Period Recession Total recession Average annual
since 1938 recession
1938-45 351 351 50
1945-50 177 528 35
1950-60 244 792 24
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
reason—Grinnell Glacier is on the eastern slope of the Continental
Divide whereas Sperry Glacier is on the western slope.
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.
[Illustration: 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
many of the icefalls which occur at the fronts of the large glaciers
along the southeast coast of Alaska.
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.