A team of scientists
takes a core sample
from the bed of Nepal’s
Taboche Lake, near
Gokyo. The sediment
layers contain clues
about when and how
the lake was formed
and allow researchers
to study changes
in seasonal conditions
over time. Get updates
from the National
Geographic and Rolex
Perpetual Planet Extreme
Expedition to Mount
Everest at natgeo.com/
perpetualplanet.
TYLER DINLEYquickly that it’s difficult for scientific teams to
agree on the precise number.
“It’s all happening much faster than we
expected it to even five or 10 years ago,” says
Alton Byers, a National Geographic explorer
and mountain geographer at the University of
Colorado Boulder.
TO UNDERSTAND HOW THESE LAKES FORM, think
of a glacier as an ice bulldozer slowly plowing
down the side of a mountain, scraping through
the earth, and leaving a ridge of debris on either
side as it pushes forward. These ridges are called
moraines, and as glaciers melt and retreat, water
fills the gouge that remains, and the moraines
serve as natural dams.
“They start as a series of meltwater ponds,”
Byers explains, and “they coalesce to form a sin-
gle pond, then a larger lake. And year by year
they get larger and larger, until you have a lake
with millions of cubic meters of water.”
And as the lake fills up, it can overspill the
moraines holding it in place or, in the worst-case
scenario, the moraines can give way. Scientists
call such an event a glacial lake outburst flood,
or GLOF, but there’s also a Sherpa word for it:
chhu-gyumha, a catastrophic flood.
One of the most spectacular Himalayan GLOFs
occurred in the Khumbu region of Nepal on
August 4, 1985, when an ice avalanche rumbled
down the Langmoche Glacier and crashed into
the mile-long, pear-shaped Dig Lake.
The lake was likely less than 25 years old—a
photo taken in 1961 by Swiss cartographer Edwin
Schneider shows only ice and debris at the foot
of Langmoche. When the avalanche hit the lake,
it created a wave 13 to 20 feet high that breached
the moraine and released more than 1.3 billion
gallons—about the equivalent of 2,000 Olympic-
size swimming pools—of water downstream.WHEN THE ROOF OF THE WORLD MELTS 137