Caption on image: Columbia Glacier, Alaska  PH Coll 247.739<br>The Columbia Glacier is one of Alaska's better known tidewater glaciers -- those that empty directly into the sea -- 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. Located about 40 kilometers (25 miles) west of Valdez near the epicenter of the great 1964 earthquake, it is rapidly losing its battle for survival. It is the last of Alaska's 52 tidewater glaciers to begin its epic retreat from the sea.   Glaciers typically follow a seasonal pattern of advance and retreat. During the late summer months, tidewater glaciers erode on the seaward edge by calving, but if the amount of ice lost is replenished during the winter and early spring months, the glacier remains stable. Scientists have reported that Columbia Glacier will be reduced to about half its present size in the next 30 to 50 years, and that its terminus will have moved about 40 kilometers up the valley in which it lies. This is an astonishing rate of disintegration.   Columbia Glacier moves toward the sea at the rate of about one to three kilometers per year (about a mile) along its center line and roughly twice that fast along the edges where the ice is thinner. In order for the leading edge to actually move backward, therefore, the calving rate in the later months of the year must be phenomenal. It is now known that Columbia Glacier is (unlikely though it sounds) moving backward into ever-deeper water. Once the instability is triggered, the process of self-destruction feeds upon itself.  In the late 1970s, the water at the glacier's face was 56 feet deep. Now, it is nearly 1000 feet deep, and icebergs almost as tall and weighing as much as a million tons are breaking off. Generally, these mammoth ice cubes are trapped by the pile of rocks that the glacier has already shoved ahead. They remain there until they break up into smaller bergs which head out to sea on their own, posing collision threats to oil tankers and other shipping in Valdez Arm.
<ul><li>Subjects (LCTGM): Columbia Glacier (Alaska); Glaciers--Alaska; Prince William Sound (Alaska)</li></ul>
<bdi><a href="https://www.wikidata.org/wiki/Q46211791" class="extiw" title="d:Q46211791"><span title="clerk for the US Railway Mail Service in Alaska">John E. Thwaites</span></a>
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<a rel="nofollow" class="external text" href="http://content.lib.washington.edu/thwaitesweb/index.html">John E. Thwaites Photographs of Alaska</a>