Geological Survey Professional Paper 387—B Recent Activity of Glaciers of Mount Rainier, Washington

The deposition of gravel and boulders by glaciers and by glacial melt water, the occurrence of slides and floods in the last few decades, and the record of similar events in the last 500 years testify to the marked changes that have occurred and are occurring in the landscape. When one views Mount Rainier from a distance, he really observes each detail for a mere instant. Because of the impressive mass of the mountain, he may depart believing that the mountain will remain in its present form forever. If, on the other hand, he could photograph the scene many times a year for 500 years and project the film on a movie screen, he would see rapid advance and melting of glaciers, frequent floods, avalanches, slides, and rapid shifting of stream channels.

Landscape features, such as hills and gullies, are silent evidence of these events, and the presence or absence of trees on some of them permit a close estimate of their age. Because trees start to grow and will survive after a landscape feature at low altitude becomes stable, the age of the oldest trees closely approaches the age of the feature. The absence of trees and other plants on a landscape feature containing fine, loose material probably means that the feature is presently active as, for example, a rock slide. If only shrubs are present on steep slopes well below timberline, trees are probably prevented from growing by frequent snow avalanches.

HYDROLOGIC INFERENCES

In initiating this study of the recent fluctuations in the positions of the glaciers on Mount Rainier, we had as a primary objective "* * * determining where modern water-supply data fit into the long-term pattern of fluctuating water supplies * * *" (Sigafoos and Hendricks, 1961, p. Al). We further stated, "The hypothesis is simple: "Glaciers advance and retreat in a manner somehow related to climate; therefore, if something is known of a glacier's movement in the past, some kind of crude inferences about the climate existing at the time of movement may be drawn." It now appears that even this cautious statement was overoptimistic.

Through the collection of additional data since the initial report, it became apparent that no evidence exists to permit the dating of glacial advances. The data show only when glaciers started to retreat from terminal and lateral moraines. Nothing is known about how far they retreated following the formation of specific moraines. They could have retreated several thousands of feet or they could have retreated only a few tens of feet. Nothing can be inferred, therefore on the magnitude of climatic and hydrologic changes that preceded or followed moraine building.

Meier (1965, p. 804-805), in summarizing existing knowledge of the relation between glacier behavior and climate, concludes:

Glacier variations may reflect or indicate variations in climate, but the connection is indirect and complex. The general meteorologic environment controls the precipitation of snow, but local influences, caused largely by topography, may greatly modify the resulting accumulation on a glacier. Slight changes in seasonal temperature or precipitation distribution may affect accumulation totals in a glacier * * * Moraines and outwash features offer physical evidence of past glacier fluctuations, even though the exact meaning of some of these deposits is unclear. Botanical evidence of glacier variations can he especially useful. However, at the present stage of knowledge one cannot trace back from glacier variations to changes in climate, except in a gross, hypothetical way.

Meier (1965, p. 800) further stated:

It should he possible in the future to determine response characteristics and delay times for different types of glaciers, up to and including ice sheets of continental dimensions It is well known that contemporary glaciers do not, in general, behave synchronously. This can result from variations in net budget resulting from different local meteorological conditions or it can result from variations in dynamic response. The delays in dynamic response for large glaciers extend into hundreds or thousands of years. It is possible that some apparently different ice-sheet advances, which have been assumed to have general chronological significance, could just be different dynamic responses to the same climatic event.

From these considerations it is clear that some risk exists in developing an overall chronology for Mount Rainier glaciers by piecing together the record of movements of all the glaciers, because of the possible anomalies in responses of adjacent glaciers. Even if one could safely do this, the present knowledge of time delays in dynamic response of individual glaciers permits only the grossest inferences on the fluctuations in climate that produced the observed sequence of glacial responses. Thus we leave climatic inferences to others, with the hope that the data recorded here may help some investigator of the future unravel the mysteries of past climatic change as indicated by these glacial histories.

If one draws hydrologic inferences from the glacial record, it is clear that, although precipitation may have varied in the past, changes in the regional water supply may also have resulted from changes in storage of water in ice. When glaciers were extended, more regional water was frozen in ice; when and if they were smaller, less water was frozen. Again, because little detail is known relating variations in glacier response to climate, we can only speculate upon the effects of greater or lesser storage of water as ice in glaciers upon discharge of water downstream.

Changes in climate and the concurrent storage of water in ice might have been in opposition in their effects upon downstream water supplies, or they might have been complementary. More water was taken from the environment and stored as ice when glaciers were larger than now so that one might expect smaller flow in streams merely because of an increase of water in storage. However, the expansion of glaciers might well be the result of an interval of greater precipitation during which there was also increased flow in streams because of increased runoff from rain and snowmelt. The two phenomena, increased flow because of increased melt-water discharge and decreased flow because more regional water was frozen in ice, tend to offset one another. Flow in streams below extended glaciers in the recent past, then, could have been virtually the same as it is now. If, on the other hand, glaciers expanded because of a general lowering of temperatures, more water would be held not only in glacier ice but in longerlasting snow cover. The effect then would be decreased flow below glaciers, with less water available for use.

Although the evidence around the mountain indicates a sequence of recession from moraines, its climatic significance and the effects upon water supply are not known. Recessions started on the fol-lowing approximate A.D. dates: 1525, 1550, 1625-60, 1715, 1730, 1765, 1820-60, 1875, and 1910. More water may have drained from receding glaciers than from the advancing glaciers of the present; certainly water drained through different channels below some glaciers than it does today.

Discharge of melt water in some valleys below some glaciers is believed to have been greater (Tahoma, Winthrop, and Emmons Glaciers, p. B7, B12, and B13) when glaciers were larger than they are now. It is not known if the total quantities of melt water from each of these glaciers were greater than it is now, because discharge could have been less in some valleys than in them today, or it could have been nonexistent. These periods when discharge was greater in some valleys occurred when glaciers were receding and melting faster than they do now. If glacier recession resulted from lowered precipitation, total discharge could have been the same as or less than it is now, and valleys that earlier carried more water may merely represent a physical shift in the site of melt-water discharge.

SUMMARY OF MORAINE AGES

During the early part of the last century, Mount Rainier presented a different face when viewed from afar by an Indian than it does today when viewed by visitors. Eight major glaciers were considerably farther downvalley 125 to 130 years ago than they are now. The upper slopes, heavily weighted with ice, exposed little bedrock to view, and valleys were filled with long tongues of ice protruding between the dark, forested slopes.

All glaciers studied receded from this maximum stand in the last century. For Nisqually, Van Trump (Crandell and Miller, 1964), Tahoma, and Puyallup Glaciers, this stand represented the farthest downvalley advance of the last 10,000 years. The others—South Tahoma, Carbon, Winthrop, Emmons, Ohanapecosh, and Cowlitz Glaciers—also were far advanced 125 to 130 years ago but earlier, within the last 750 years, were even larger and more extended (table 10). A lateral moraine of Carbon Glacier was formed before A.D. 1217 (Crandell and Miller, 1964). These 125- to 750-year-old moraines were formed during the Garda Stade of the Winthrop Creek Glaciation, recognized by drift laid down since deposition of pumice layer C (Crandell and Miller, 1964). Some of these moraines are separated into old Garda drift overlain by pumice layer W (deposited about 450 years ago) and young Garda drift not overlain by layer W (Crandell, 1969, p. 30).

TABLE 10.—Ages of glacial or pyroclastic deposits at Mount Rainier, Washington

[E., end; L, lateral]

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End and lateral moraines, from which eight glaciers started to recede between 1830 and 1850, indicate a consistent pattern of recession around the mountain for this period of time. The older moraines, however, do not show this correlation. This may be the result of insufficient data or may indicate different recession patterns of the glaciers. The older moraines generally are small segments that remain after the more extensive parts were destroyed by later glacier advances, by landslides, or by stream erosion. Some glaciers, such as Emmons, Carbon, and both Tahoma Glaciers behaved differently at different times. Earlier they must have been thicker and not as long as during later times (Sigafoos and Hendricks, 1961, p. A7-A8). In spite of the vagaries of Mount Rainier glaciers, their dynamic nature is clear. That we are able with considerable accuracy to date the modern moraines of Mount Rainier by using botanical evidence is a step toward solving the problems of dynamic glacial processes.