Recession measurements on the Emmons Glacier, largest on Mount Rainier and largest in the United States (exclusive of Alaska), were initiated on September 22, 1930, when, with the cooperation of the engineering department, a metal NPS marker was cemented in a fissure on top of a large boulder. This boulder was on the south side of the White River and at the base of a cliff at that point. It can be readily recognized from a distance by a prominent zig-zag stripe of lighter colored material that runs through. its approximate center.
From this point a base line was surveyed, bearing N. 20 W. The line cut across the glacier above the point from which the main stream issued and consequently, in accordance with our original policy of considering the actual terminus of the ice as the point from which the main stream issued, the distance from the base line to the terminus was considered as nil.
The use of a base line was deemed advisable in this case inasmuch as it was apparent that the Emmons Glacier would be a difficult one upon which to measure the recession with any degree of accuracy by any other method. No definite point can really be taken to represent the actual terminus. The ice, at the end of the glacier, is wide and irregular in form. In addition it possesses a heavy covering of morainal material that promotes very uneven melting and further complicates matters pertaining to the determination of the end of the ice. However, in later years, when boulders of sufficient size and stability presented themselves, they were marked so that check data could be taken from them. Few of the boulders so marked were able to resist the flood periods of the White River until about 1936 and consequently most recession data were derived from measurements made from the ice face to points upon the base line.
Because of the fact that the retreat of this glacier presents numerous problems, caused by the above mentioned features characteristic of the terminus, which are reflected in the results of the recession computations, a description of conditions encountered at the terminus of the Emmons Glacier each year, is necessary. In the fall of 1931 it was obvious that the recession for the year 1930-31, as well as the year following, would have little significance if considered individually. At this time the ice face presented a very uneven appearance. A deep and very narrow cleft had formed in the vicinity of the stream with lobes of ice extending downstream on both sides of the White River, almost to the base line. This was undoubtedly due to the protection offered by the covering of morainal debris which was quite heavy upon these lobes. The presence of the stream, with other associated factors, may account for the more rapid melting at that point. At any rate, in following the pre-determined decision to regard the point in the ice from which the main stream emerged as the true terminus, a recession of 142 feet was recorded. While this may appear as a discrepancy, a more exact understanding of the rate of retreat of this glacier in future years was obtained by adhering to our original definition of the true terminus. Irregularities such as this one were absorbed in the computation of the average annual recession - a figure which presents the retreat of the Emmons Glacier in its true light.
In the fall of 1932 it was found that extreme changes had taken place at the terminus of the Emmons Glacier since its recession was measured the year previous. The stream issued, not from a point in the ice identical to its position of the year previous, but from the side of the left lobe of ice. Measurements from this point to the base line indicated a recession of but 2-1/2 feet for the year 1931-32. Supplementary measurements from the base line to the point on the ice from which the stream issued in the previous fall indicated a retreat of 19 feet.
When recession was tabulated in 1933 we found that the stream had changed once more and was emerging from the ice at a position similar to that in 1931. Measurements made from this point to the base line indicated a recession of 48-1/2 feet for the year 1932-33. Supplementary measurements were taken also from the point on the left lobe of ice, relatively the position from which the stream emerged in the previous fall, and in this case a retreat of 32 feet was recorded.
Another change in the stream was noted in the fall of 1934. Again it emerged from the left lobe of the ice at a position relatively the same as that in 1932. Measurements made from this point on the glacier to the line of site indicated an apparent advance of 1-1/2 feet. However this apparent "advance" was due, not to the downward movement of the glacier but to the change in the position of the stream and factors related to it. Measurements made from the base line to a point on the ice face relatively the same as the position from which the stream emerged in the fall of 1933 indicated a retreat of 132 feet.
No change was observed in the position of the stream when recession was measured in 1935. Measurements from the base line to where the stream emerged indicated a recession, for the year 1934-35, of 67 feet. Supplementary measurements from the base line to a point on the ice face, similar to that which served in the 1934 supplementary measurements, indicated a retreat of 30 feet.
The recession of the Emmons Glacier for the year 1935-36 was very great. Although no change had occurred in the stream, the lobe from which it issued had lost considerable ground and a recession of 193 feet was recorded from measurements made from the point from which the stream emerged to the base line. Supplementary measurements, taken from a point similar to that of the year previous, also indicated a considerable retreat; a 100 foot recession being recorded in in that case.
Several large rocks; were marked, for use in future recession measurements, in the fall of 1936. These were the first of the supplementary points established to be found intact the following year.
In 1937 measurements made from the base line to the point where the stream emerged indicated. a recession of but 10 feet for 1936-37. Supplementary measurements from a point on the ice face, similar to that used in 1936, to one of the rocks established in the previous year indicated a recession of 16 feet.
Tabulation of recession data.
The above tabulation may be interpreted in four different ways. Computed on the basis of the recession figures which consider the point on the ice from which the stream emerges as the true terminus, the average recession of this glacier is 65.93 feet per year. On the basis of supplementary measurements, which were sometimes dependent upon poorly defined points on the glacier, the average recession is 54.83 feet per year, while if the recession indicated by each method is averaged for each year and the average annual recession computed from these average figures the average annual recession of the Emmons Glacier is 54.04 feet. Lastly, the average of the three results just noted indicates an average recession of 58.27 feet per year.
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