| Period | Recession during period (feet) | Total Recession since 1937 (feet) | Average annual 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 (Acres) | Remarks |
| 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. |
Recession, in feet, of central half-mile section of terminus
| Period | Recession | Total recession since 1938 | Average annual 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.