MELTING ICE: As glaciers melt, risk of flash floods on rise

Suzanne O'Connell & Alton C Byers

In mountain ranges around the world, glaciers are melting as global temperatures rise. Europe's Alps and Pyrenees lost 40 per cent of their glacier volume from 2000 to 2023.

These and other icy regions have provided freshwater for people living downstream for centuries – almost 2 billion people rely on glaciers today. But as glaciers melt faster, they also pose potentially lethal risks.

Water from the melting ice often drains into depressions once occupied by the glacier, creating large lakes. Many of these expanding lakes are held in place by precarious ice dams or rock moraines deposited by the glacier over centuries.

Too much water behind these dams or a landslide into the lake can break the dam, sending huge volumes of water and debris sweeping down the mountain valleys, wiping out everything in the way.

These risks and the loss of freshwater supplies are some of the reasons the United Nations declared 2025 the International Year of Glaciers' Preservation and March 21 the first World Day for Glaciers.

Erupting ice dams and landslides

Most glacial lakes began forming over a century ago as a result of warming trends since the 1860s, but their abundance and rates of growth have risen rapidly since the 1960s.

Many people living in the Himalayas, Andes, Alps, Rocky Mountains, Iceland and Alaska have experienced glacial lake outburst floods of one type or another.

A glacial lake outburst flood in the Himalayas in October 2023 damaged over 30 bridges and destroyed a 200-foot-high (60-metre) hydropower plant. Residents had little warning. By the time the disaster was over, more than 50 people had died.

Juneau, Alaska, has been hit by several flash floods in recent years from a glacial lake dammed by ice on an arm of Mendenhall Glacier. Those floods, including the one in 2024, were driven by a melting glacier slowly filling a basin below it until its ice dam broke.

Avalanches, rockfalls and slope failures can also trigger glacial lake outburst floods. These are growing more common as frozen ground known as permafrost thaws, robbing mountain landscapes of the cryospheric glue that formerly held them together.

These slides can create massive waves when they plummet into a lake. The waves can then rupture the ice dam or moraine, unleashing a flood of water, sediment and debris.

That dangerous mix can rush downstream at speeds of 20-60 mph (30-100 kph), destroying homes and anything else in its path.

The casualties of such an event can be staggering. In 1941, a huge wave caused by a snow and ice avalanche that fell into Laguna Palcacocha, a glacial lake in the Peruvian Andes, overtopped the moraine dam that had contained the lake for decades.

The resulting flood destroyed one-third of the downstream city of Huaraz and killed between 1,800 and 5,000 people.

Governments have responded to this widespread and growing threat by developing early warning systems and programs to identify potentially dangerous glacial lakes. Some governments have taken steps to lower water levels in the lakes or built flood diversion structures, such as walls of rock-filled wire cages, known as gabions, that divert floodwaters from villages, infrastructure or agricultural fields. Public education has helped build awareness of the flood risk, but the disasters continue.

Flooding and thawing permafrost

The dramatic nature of glacial lake outburst floods captures headlines, but those aren't the only risks. As scientists expand their understanding of how the world's icy regions interact with global warming, they are identifying several other phenomena that can lead to similarly disastrous events.

Englacial conduit floods, for instance, originate inside glaciers, commonly on steep slopes. Meltwater can collect inside massive systems of ice caves or conduits. A sudden surge of water from one cave to another, perhaps triggered by the rapid drainage of a surface pond, can set off a chain reaction that bursts out of the ice as a full-fledged flood.

Thawing mountain permafrost can also trigger floods. This permanently frozen mass of rock, ice and soil has been a fixture at altitudes above 19,685 feet (6,000 metres) for millennia.

Freezing helps keep mountains together. But as permafrost thaws, even solid rock becomes less stable and is more prone to breaking, while ice and debris are more likely to become detached and turn into destructive and dangerous debris flows. Thawing permafrost has been increasingly implicated in glacial lake outburst floods because of these new sources of potential triggers.

In 2017, nearly a third of the solid rock face of Nepal's 20,935-foot (6,374-metre) Saldim Peak collapsed and fell onto the Langmale glacier below. The heat generated by the friction of rock falling through air melted ice, creating a slurry of rock, debris and sediment that plummeted into Langmale glacial lake below, resulting in a massive flood.

These and other forms of glacier-related floods and hazards are being exacerbated by climate change.