The Columbia Glacier
The Columbia Glacier is one of Alaska's better known tidewater glaciers, both from the standpoint of tourist attraction and the model it provides for scientific investigation. In 1973 it became the object of close scientific scrutiny.
In Alaska, some 50 to 60 glaciers calve into the sea, but exhibit such diversified behavior that they have baffled glaciologists for decades. For instance, some will recede from 10 to 20 miles in a number of years, at the same time that others will advance into standing forests. Subtle variations in climate should not produce such disparities.
On investigation, it was found that many glaciers built underwater moraines across the necks of their fiords, after which they retreated great distances up the valleys. The Columbia Glacier did not behave in this manner. it was found to be the only sea-calving glacier in Alaska that terminated at a mature coastal forest of at least 4000 years of age, judging from peat deposits. Assisted by the University of Alaska's research vessel ACONA in 1973, it was discovered that the glacier was still grounded on the crest of its submarine moraine located at the north end of Heather Island, and was thus protected from rapid calving by ocean water. Soon, however, large embayments began forming each summer and the glacier began retreating from its moraine.
During the past decade, it has been found that 3 to 4 cubic kilometers (about 1.5 cubic miles) of ice accumulate on the glacier from snowfall alone. Nowhere else in Alaska have depths of up to ten meters (33 feet) of snow been found remaining on glaciers at the end of a summer. The mass of ice generated from year-round snowfall high in the Chugach Mountains flows seaward along the main ice stream, which glacier-radar reveals to extend as far below sea level as the glacier surface rises above it. As the ice stream rides up the terminal moraine from well below sea level, it thins, so must accelerate to higher speeds. This past winter, automatic cameras near the terminus measured speeds of over 15 meters per day, or 5000 meters per year (50 feet per day, or 3 miles per year).
The depth of water at the calving terminus was found to be critical to the stability of the glacier. If the ice face recedes from the protection of the submarine shoal and is exposed to deeper water, the calving rate can exceed the ice flow rate and the glacier will retreat into deeper water and its attendant higher erosion rates. An estimate of this behavior was predicted by the U.S. Geological Survey in 1980 and refined in 1982, when it warned that the Columbia G lacier could retreat by 2 to 10 kilometers ( 1 to 6 miles) by 1984.
As it developed, 1977, 1980 and 1981 produced about twice the normal snowfall levels in the mountains, and the upper glacier actually thickened despite the rapid rate of flow. A storm in August, 1981, washed out the highway to Valdez and deposited 8 meters (26 feet) of new snow high on the glacier. This and other significant snowfalls reduced the total amount of retreat to only about 800 meters (2600 feet) in the past 5 years.
Measurements made from January to June of this year revealed that the glacier was still advancing, and that the period of rapid, continuous retreat has not yet begun. This is in spite of the fact that it has receded from the protection of its submarine shoal and the calving rate has increased several times over that of 5 years ago.
A new development occurred during the past winter, when a surge of glacier ice filled part of the sea between the ice cliff and Heather Island with icebergs. These became trapped and, frozen together with winter sea ice, have been squeezed into a reconstituted ice tongue. This type of ice formation has not previously been observed in Alaska. In regaining some of the ground that it had lost over the past several years, this may signify that the glacier is capable of advancing to its submarine moraine and becoming stable again, although it is suspected that the gain is merely temporary.
It has been found that the shape, size, speed and mass of the Columbia Glacier can change rapidly with time, depending on existing conditions. Observations of these phenomena have led to increased interest by scientists and others with a critical interest, such as oil tanker captains.
