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Denali’s Muldrow Glacier Surged in 2021 for the First Time in 64 Years
On March 4, 2021, a K2 Aviation pilot, Chris Palm, flying along the north side of Mount McKinley saw something wrong with the Muldrow Glacier. A glacier that is usually slow and relatively smooth was torn by crevasses along nearly its whole length. The long-awaited surge had begun, and Denali National Park scientists rushed to record it. The Muldrow Glacier (Henteel No' Loo') is a 39-mile-long glacier that starts high on the northeastern slope of Mount McKinley and flows to the McKinley River.
The Muldrow Glacier on March 17, 2021, with the Traleika and Brooks Glaciers feeding the main trunk (National Park Service).
What a surge is
Most glaciers flow at a fairly steady rate, but surge-type glaciers alternate short bursts of rapid flow with long stretches of near standstill, called quiescence. In a surge, ice moves from a reservoir area high on the glacier to the toe, at speeds that can be 10 to 100 times normal. Only about one percent of the world's glaciers are thought to surge, and several of Denali's do.
The likely cause is a balance between ice buildup and meltwater. Over the quiet years ice accumulates and thickens in the upper glacier. When it passes a threshold, the glacier's internal drainage breaks down and meltwater that normally exits at the toe gets trapped at the base. It lubricates the ice, perhaps even lifting it, so friction falls and the ice speeds up.
The Muldrow's last surge was in 1956-57. Evidence of an earlier one between 1906 and 1912 suggests a cycle of roughly 50 years. The park's scientists had been collecting baseline data since the 1990s, but nobody knew when the next surge would come. An earlier study of elevation change from 1952 to 2010 found the glacier's surface in 2006 some 30 to 40 meters below its 1952 level, which suggested it might not build enough ice to surge again under today's climate. It did.
What changed
During the quiet decades the glacier was covered in debris, with ponds and flowing water on its surface and few crevasses. The surge transformed it: pervasive crevasses, sheared tributary glaciers and an advancing toe. Satellite and aerial imagery showed it flowing 10 to 20 meters per day, 50 to 100 times faster than the 0.16 to 0.25 meters per day of the previous 60 years. Transverse crevasses, which cut across the glacier, appear where the ice is speeding up and stretching.
The glacier's bend and toe, where the ice surface bulges around the inside corner, March 13, 2021 (National Park Service).
Transverse crevasses across the main trunk (National Park Service).
How scientists followed it
| Tool | What it showed |
|---|---|
| Satellite radar (Sentinel-1), analyzed by researchers at the University of Alaska Fairbanks | The surge started sometime in January. Radar sees through clouds and winter darkness, and the ice was moving 50 to 60 feet a day by the end of the animation |
| Aerial photos and Structure from Motion mapping | About 1,000 geotagged photos make a detailed elevation model. Flights on March 13 and 17, planned at least twice a month, let computers match features and compute velocities |
| Two continuous GPS units, placed March 28 | Positions every 30 seconds, to see whether the glacier moves smoothly and how the surge ends. Power and safety on a changing glacier are hard, and one tripod nearly fell into a new crevasse |
| Time-lapse cameras | Four placed March 28: two at the bend near Anderson Pass, one at the toe and one on the McKinley River to catch any outburst flood |
| Sound station | Records the noise of an ice-falling, grinding glacier, to compare with its normal soundscape |
| Seismometer at the Eielson Visitor Center | Baseline noise rose from December 2020 as the ice cracked and ground |
| River gauge | A radar gauge measures the McKinley River's level as a baseline for any outburst flood |
A detailed image of the surging glacier stitched from the first mapping flight (National Park Service).
Glacier surges can stop suddenly, often within hours or days, usually with a dangerous outburst flood.
Impacts for visitors
The park warned that debris-covered ice visible from Eielson might break up, the swollen toe would be visible, and outburst floods could make the McKinley River dangerous or impossible to cross. The backpacking route over the lower glacier to the Pirate Creek area might be impassable, and so might the common northern route up Mount McKinley across the glacier. Visitors were told to avoid the jumbled, unstable ice.
How far, and how it ended
From September 2020 to March 2021 the ice moved 1.1 to 3.1 kilometers in the main stem, and by March 17 the toe had advanced 3.5 km and was within 800 meters of the 1957 limit. That surge moved ice by about 6.6 km, so the team thought this one might be about halfway. About 3.3 cubic kilometers of ice moved from the upper and middle glacier to the toe in 1956-57, lowering the upper surface by 170 meters and leaving a wall of ice 200 feet tall at the toe.
A time-lapse camera at the toe, set up March 28 (National Park Service).
| Date, 2021 | Update |
|---|---|
| May 2 | The front advancing at 24 m a day in the center; fast flow of 17 m a day over the first 4 km up the Traleika |
| May 7 | Surface down 50 m at Gunsight Pass and up to 120 m higher near the toe |
| May 1 to 8 | The toe moved 175 m at 25 m a day |
| May 8 to 25 | It advanced about 340 m in 17 days |
| June 6 | The McKinley River rose 1 m in 3 hours, a small outburst |
| Early July | Lower glacier speed about halved; the Traleika trunk still at 20 m a day |
| July 26 | The toe had nearly stopped. The river gauge failed on July 22, and the glacier toe stopped surging on or soon after that date |
A GPS tripod that nearly fell into a crevasse formed under it (National Park Service, Chad Hults).
Researchers from the University of Alaska Fairbanks spent about two weeks in May beside the glacier with six seismometers and a ground radar scanning every two minutes. Comparing scans gives interferograms that show ice moving toward and away from the radar, which will allow precise surface velocities. By the end, the toe had advanced more than 1 kilometer over 54 days and risen about 200 meters over the summer, pushing ahead a moraine of dead ice left since 1957.
Sources
- National Park Service, Denali's Muldrow Glacier, Denali National Park and Preserve; rewritten in hubnx's own words.