Question: Seems like we’ve been getting a lot of those glacier floods in the mountains lately, especially around the Himalayas. I imagine it’s somehow related to global warming, but I’m unsure about the exact mechanism. It can’t be just that they’re melting so fast they cause a flood. That would take a catastrophic increase in melt rate. The ice in our Glacier National Park has been melting a lot too, but we haven’t had any flooding. Just an increase of flow in the meltwater streams. Makes for some great fishing! — FS, Columbia Falls, MT
Answer: Glacial flooding does indeed require a catastrophic event, but it’s typically not just a rapid increase in melt rate. This phenomenon technically called a “glacial lake outburst flood” (GLOF) follows a chain of related events as shown in the slide show.
Many civilizations around the globe depend on the runoff of melting glaciers for their fresh water supply, and have done so for millennia. Even before global warming, there was a reliable cycle of winter ice replenishment and summer ice loss that kept the meltwater flowing. Slide 1 shows a healthy and stable system doing just that. Some systems have been running for for ages without experiencing a GLOF.
The recent tragedy in Nepal was the result of an interruption to the usual cycle. A landslide triggered by melting permafrost and seismic activity temporarily dammed the meltwater forming a large lake behind the dam. Slide 2 shows this unstable configuration. Occasionally, slabs of ice can calve off the terminus of the glacier and add to the dam.
Slide 3 shows what further exacerbated the situation. Heavy rainfall added to the lake and caused an increase in the amount of meltwater flowing from the glacier. This is an unsustainable scenario that usually results in a GLOF. Rarely, the meltwater can find a way around or through the dam and the lake can become more or less permanent and continue to feed the river downstream. But more often than not, the dam gives way catastrophically due to overloading, over-topping, additional seismic activity, or a tidal wave caused by a large piece of ice falling from the glacier’s terminus. Slide 4 shows an over-topping GLOF.
GLOFs may have once occurred in Glacier National Park but are unlikely in the future. There’s no real unstable permafrost in the park, and it has little seismic activity to cause landslides in the solid granite mountains. This composite photo shows a comparison of Grinnell Glacier 100 years ago and today, and as you can see, most of the ice is already gone. It’s been estimated that by the 2040s all but small sheltered pockets of ice will be gone. Sadly, people should visit this disappearing treasure while they can. Click to enlarge:
Conditions in the Himalayas will remain conducive to GLOFs for some time, unfortunately, on both the Nepal and China side of the peaks. What happened in Nepal in August 2026 was a largely avoidable tragedy, given the use of early warning sensors and planned evacuation routes. Alas, Nepal does not have access to much of this technology for reasons of cost and infrastructure. Huge amounts of emergency aid have already poured in to the country, but a sustained effort will be required to solve the problem and prevent repeats of this tragedy.
Next Week in Sky Lights ⇒ Unexpected Sabbatical