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 the
year by year changes in the ice front of each of the several glaciers.
From 1937 to 1939, inclusive, the program was expanded to include the
detailed mapping of Grinnell, Sperry, and Jackson Glaciers to serve as a
basis for comparisons in future years. Aerial photographs were obtained
of all the known Park glaciers in 1950 and 1952 and again in 1960. Maps
have been compiled and published of the Grinnell and Sperry Glaciers
based on the 1950 and 1960 aerial photography. The 1950 and 1960 maps of
each glacier are shown on one sheet for convenience in comparison.
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:
GRINNELL GLACIER
Year Area Remarks
(Acres)
1901 525 From Chief Mountain topographic quadrangle map.
1937 384 From mapping by J. L. Dyson and Gibson of lower portion
of glacier plus area of upper glacier (56 acres), as
shown on 1950 USGS map.
1946 336 As above.
1950 328 From USGS map compiled from aerial photography.
1960 315 As above.
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:
Period Recession during Total Recession Average annual
period (feet) since 1937 (feet) recession (feet)
1937-45 270 270 34
1945-50 75 345 15
1950-60 85 430 8
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:
SPERRY GLACIER
Year Area Remarks
(Acres)
1901 810 From Chief Mountain topographic quadrangle map.
1938 390 From mapping by J. L. Dyson and Gibson.
1946 330 From mapping by J. L. Dyson.
1950 305 From USGS map compiled from aerial photography.
1960 287 From USGS map compiled from aerial photography.