The shrinking Mendenhall Glacier

Posted: Friday, January 05, 2001

Mendenhall Glacier is shrinking and scientists want to know why.

While George Washington rowed around ice floes on the Delaware River and Hans Brinker skated toward Amsterdam on the frozen canals of Holland, the Mendenhall Glacier was beginning its retreat from its furthest forward position, known as its Little Ice Age Maximum.

Between 1767 and 1909, as the world was thawing out from the Little Ice Age, the glacier's terminus retreated nearly a mile, leaving behind a terminal moraine ridge, composed of rocks, sand and silt at the southern end of River Road.

Successive recessional east-west trending moraines were deposited in places like Mendenhall River School, along Taku Boulevard, and across the Loop Road near Threadneedle and Garnet Streets. Melt water pooled behind the moraines and formed Dredge Lakes. Subdivisions, roads, campgrounds and a visitors' center now occupy sites that were covered by 500 to 1,000 feet of ice merely 100 to 200 years ago. We live in a unique community that has a glacier flowing into its own backyard!

A U.S. Geological Survey map made in 1909 shows a small pond at the edge of the glacier, south of the present Skaters Cabin. The basin that was to become Mendenhall Lake was still occupied entirely by ice, and both Duck and Jordan Creeks received a steady supply of glacier melt water.

Glacier retreat accelerated in the decades that followed, receding another one-half mile by 1931, and Mendenhall Lake was born.

While the world struggled with the Great Depression and World War II between 1931 and 1949, Mendenhall Glacier retreat was wreaking havoc on rivers and water flow in Mendenhall Valley.

A large, abandoned riverbed and delta north of Loop Road marks the old runoff channel for glacier melt water and Steep Creek. Flowing water, which for decades had emerged from the east side of the glacier, switched to the west side and now drained directly into Mendenhall River as the glacier receded into the lake basin.

By 1949, the glacier had receded another one-half mile and the terminus stretched across a one-mile-wide lake filled with icebergs. Today, after retreating another three-quarters of a mile, only the northeast corner of the terminus calves into the lake, and the erosion-resistant ridge uncovered to the west has become a popular breeding ground for gulls and arctic terns.

During the last 52 years, the glacier has lost an average of 110 million cubic meters of ice per year. This is equal to about twice the volume of Mendenhall Lake in 2000.

The excess ice melt contributes about 13 percent of the water annually discharged from Mendenhall Lake and is about 19,000 times the annual fresh water usage of the Juneau community. The glacier has retreated 2.6 km (1.6 miles) in the 20th century and its ice has thinned considerably. Its lower end ice currently loses 8 meters (26 feet) in thickness annually.

What is causing this dramatic shrinking of the glacier?

Global warming is certainly a culprit: Most alpine glaciers in the world have been retreating for some time, some completely disappearing.

Does it seem like Juneau has been getting warmer and wetter? Well, it has.

The average annual temperature at the Juneau Airport has risen nearly 3 degrees Fahrenheit (1.6 degrees Celsius) since 1943, while the average annual precipitation has increased by 6 inches (15 centimeters). But iceberg calving has also taken its toll on the Mendenhall Glacier by speeding up the recession process. Thus, climatic warming coupled with ice loss through iceberg calving are the reasons the Mendenhall Glacier is retreating and shrinking.

Mendenhall Lake is 70 meters (220 feet) deep along the present ice terminus. Behind the Mendenhall Glacier terminus, the basin drops to 110 meters (360 feet) and extends up the valley another one-half kilometer (0.3 miles).

Icebergs will continue to calve into the lake for another 10 to 15 years and then, if present climate conditions persist, the glacier will recede out of the lake and look more like Herbert Glacier.

The lake has become an important habitat for juvenile salmon and other fish species. Spawning sockeye salmon in Steep Creek are a popular visitor attraction. The lake has also become a recreational mecca for outdoor enthusiasts, with kayaking and rafting in the summer, and excellent skating and cross-country skiing in the winter. However, users should be cautious of ice conditions, particularly near the terminus where glacier movement can buckle lake ice and the glacier can calve, even in the winter.

It is important to remember that the glacier ice continues to flow down the valley slopes even while the terminus position retreats up the valley. Glacial speed on the Mendenhall can be as fast as 160 meters per year at the 700 meter (2,296 foot) elevation, where the ice is 630 meters (2,100 feet) deep.

This velocity would be the equivalent of motoring along the Egan Highway at a rate of 1.4 feet per day or taking 93 years to reach downtown Juneau from the Mendenhall Mall; a lifetime for a person but not bad for a glacier.

Cathy Connor is an Assistant Professor of Geology and coordinator of the Environmental Science Program at the University of Alaska Southeast. Roman Motyka is a University of Alaska Research Professor with the University of Alaska Fairbanks Geophysical Institute and a Faculty Affiliate at UAS. Laurie Craig Ferguson will speak on her "Wetland Watch" project on the airport dike trail at the Juneau Audubon Society's monthly meeting at 7:30 p.m. Thursday at Dzantik'i Heeni Middle School.



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