News Release

Denali's Muldrow Glacier Currently Experiencing an Historic Surge Event

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Date: April 7, 2021
Contact: Paul Ollig, (907) 683-9583

For the first time in 64 years, Denali National Park’s Muldrow Glacier, which originates high on the northeastern slope of Denali and flows north to form the McKinley River, is dramatically surging. Local pilot Chris Palms of K2 Aviation first noticed this unique geologic event during a recent overflight of the mountain. Since then, park scientists have begun a flurry of observations of this now rapidly flowing glacier.  

“It’s a spectacle of nature, and especially interesting because it’s rare to have this kind of event on such a historically prominent glacier that is a centerpiece of Alaskan visitation,” says Guy Adema, Deputy Director for Natural Resources Stewardship and Science for the National Park Service and former glaciologist for the Central Alaska Inventory and Monitoring Network. Glacial surges like this are typically short-lived events where ice within a glacier can advance suddenly and substantially, sometimes moving at speeds 10-100 times faster than normal. This sudden surge has quickly and dramatically altered the appearance of the previously slow moving Muldrow Glacier, with heavy crevassing occurring throughout most of the length of the glacier.  

As a result, mountaineers planning to use the north approach to Denali this climbing season will likely be impacted. Route finding this spring may be nearly impossible due to unstable ice conditions and the dramatic increase in crevasse hazards. Additionally, there is a heightened risk of outburst flooding along the McKinley River beyond the terminus of the Muldrow Glacier. Outburst flooding can occur with no warning, and for safety reasons backpacking in these areas of the park may be restricted.  

In a prior surge event during the winter of 1956-57, the glacier advanced just over 4 miles in a few months’ time. Today, preliminary satellite radar image analysis of the Muldrow Glacier completed by University of Alaska Fairbanks researchers Mark Fahnestock and Bretwood Higman suggests it is currently flowing at a rate of 10-20 meters, or 30-60 feet, per day which is about 100 times faster than normal. This surge is believed to have begun sometime in January and is expected to continue for several months.    

Scientists had been anticipating this surge for a while now, as the glacier displays evidence of a 50-year surge cycle, but no one knew exactly when it would occur. Adema, who has studied the Muldrow Glacier for decades, is particularly interested in investigating this event more closely, “I’ve been waiting for the Muldrow to surge for a long time. This is an incredibly exciting event to witness and to study!”  

The current surge of the Muldrow Glacier is likely not driven by impacts of climate change, but rather by the glacier’s geomorphology, the unique nature of the structure and composition of the glacier’s ice, its surrounding rocks, and the topography it is moving through.   

Surges like this may be caused by the interplay of ice buildup at higher elevations over time, and supply of meltwater to the base of a glacier. Due to the geometry of many surge type glaciers, ice accumulates and thickens on the upper glacier over many decades with only a slow flow or transfer of ice to the lower glacier. At some point during the quiet, accumulation phase, the mass reaches a threshold where the internal hydrology of the glacier is disrupted. Rather than running out at the terminus, meltwater is retained at the base of the glacier. The soft metamorphic bedrock that underlies the Muldrow, along with the massive quantities of retained water reduces friction and “floats” the glacier causing it to surge. Once a surge begins, positive feedback loops and momentum can often contribute to the speed and duration of the surge. 

“The fact that it’s happening in a place that people know and care about makes it rather interesting,” said Adema. Overall, while it is estimated that only one percent of all the world’s glaciers ever surge, Adema points out that Denali is a bit unique in this respect. “A relatively large portion of Denali glaciers are surge type due to the dramatic relief of the mountain. The intermittent surging makes these glacier less ideal for monitoring the impacts of climate change, but are fascinating to watch when surges like this occur.”   

Park scientists will continue to monitor and map this dynamic situation both during and after the surge event, with the hope of increasing our understanding of how glaciers in Alaska are changing. The NPS also monitors the mass balance of a small set of glaciers in several national parks. Non surge type glacier dynamics are a long-term indicator of climate change. On the south side of Denali, opposite from the Muldrow, the Kahiltna glacier is a long-term index glacier which is slowly retreating. Overall, ice cover in Denali has declined by 8% since 1952.   

Visitors to Denali National Park this summer should temper expectations of what they may be able to see without a backcountry excursion or overflight, however. Evidence of this surge is likely to be less dramatic when viewed from the Park Road or the Eielson Visitor Center, with only a bit more jumbled ice visible in the distance along the edge of the glacier.  

Learn more about the Muldrow Glacier



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A brief intro to the surge of the Muldrow in 2021

Last updated: April 7, 2021

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