Recession studies were initiated at the terminus of the South Tahoma Glacier on October 1, 1931, when a large rock, immediately in front of the ice face, was marked. This rock, at that time, was 140 feet from the ice at the point from which the main stream (Tahoma Creek) emerged.
The terminus in question is actually the combined mass of the lower part of the South Tahoma Glacier and a portion of the Tahoma Glacier, these two merging below Glacier Island. A glance at the map of the park (see page 153) will reveal the fact that the South Tahoma Glacier may be considered as the logical occupant of the canyon on the south and east sides of Glacier Island. The ice on the west side will be noted to be a connecting link uniting the Tahoma Glacier, from which the South Puyallup River arises, and the South Tahoma Glacier, which gives rise to Tahoma Creek. Eventually these two bodies of ice will separate and each will retreat up its own canyon. This may complicate recession measurements in the future by necessitating a decision as to which ice stream to follow. However, when this decision becomes necessary recession measurements will be continued on the more important of the two glaciers at that time, or, if desirable, the retreat of both may be noted annually.
As the terminus of this glacier shows considerable variation from year to year, it was deemed advisable, on September 7, 1934, to establish a second point at a different place for use in future measurements. This was done in anticipation of a change in the stream which would cause it to emerge from a different place in the ice face. The second point, termed Point B, was 142 feet from the ice in 1934. By 1935 the anticipated change in the stream course had taken place, and the point from which the stream emerged in former years was entirely covered with debris. In 1936 and 1937 a greater amount of debris had accumulated and this point on the ice has, since 1935, been very poorly defined. On the other hand the main stream has continued to emerge from the same place in the ice since 1935. In view of these facts the measurements made from Point A, in 1932, 1933, and 1934, may be considered as representative of the recession of this glacier, while, since 1934, the measurements made from Point B are more typical.
On September 25, 1936, a third point was established in order to determine the annual recession at another ice face that had developed some distance from Point A and Point B. This marker was washed out during the following year. However Point C, as it is known, was re-established in the fall of 1937.
Tabulation of recession data.
The above tabulation indicates an average annual retreat of the South Tahoma Glacier of 50 feet (from Point A) and 30.67 feet (from Point B). However, if the point from which the main stream emerges is arbitrarily considered as the true terminus (see measurements made from Point A in 1932, 1933, and 1934, and, from Point B in 1935, 1936, and 1937), the average annual recession of the South Tahoma Glacier is 35.83 feet.
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