Habitat of Steep Creek in Mendenhall Valley
The Juneau Icefield
Embark upon a trip back in time during a visit to the Juneau Icefield. Located in the Coast Mountain Range, North America's fifth largest icefield blankets more than 1,500 square miles of land stretching nearly 100 miles north to south and 45 miles east to west. Proceed up valley and observe the transformation. Watch the temperate rainforest diminish as the ice spreads like tentacles among the jagged mountain peaks. What ancient process fashioned this stark land? How will it be transformed in the centuries yet to come?
Step back two million years to the Pleistocene when mammoths roamed the West and a cooling trend locked moisture into ice. During the Great Ice Age several climatic fluctuations nourished glacial advance and retreat, and vast sheets of ice enshrouded nearly a third of the Earth's land mass and one half of Alaska. Twenty thousand years ago, as the climate warmed at the dawning of the Holocene, the ice released its hold on the land and retreated. In Alaska, ice remained at only the highest elevations. Continuing variations in climate prompted four smaller scale glacial advances and retreats. The most recent period of neo-glaciation to shape the Juneau Icefield began 3,000 years ago and ended in the mid-1700's. During this time, many glaciers in Alaska, including those which flow from the Juneau Icefield, fluctuated with the climate, advanced, and again retreated after reaching their glacial maximums in the mid-1700's.
Today ice covers only 5% of Alaska, though this region has actually hosted a glacier-favoring mixture of climate and topography for the last 12.5 million years. Weather and terrain are not the only factors that make glaciation possible. One widely accepted theory suggests that Pleistocene glacial and inter-glacial periods result from the Earth's orbital-rotational cycles. The fluctuation in the tilt of the Earth's spin axis and the shape of the Earth's orbit interact, varying the amount of seasonal sunshine which the Earth receives in certain areas. These changes in seasonal intensity may affect ocean currents, which ultimately influence the climate.
In southeast Alaska, maritime climate and coastal mountains work together to create favorable conditions for glaciation. The Juneau Icefield straddles the Coast Mountain Range on the United States-Canadian border, directly in the path of the Pacific Ocean's prevailing winds. Moist air rushes toward the mountains, rises, cools, and releases snow and rain. Annual snowfall on the Juneau Icefield exceeds 100 feet, and mild Southeast summers assure that snow accumulation exceeds snow melt at higher elevations. As the snow continues to accumulate, its own weight compacts snow layers from previous years into solid ice, causing changes in volume, density and crystal structure. Glacial ice absorbs all color of the visible light spectrum except blue, which it transmits. Scientists estimate the icefield's snow and ice depth to be from 800 to over 4,500 feet deep. As snow and ice continue to accumulate, gravity eventually pulls the ice mass into motion.
Terrain determines the flow and boundaries of an icefield. Icefields form where numerous tongues of ice known as valley glaciers interconnect around mountain peaks called nunataks which push through the ice. Devil's Paw, the icefield's highest peak, stands at 8,584 feet. Many small glaciers and at least 40 larger valley glaciers flow from the icefield. These glaciers from where annual snowfall exceeds annual snowmelt. Climate, geography and snowfall determine the advance or retreat of a glacier's face or terminus. A glacier's accumulation zone, located in higher elevations, accrues a wealth of snow and ice. The ablation zone, located in lower elevations, loses ice through melting or downwasting. A glacier's terminus advances when more snow and ice ammass than melt, and it retreats when melt exceeds accumulation. When melt equals accumulation, the glacier's terminus remain stationary. Regardless of a glacier's advance or retreat, glacial ice persistently glides down valley. Although the Juneau Icefield is at least 3,000 years old, the ice remains young because its steady flow perpetually renews itself through snowfall at upper elevations. Glacial ice at the terminus of Mendenhall Glacier flows only 80 - 120 years on its twelve-mile trek to Mendenhall Lake.
Each episode of glacial advance and retreat also shuffles the mix of flora and fauna. Fragile vegetation ventures into a seemingly barren wasteland. Carried by the wind, seeds and spores of pioneering plants cling tenaciously to life in the hostile environment. As lichen and moss clothe the exposed rock, the rebirth of the temperate rainforest begins, with alder, willow, cottonwood, spruce and hemlock systematically reclaiming the land they inhabited before the most recent glacial advance. Glacial debris, poor in nutrients, depends on flowering lupine, decomposing alder leaves, and alder root nodules to fix nitrogen into the developing soil. Overshadowed by cottonwood and spruce, decaying alder adds additional fertilizer to the forest flower, while hemlock ultimately rises to close the canopy, shading out most spruce and creating an old growth stand or climax forest. Encompassing almost 350 years, this sequence of plant succession nurtures the development of the forest community and provides habitat for an increasing number of plant and animal species.
Barriers, created by the geography and the brief span of time since the Great Ice Age, inhibit the repaid re-establishment of animal communities in Southeast Alaska. River valley provide primary routes into recently deglaciated areas. Several species venture rapidly into the developing landscape. Migrating songbirds, snowshoe hare and mice build homes in the young forest. During the summer months, mountain goats favor the rocky terrain which skirts the icefield and provides protection from less sure-footed predators. Salmon establish spawning areas inlakes and streams formed by retreating glaciers, while wolf and wolverine occasionally journey onto the ice from the adjacent ridges and forest. Many other species including Sitka black-tailed deer, black bear, goshawk and weasel wait to take residence during the middle to later stages of plant succession.
As the soil is replenished and the time since the last glacial advance continues to pass, additional species repopulate the land. Each episode of glacial advance and retreat renews the cyclic tug-of-war between ice and vegetation.
This material was taken from the USDA Forest Service brochure, "The Juneau Icefield", R10-RG-124. You can get this excellent publication by contacting the Juneau Ranger District at (907) 586-8800.