  Prior to the beginning of the present century all glaciers in the Park, and most of those in the rest of the world, began to shrink in response to a slight change in climate, probably involving both a temperature rise and a decrease in annual snowfall. From about 1900 to 1945 shrinkage of Park glaciers was very rapid. In other words these glaciers were not in equilibrium with the climate, for less ice was added to them each winter than disappeared by melting and evaporation during the remainder of the year. Over a period of several years such shrinkage is apparent to the eye of an observer and is manifest by a lowering of the glacier’s surface, and more particularly by a “retreat” of the lower edge of the glacier. This part of the ice is generally referred to as the ice front. When sufficient snow is added to the upper part of the glacier to cause the ice at the front to move forward equal to the rate at which it melts away, the glacier is in equilibrium with the climate. When the yearly added snow decreases in amount the ice front seems to retreat or move back, whereas the mass of the glacier is merely decreasing by melting on top and along the edges, just as a cube of ice left in the kitchen sink decreases in size. The National Park Service initiated observations on glacier variations in 1931. At first the work consisted only of the determination of Since 1945, the glacier observations have been carried on in cooperation with the U. S. Geological Survey. The work has included the periodic measurement of profiles to determine changes occurring in the surface elevation of Grinnell and Sperry Glaciers and also the determination of the rate of annual movement. Some of the more important data yielded by surveys on Grinnell and Sperry, the two largest glaciers in the Park, are summarized in the following tabulations:
The Grinnell Glacier originally consisted of an upper and lower portion connected by an ice tongue. This tongue disappeared in 1926 and since then the two portions have been separate. The area of the upper portion of the glacier was essentially the same in 1960 as in 1956—56 acres. The upper section is known as Salamander Glacier because of its shape as viewed from a distance. The terminal recession of the Grinnell Glacier is somewhat difficult to determine accurately as a part of the terminal portion ends in a lake, the shore of which varies from year to year. The recession for a half-mile section extending southeast from the lake is shown below:
The values for area and recession shown above indicate that changes in the area of the glacier have not been as pronounced since the mid-1940’s as they were prior to that time. Profile measurements starting in 1950 indicate a general trend of continued shrinkage although annual changes have been both positive and negative. The 1965 observations showed a surface lowering of 20 to 25 feet, since 1950. The movement of the Grinnell Glacier, based on observations since 1947, has been about 35 to 40 feet per year. The Sperry Glacier is located 9 miles from the Grinnell Glacier, on the opposite side of the Continental Divide and at an altitude approximately 1,000 feet higher. It has also shown a continual shrinkage in area and recession of the terminus as shown by the following tabulations:
Recession, in feet, of central half-mile section of terminus
Profile measurements, starting in 1949, indicate a continued lowering of the glacier surface below an altitude of about 7,500 feet. Above this altitude it has remained much the same during the period of observations with annual changes, both positive and negative, with a possible slight net increase since 1949. The forward movement in the central portion of the Sperry Glacier, based on observations since 1949, has averaged about 15 feet per year. The rate of movement is presumed to be greater in the upper reaches of the glacier. It is of interest to note from the data that the changes in Sperry Glacier are more pronounced than those in Grinnell Glacier although the straight-line distance between them is only 9 miles. One possible Even more significant is the lowering of the glacier’s surface, from which volume shrinkage may be obtained. In 1938 Sperry Glacier had a thickness of 108 feet at the site of the 1946 ice margin. At this same place in 1913 the thickness was nearly 500 feet, and the average thickness of the glacier over the area from which it has since disappeared was at least 300 feet. The average thickness of Grinnell Glacier in 1937 at the site of the 1946 ice front was 73 feet. The surface of the entire glacier was lowered 56 feet during that nine-year period. This means that each year the glacier was reduced in volume by an amount of ice equivalent to a cube 450 feet high. GRINNELL GLACIER AS IT LOOKED PRIOR TO 1926 WHEN THE LOWER AND UPPER SEGMENTS WERE STILL CONNECTED. At the northern terminus of Grinnell Glacier, which is bordered by a small marginal lake, a large section of the ice front fell into the water on or about August 14, 1946, completely filling it with icebergs. This event, although witnessed by no one, must have been comparable to The volume of Grinnell Glacier was reduced by about one-third from September 1937 to September 1946. Several other glaciers have exhibited a more phenomenal shrinkage than Sperry or Grinnell. The topographic map of Glacier National Park, prepared in 1900-1902, shows several comparatively large glaciers such as Agassiz, Blackfoot and Harrison. Their shrinkage has been so pronounced that today Agassiz has virtually disappeared and the other two are pitifully small remnants, probably less than one-fifth the size they had been when originally mapped. Since 1945, because of above-normal snowfall and subnormal temperatures, glacier shrinkage has slowed down appreciably, coming virtually to a standstill in 1950; and in 1951, for the first time since glacier changes have been recorded in the Park, Grinnell Glacier increased slightly in volume. This was also reflected by a readvance of the front. Although no measurements were made in 1951 on other Park glaciers some of them certainly made similar readvances. Thus the climatic conditions which caused glaciers to shrink for fifty or more years seem to have been replaced by conditions more favorable to the glaciers. Time alone will tell whether the new conditions are temporary or mark the beginning of a long cycle of wetter and cooler climate. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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