Mendenhall Glacier FAQs
Why do glaciers form?
In Southeast Alaska, maritime climate and coastal mountains create favorable conditions for glaciation. Moist air flows toward the mountains, rises and releases snow and rain. Average annual snowfall on the Juneau Icefield exceeds 100 feet. Mild Southeast Alaskan summers cause winter snow accumulation to exceed summer snowmelt at higher elevations. Year after year, snow accumulates, compacting underlying snow layers from previous years into solid ice. Mendenhall Glacier is one of the 38 large glaciers that flow from the 1500 square mile expanse of rock, snow and ice known as the Juneau Icefield. As glacial ice continues to build, gravity pulls the ice down slope. The glacier slowly scours the bedrock and grinds down its 13-mile journey to Mendenhall Lake.
Is the glacier retreating?
A neo-glaciation period began 3,000 years ago and ended in the mid-1700s. At this time, Mendenhall Glacier reached its point on maximum advance, and its terminus rested almost 2.5 miles down valley from its present position. Mendenhall Glacier started retreating in the mid-1700s because its annual rate of melt began to exceed its annual total accumulation. The icefield's snowfall perpetually creates new glacial ice for Mendenhall Glacier and this ice takes 200-250 years to travel from the Juneau Icefield to Mendenhall Lake. Water depth at the glacier's terminus is 220 feet. At this rate, the glacier would take several centuries to completely disappear. For Mendenhall Glacier to advance, the icefield's snowfall needs to increase, the glacier's rate of melt needs to decrease, or both.
What happens after the glacier retreats?
As Mendenhall Glacier retreats and uncovers bare rock, the wind carries seeds and spores of moss onto barren land. Alder, willow and cottonwood tree seeds systematically grow in degalciated landscapes. Glacier debris, poor in nutrients, depends on flowering lupine and alder to fix nitrogen in the soil, and all species add organic matter to the soil as they are overtopped and shaded out by other species. Spruce and hemlock ultimately rise to close the forest canopy, eventually creating an old growth forest. Encompassing almost 350 years, this sequence of plant succession provides habitat for an increasing number of plants and animal species.
What evidence do glaciers leave behind?
The base of Mendenhall Glacier works like a giant piece of sandpaper. As the ice flows towards Mendenhall Lake, the glacier plucks rocks that become imbedded in the ice from the valley floor. The glacier scrapes these rocks across the bedrock creating grooves and striations. The glacier's erosive power changes the landscape and scrapes much of the soil and rock from the valley walls. Rocks scoured from the surrounding valley walls create dark debris lines called moraines on the edges and down the center of the glacier. As the glacier continues its path towards Mendenhall Lake, it grinds rock to a fine powder called rock flour that escapes with glacial melt water and creates the lake's murky color. Mendenhall Glacier's retreat exposes its trimline, marked by slightly sloping changes in vegetation on the valley walls that indicate the glacier's height at its point of maximum advance. As the glacial ice melts or calves icebergs, the glacier drops large rocks called erratics that its ice either quarried further up the valley or that fell onto the ice from rock walls above the glacier. These granite boulders can be seen lying on the metamorphic rock around the visitor center.
What wildlife lives near the glacier?
Coyote, porcupine, squirrel, snowshoe hare, and short-tailed weasel build homes on the valley floor, and migrating songbirds build nests in the deciduous shrubs in the young forest. In Steep Creek, beavers work to create ponds while spawning sockeye and coho salmom provide a food source for black bears and eagles. Loons, gull and Arctic terns nest around Mendenhall Lake, and mountain goats favor the rocky terrain and alpine meadows on the surrounding peaks.
Why is the ice blue?
Glacial ice appears blue because it absorbs all colors of the visible light spectrum except blue, which it transmits. The transmission of this blue wavelength gives glacial ice its blue appearance. Glacier ice may also appear white because some ice is highly fractured with air pockets and indiscriminately scatters the visible light spectrum.
Who was Mendenhall?
Appointed by President Harrison, Thomas Corwin Mendenhall (1841-1924) served as Superintendent of the U.S. Coast and Geodetic Survey from 1889 to 1894. A noted scientist, Mendenhall also served on the Alaska Boundary Commission that was responsible for surveying the international boundary between Canada and Alaska. In 1892, this glacier was renamed to honor Mendenhall. Naturalist John Muir first named the glacier Auke Glacier in 1879 after the Aak'w Kwaan of the Tlingít Indians.