This post was originally published on snow.news, my free Substack on snow science and the state of the snowpack.
A jökulhlaup is a type of flood emanating from glaciers that’s pronounced “yo-KOOL-lahp.” The word is Icelandic in origin and means “glacial run” in its native tongue.
Initially, the term referred to floods in Iceland that were triggered by geothermal heating or a volcanic eruption, but it’s now sometimes used more broadly to refer to any big release of water from a glacial lake.
“Glacial outburst floods or ‘jökulhlaups’ are a key part of deglaciation of both alpine mountains and of ice sheet margins,” according to a review in Geology Today. “They produce widespread and intense landscape change through erosion and deposition and are a hazard to populations and infrastructure.”
The paper concludes that “with increased intense rainfall events and with increased glacier melt due to climate change it is likely that outburst floods—including jökulhlaups—will increase in frequency and also in magnitude in the near future.”
The term “glacial lake outburst flood” (GLOF) is what you’ll see more often in the scientific literature. A 2023 study in Nature Communications concluded that GLOFs threaten 15 million people around the globe, with more than half of those at risk living in four countries: India, Pakistan, Peru, and China. A 2024 study in Nature Reviews Earth & Environment put the number at more than 10 million, projected a tripling of the hazard in high-mountain Asia by 2100, and included the instructive graphic below illustrating triggers/impacts.
“Jökulhlaups may occur with a somewhat regular periodicity, but others drain without warning,” according to the National Park Service, which also notes:
In order for a jökulhlaup to occur, the lake water levels must reach a critical point such that the lake:
Causes its ice dam to float
Overtops its dam, causing rapid incision into the sediment, rock, or ice that contains it
Carves large meltwater channels beneath the glacier ice that allow for rapid drainage
“Avalanches, rockfalls and slope failures can also trigger glacial lake outburst floods,” according to a recent piece in The Conversation. “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.”
One type of jökulhlaup is a monster-sized megaflood, in which the dissolution of a glacial dam leads to the drainage of a ginormous lake, such as those that existed in North America toward the end of the last ice age.
The Missoula Floods, which swept across the Pacific Northwest around 15,000 years ago, are one of the best-known examples. Glacial Lake Missoula extended for around 200 miles and “contained more water than Lake Erie and Lake Ontario combined,” according to the U.S. Geological Survey. “When the highest of these ice dams failed, lake water burst through, shooting out at a rate 10 times the combined flow of all the rivers of the world.”
