Geological Survey Professional Paper 387-A Botanical Evidence of the Modern History of Nisqually Glacier Washington

Nisqually Glacier, on the south slope of Mount Rainier, was chosen as the initial subject for study because of the large volume of data on its terminal positions during the last 60 years (Bender and Haines, 1955, p. 275—280). Also, it is the most accessible glacier and is part of the largest single-peak system in the conterminous United States. The recorded positions of the terminus together with the ages of trees at these positions provide data for use in interpreting tree ages as a means of dating the ice positions marked by moraines deposited before detailed observations began. In addition, a reconnaissance was made of both Emmons and Tahoma Glaciers to determine whether botanical evidence shows their history to be similar to that of Nisqually Glacier.

Nisqually, Emmons, and Tahoma Glaciers are valley glaciers that radiate from the central ice cap of Mount Rainier. Nisqually Glacier extends down the south side of the mountain, Emmons down the northeast side, and Tahoma down the southwest side (fig. 1). The present terminus of Nisqually Glacier is at an altitude of about 4,500 feet, the terminus of Emmons Glacier is at about 5,300 feet, and that of Tahoma Glacier is somewhat higher, possibly at about 5,600 feet. The valleys are U-shaped with steep sides that are nearly vertical at several places. At other places slopes are so steep that perennial rock slides and snow avalanches are common. Trees grow in profusion where the valley slopes are gentler.

FIGURE 1.—Index map of Mount Rainier National Park, Wash. (click on image for an enlargement in a new window)

The flood plains immediately downvalley from existing glaciers are largely the products of streams that now occupy the valleys. Nisqually River emerges from an ice cave in the terminus of Nisqually Glacier. White River originates at a few points along the broader terminus of Emmons Glacier. South Puyallup River, which flows from Tahoma Glacier, was not examined in much detail during the short reconnaissance. Nisqually and White Rivers meander over boulder strewn flood plains built up largely from debris deposited by glaciers, floods, and mudflows. Floods, like the one of October 25, 1955 on Nisqually River within the park boundary, consist of surges of water released at the snout of Nisqually Glacier following exceptionally heavy rain. This flood roared down the steep valley at a high velocity carrying a large amount of bedload material that included boulders more than 6 feet in diameter. The highway bridge, a short distance below the glacier was swept away, as were previous bridges at the same location. The volume of water over the flood plain was so great for about 4 miles downstream from the glacier that virtually all vegetation was either buried or swept away in this reach.

For the greater part of the summer season, however, the main streams and small tributaries entering below the glaciers' termini derive their water from melting snow and ice. As would be expected the rate of flow fluctuates diurnally in direct response to the heat available for melting and to the area of exposed ice and snow. Owing to the steep longitudinal gradients of the valleys, much energy is available in melt water for the movement of flood-plain gravel and boulders. These streams at all times, and especially during periods of high flow, are working and reworking the flood-plain sediments by the constant migration of meanders from one side of the valley to the other.

As a result of lateral migration and flooding, vegetation on the flood plains stands little or no chance of growing old. Thus, in order to find both topographic and botanical evidence of past glacier positions, this study is limited to the vegetation growing on the valley sides.

Below an altitude of about 5,000 feet the vegetation on these side slopes, where not destroyed by recent floods, consists of a dense forest of coniferous trees (Brockman, 1949, p. 3—6). Above 6,500 feet, the country is treeless, and the ground is covered with alpine meadow vegetation or is bare. Between 5,000 and 6500 feet, the forest is open and parklike. Thus the modern terminal moraines and present termini of glaciers (p. A—11) are below the upper timberline. This fact permits the use of botanical methods in dating surfaces once occupied by glaciers.

The old forest on slopes above and beyond the trim line in the vicinity of Nisqually, Emmons, and Tahoma Glaciers consists primarily of western white pine, western hemlock, Douglas-fir, Pacific silver fir, noble fir, and Alaska-cedar. Trimline is a term suggested by R. P. Sharp, according to Lawrence (1948, p. 26), to designate a boundary between types of vegetation differing in age of plants, in species, or in the density of the stand marking a position from which a valley glacier has recently receded. Lodgepole pine and Engelman spruce are common on the modern moraines of Emmons Glacier. Whitebark pine grows on the moraines of Tahoma Glacier. Sitka alder forms dense thickets in places on the younger morainic surfaces where coniferous trees are small and widely scattered. Vine maple and Douglas maple grow in the forest and form dense thickets in most open areas within the forest. Black cottonwood trees are scattered along some of the streams. A complete list of comnion and Latin names is given in table 1; these names are the ones used by Little (1953).