glacial lake and 1 kilometer of a runout path, close enough to be in danger if an outburst were to occur. The risk analysis also drew on socioeconomic data to evaluate communities’ vulnerability and resources to cope with a disaster.

High Mountain Asia, home to China’s numerous glacial lakes and Pakistan’s huge vulnerable population, is the world’s most at-risk region, with 9.3 million people exposed to potential GLOFs in 2,211 lakes, the study found. Pakistan hosts the world’s most treacherous glacial basin—in the province of Khyber Pakhtunkhwa—with 1.2 million people in possible flash flood paths. Meanwhile, the Andes region is an understudied GLOF hot spot, the paper stated.

To quantify each lake’s hazard, the team measured the lake’s total area. “Bigger lakes present a greater threat, as there is simply more water stored upstream waiting to come down,” said study coauthor Tom Robinson, a disaster risk researcher at the University of Canterbury in New Zealand. In reality, when dams fail, most glacial lakes drain slowly and partially. Fewer than 1% are sudden, complete blowouts.

Caroline Taylor, lead author of the study and a doctoral student at United Kingdom’s Newcastle University, admitted that the approach overstates hazard. “Most lake failures are not complete. Thus, presented here is the worst-case scenario,” Taylor said. However, she added, the aim was to identify glacial basins with the greatest potential GLOF impacts to prioritize mitigation efforts.

Extreme Outbursts Declining

It’s not clear how GLOF threats will shift with a changing climate. Previous research has found no consistent patterns linked to current warming. In a new study, published last month in Nature, researchers compiled a record of the most common type of GLOF—caused by the failure of an ice dam—reported from 1900 to 2021. They found that ice dam floods are starting about 6 weeks earlier and at higher elevations today than in 1900. But despite rapid growth in the numbers and sizes of glacial lakes, there’s no sign that outbursts are increasing as the climate warms.

In fact, the volume of the largest ice dam GLOFs has declined “by an order of magnitude” in most regions since 1900, and the size of moderate flows is unchanged, said geomorphologist and lead author Georg Veh of the University of Potsdam in Germany. Preliminary research found the same trends for moraine-dammed GLOFs, said Veh, who is compiling a comprehensive global inventory of glacial lake outburst floods.

The reasons for the decreasing discharge are unclear, Veh said. The reduction is only moderately correlated with glacial thinning. Changes in glacier drainage systems may play a role, as could shifting weather patterns. Looking forward, the study projected GLOF threats continuing in the near future, then possibly fading by the end of the century as glaciers dwindle away.

Small Flood, Huge Damage

Even as glacial lake outbursts diminish, researchers said the disaster threat could increase as mountain populations climb.

Even as glacial lake outbursts diminish, researchers said the disaster threat could increase as mountain populations climb. “Trying to understand how that danger has changed over the last several decades, and how it will continue to change in the future…will really help us better understand what we need to do to reduce the threat,” Taylor said.

Incorporating population exposure and vulnerability is essential for glacial flood risk management, said Miriam Jackson, a glaciologist with the International Centre for Integrated Mountain Development (ICIMOD) in the Hindu Kush region. Jackson contributed data for the Nature paper but was not otherwise involved in either study.

“A relatively small volume flood can cause a huge amount of damage if there are a lot of people living downstream and if there is a lot of infrastructure, and if those people are vulnerable to start with, having little resource to respond to sudden disasters,” Jackson said.

Monitoring and mitigation can reduce the risk. In Nepal, for instance, which Robinson said ranks ninth in the world in GLOF danger, strategies implemented include identifying potentially dangerous glacial lakes, lowering lake levels, and installing early-warning systems.

The biggest challenge, however, is prediction. “Actually knowing the next one, the next location, the size of that event—this is a huge challenge,” said Veh. “And this is where we still struggle.”

—Cheryl Katz (@ckatz99), Science Writer

Citation: Katz, C. (2023), Glacial lakes can unleash deadly deluges. How risky are they?, Eos, 104, https://doi.org/10.1029/2023EO230110. Published on 21 March 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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