Tongass National Forest
Frozen Rivers Shaping the Land
The Stikine Icefield covers 2,900 square miles along the crest of the Coastal Mountains that separate Canada and the U.S. It extends 120 miles from the Whiting River to the Stikine River.
During the Pleistocene age, when the climate was 3 to 9 degrees Fahrenheit colder than it is today, an ice sheet covered a large expanse of the earth, including the islands of southeastern Alaska. Periods of cooling alternated with warming intervals. Ice sheets advanced or retreated in response to climate changes.
With most of earth's water locked up in ice, sea levels lowered. Animals and the first people used newly exposed land bridges to migrate from one continent to another. Each episode of accumulation and melting created new mixes of animal and plant life. Those that adapted, survived; others vanished, leaving only fossils to tell us of their existence.
Since the end of the Pleistocene age, about 10,000 years ago, the climate has become gradually warmer. The continental glacier retreated, forming the plains of the Dakotas, hills of the Midwest, and Finger Lakes of New York. The Stikine Icefield is one of the few remnants of the once vast ice sheets.
What is a Glacier?
Glaciers, perennial accumulations of ice, snow, sediment, rock and water, respond to changes in temperature, snowfall and geologic forces. Several components make up a glacial system: the ice and sediment contained in the glacier; the valleys, fjords and rock features it flows over, on, or around; and the deposits left by its retreat or advance.
New snow layers create pressure on existing layers of snow and ice. This process, "firnification," changes snow to firn, a dense granular snow (like corn snow). After the first season's melt, snow becomes firn. As it is compressed further, firn becomes ice.
As the snow collects over many years, an ice field forms. Ice flows down the valleys and slopes of the mountains to the lower elevations, terminating on land or water.
Glaciers Shaping the Land
Glaciers flow under the force of gravity, following the path of least resistance, often along stream valleys. Moving downward, they modify valleys into a characteristic U-shape.
Cracked pieces of rock, plucked or torn from the bedrock, are carried with other debris in and on the glacier. This debris scrapes the valley walls and floors, leaving grooves and striations. Rock debris is crushed and ground into fine grains, called rock flour.
How do Glaciers Move?
The depth of ice and steepness of slope are primary factors affecting the flow. Alaskan glaciers vary in thickness from several hundred feet to 2,000 feet. Movement creates elongated cracks, called crevasses, in the more brittle upper crust of the glacier. Below depths of 100 to 150 feet, the ice, under more pressure, behaves more plastically.
Mountain goats use the rocky, high elevations of Horn Cliffs. The terrain protects the goats from predators who need more secure footing. The sparse vegetation is high quality, enriched by the nutrient-laden soil left by the glaciers.
The Stikine River Delta, a haven for migrating shorebirds and waterfowl, is formed from glacial silt carried downstream by the river. The silt settled around islands near the mouth of the river and formed the delta. The river plays an important role as a migratory stopover. During the spring and fall migrations, thousands of snow geese, green-winged teal and other waterfowl feed on the delta grasses and crustaceans they find in the mud. Approximately a million shorebirds, such as the western sandpipers, also use the delta.
Harbor seals choose LeConte's protected water and abundant icebergs as a breeding, birthing, and rearing area. Remember not to disturb these little ones. Abandonment of a young pup by its mother is a common occurrence, particularly if they are disturbed by hunting or other activities by humans.
An arctic tern takes advantage of natural camouflage to lay her eggs along a glacier's outwash plain. Mimicking the colors and shape of the smooth polished rocks, the eggs blend into their environment, receiving excellent protection.
People of the Ice Fields
Glaciers have always played a part in the peopling of southeast Alaska. Melting ice created streams that flowed beside and underneath glaciers. Migration histories of Tlingit people tell of many who crossed under (yes! under) glaciers in order to reach the lands of southeastern Alaska from the interior.
Thousands of years passed. Glaciers continued to affect the area, this time influencing the economic history. Russian ships transported ice to be sold in San Francisco. Later the fishing industry grew. Fishermen received payment only for fish arriving in good condition in Seattle. Knowing this, they would fish for halibut and salmon just before the steamer from Seattle arrived. Fish, packed in glacier ice harvested from LeConte, arrived at Seattle markets in good condition. In 1926, the cannery built a cold storage in Petersburg. If manufactured ice ran out, harvested icebergs were ground up and blown into the holds of the boats. The compact ice melted much more slowly and provided an excellent means of preserving the fish on the long trips to Seattle.
Today, there is some speculation about harvesting glacial ice to sell for cocktail ice in large metropolitan areas. Locally, icebergs blown on shore are often broken up and saved to serve later in cold drinks. Some folks even use glacial ice to pack into coolers because it lasts so long.
Baird Glacier to the Stikine River
With Baird Glacier draining into its north end and Patterson Glacier into its south end, Thomas Bay provides a unique opportunity to see outwash plains, buried forests, and timber management areas.
Patterson Glacier has retreated about a mile and a half in the past 50 years. Like Baird, Patterson has a braided outwash plain, but unlike Baird, it reveals the remains of a buried forest once covered by the glacier.
Meltwater and sub-glacial water move rock flour, pebbles and larger rocks beyond the terminus to form outwash plains like these seen at Baird Glacier.
Near the south end of the Stikine Icefield, LeConte Glacier is the southernmost active tidewater glacier in the northern hemisphere. Since first charted in 1887, it has retreated almost 2.5 miles. Today, LeConte is considered in a stable position. Due to the deep water (810 feet) of the bay, LeConte calves instead of advancing. (Thomas Bay, in contrast, is about 300 feet deep.)
Beginning in 1983, measurements of LeConte's terminus (the point farthest from the head of the glacier) have been taken by Petersburg High School students. Results show the glacier moves forward in the spring after the cold winter weather decreases melting. In the fall, after warmer summer temperatures, it retreats.
Details mentioned in this article were accurate at the time of publication
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