Scientists measure winter snow accumulation and summer melt of snow and ice to analyze the response of glaciers to climate; however, it is very time-consuming and potentially a hazardous task. Consequently, alternative data, which are obtained by mapping of terminus positions and surveying of glacier surface elevations, are commonly used. At Mount Rainier, annual measurements of Nisqually Glacier's terminus position were begun in 1918 by National Park Service (NPS) personnel and are currently made by U.S. Geological Survey (USGS) personnel.
Changes in terminus position may actually be forecast by precise surveys of a glacier's surface elevation. For example, a rise in surface elevation, which reflects an increase in ice thickness, is typically followed within a few years or decades by terminus advance. The surface-elevation record at Nisqually Glacier is the lengthiest of any made in North America. The record, which was started in 1931, shows the glacier's dramatic responses to about half a century of small but significant climatic variations. These measurements of surface elevation were begun by personnel of the Tacoma City Light Co. because of their interest in water for hydroelectric power. Measurements were later done by USGS personnel and most recently by NPS personnel.
Geologists can determine the former extent of glaciers on Mount Rainier by mapping the outline of glacial deposits and by noting the position of trimlines, the distinct boundaries between older and younger forests or between forests and pioneering vegetation. Geologists determine the age of some of the deposits by noting the age of the oldest trees and lichens growing on them and the degree of weathering on boulders.
Between the 14th century and A.D. 1850, many of the glaciers on Mount Rainier advanced to their farthest extent downvalley since the last ice age. Many advances of this sort occurred worldwide during this time period known to geologists as the Little Ice Age. During the Little Ice Age, the Nisqually Glacier advanced to a position 650 feet to 800 feet downvalley from the site of the Glacier Bridge, Tahoma and South Tahoma Glaciers merged at the base of Glacier Island, and the terminus of Emmons Glacier reached within 1.2 miles of the White River Campground.
Retreat of the Little Ice Age glaciers was slow until about 1920 when retreat became more rapid. Between the height of the Little Ice Age and 1950, Mount Rainier's glaciers lost about one-quarter of their length. Beginning in 1950 and continuing through the early 1980's, however, many of the major glaciers advanced in response to relatively cooler temperatures of the mid-century. The Carbon, Cowlitz, Emmons, and Nisqually Glaciers advanced during the late 1970's and early 1980's as a result of high snowfalls during the 1960's and 1970's. Since the early-1980's and through 1992, however, many glaciers have been thinning and retreating and some advances have slowed, perhaps in response to drier conditions that have prevailed at Mount Rainier since 1977.
Cowlitz-lngraham Glacier is best seen from the upper slopes of the mountain, either from Cowlitz Rocks (above Paradise Glacier) or from the summit climbing route by way of Camp Muir. At its farthest extent perhaps more than 35,000 years ago, the Cowlitz-Ingraham Glacier terminated approximately 65 miles downvalley of the mountain near the town of Mossyrock, Washington. The Cowlitz-lngraham Glacier made a notable advance in the mid-1970's and continued to advance slowly until the mid-1980's. It is currently thinning and retreating.
Emmons Glacier, on the east slope of Mount Rainier, has a surface area of 43 square miles, the largest area of any glacier in the contiguous United States. A 0.2-mile walk Emmons Glacier. For a closer look, hike the 1-mile trail from White River Campground to the crest of the lateral moraine. In 1963, a rockfall from Little Tahoma Peak covered the lower glacier with rock debris. The debris cover insulates the ice from melting. As a result of decreased melting, the glacier advanced rapidly in the early 1980'S. That advance continues today, but at a slower rate. Ice beneath the rock debris is melting irregularly and forming a vast hummocky area.
Carbon Glacier has the greatest measured thickness (700 feet) and volume (0.2 cubic miles) of any glacier in the contiguous United States. It is best viewed via an easy 4 mile trail from Ipsut Creek Campground on the north side of Mount Rainier. The glacier has retreated less than 0.6 miles since the Little Ice Age. The glacier terminus is at a relatively low elevation and is surrounded by mature forest and shrubbery. During the advance of this heavily debris- laden glacier in the late 1970's, visitors watched vegetation being crushed by rocks rolling off the advancing terminus. Currently, the Carbon Glacier terminus is undergoing a minor retreat.
Crandell D.R., and Miller, R.D., 1974, Quaternary stratigraphy and extent of glaciation in the Mount Rainier region, Washington: U.S. Geological Survey Professional Paper 847, 59 p. Driedger, C.L., 1986, A visitors guide to mount Rainier glaciers: Longmire, Pacific Northwest National Parks and Forests Association, 80 p. Helliker, C.C., Johnson, A., and Hodge, S.M., 1983, The Nisqually Glacier, Mount Rainier, Washington, 1857-1979:A summary of long-term observations and a comprehensive bibliography: U.S. Geological Survey Open-File Report 83-541, 20 p. Hodge, S.M., 1972, The movement and sliding of the Nisqually Glacier, Mount Rainier: unpublished doctoral thesis, University of Washington, 409 p. Porter, S.C., and Denton, G.D., 1967, Chronology of neoglaciation in the North American cordillera: American Journal of Science, v. 265,p.177-210.Open-File Report 92-474/C.L.Driedger, 1993