Figure1 (272Kb JPG)Mount Rainier is a young volcano whose slopes are undergoing rapid change by a variety of geologic processes, including debris flows. Debris flows are churning masses of water, rock and mud that travel rapidly down the volcano's steep, glacially carved valleys, leaving in their wake splintered trees, picnic sites buried in mud, and damaged roads. Debris flows typically contain as much as 65 to 70 percent rock and soil by volume and have the appearance of wet concrete. At Mount Rainier National Park, these flows invariably begin in remote areas nearly inaccessible to people, but may move rapidly downstream into areas frequented by visitors.
The smallest, but most frequent, debris flows at Mount Rainier begin as glacial outburst floods, also called by the Icelandic term "j”kulhlaup" (pronounced "yo-kul-h-loip"). Outburst floods at Mount Rainier form from sudden release of water stored at the base of glaciers or within the glacier ice. Outburst floods have been recorded from four glaciers on Mount Rainier: the Nisqually, Kautz, South Tahoma, and Winthrop glaciers. From 1986 through 1992, South Tahoma Glacier released a total of 15 outburst floods, including at least one every year. These outburst floods from South Tahoma Glacier occurred during periods of unusually hot or rainy weather in summer or early autumn, and were apparently caused by rapid input of meltwater or rainwater to the base of the glacier. The exact timing of such outburst floods is unpredictable, however.
Outburst floods become debris flows by incorporating large quantities of sediment from valley floors and walls, often by triggering landslides that mix with the flood waters. The transformation from water flood to debris flow occurs in areas where streams have eroded glacially derived sediments and sediment-rich, stagnant glacier ice that was stranded in valleys as glaciers thinned and retreated earlier in this century. As the stagnant ice melts over the next several decades, it will release its charge of sediment into the stream valleys. That sediment will potentially be incorporated into more debris flows if it is mobilized by outburst floods.
Glacier-generated debris flows at Mt. Rainier travel downstream at speeds of 5-10 meters per second (10-20 miles per hour) or more. People who have witnessed them report that only 1-2 minutes may pass between the time the roaring sound of an approaching debris flow is heard and the time the flow rushes past. These flows typically have steep, bouldery snouts--up to 10-20 meters (30-60 feet) high in the most constricted parts of a stream valley--followed by a churning mass of mud, rock, and vegetation. Their deafening noise is often accompanied by strong local wind, thick dust clouds, and violent ground shaking.
Debris flows usually follow stream channels and construct their own levees as they move, but their exact paths are unpredictable. As a debris flow moves downstream from Mount Rainier's steep flanks onto relatively gentle slopes, the flow's bouldery snout may clog the stream channel; the moving mass behind the snout may then overtop the banks and cut a new channel, perhaps through forest or across trails and roads. Debris flows at Mount Rainier typically come to rest after perhaps 30 minutes to an hour, leaving muddy, bouldery deposits from which muddy water drains for a period of a few hours to a few days. Near the valley mouth, however, the only hint of the havoc upstream might be a small increase in water level and a change in the stream's color to a murky brown or gray.
The largest debris flows at Mount Rainier are unrelated to glacial outburst floods. Several times during the last 6000 years, debris flows enormously larger than any caused by outburst floods were triggered by huge rock avalanches and travelled far beyond the park boundaries. See the reports by Crandell and by Scott and others (cited below) for discussion of potential hazards of these giant debris flows.
Visitors can see effects of recent glacier-generated debris flows at the following sites:
KAUTZ CREEK--The largest debris flow since the establishment of the park occurred October 2-3, 1947, when heavy rains apparently triggered an outburst flood from Kautz Glacier. The flood passed over the lowest part of the glacier, eroding a gorge through the ice, then mobilized sediment and transformed into a debris flow as it continued downvalley. Nine kilometers (5.5 miles) downstream from the glacier, the Nisqually-Longmire Road (equivalent to Highway 706 west of the park entrance) was buried by 9 meters (28 feet) of mud and debris. About 40 million cubic meters (50 million cubic yards) of sediment were moved, including boulders up to 4 meters (13 feet) in diameter. Although Kautz Creek has subsequently cut down through those deposits, visitors to the park will notice that they are driving uphill as they approach the creek. To observe deposits of the 1947 event, along with upright dead trees partly buried by those deposits, stop at the parking lot on the east side of Kautz Creek. Smaller debris flows have moved along Kautz Creek in 1961, 1985, 1986, and perhaps at other times.
From Longmire, hike the Wonderland Trail about 3 kilometers (2 miles) to where it crosses Kautz Creek. Boulders strewn across the valley there were deposited by the 1947 debris flow. Note the splintered trees that lined a former stream channel, and trees that lie buried horizontally in older debris-flow deposits. Above the boulder-strewn region and amid the forest lie moss-covered logs downed by previous debris flows.
NISQUALLY RIVER--Debris flows triggered by outburst floods from Nisqually Glacier damaged or destroyed bridges over the Nisqually River in 1926, 1932, 1934, and 1955. The present bridge has not been damaged by subsequent floods, which occurred in 1968, 1970, 1972, 1985, and perhaps on other occasions. At the Glacier Bridge over the Nisqually River, observe the boulder berms constructed by debris flows in the 1930's and 1950's. Twisted reinforcing bar and weathered concrete are all that remain of the 1930's-vintage bridge foundation about 100 meters (100 yards) upstream.
TAHOMA CREEK--At least 23 debris flows triggered by outburst floods from South Tahoma Glacier have moved down Tahoma Creek since 1967. These flows have carved a gorge as much as 40 meters (130 feet) deep into sediments and stagnant ice below the terminus of South Tahoma Glacier. The hazard of potential debris flows has prompted the National Park Service to close the Westside Road to visitors' automobiles at a point about 4 kilometers (2.5 miles) from the junction with the Nisqually-Longmire Road. Bouldery debris-flow deposits have buried the Westside Road about 600 meters (660 yards) farther upstream, near the confluence of Tahoma Creek and Fish Creek, on several occasions since 1988. In the vicinity of a former picnic area, about 1.6 kilometers (1 mile) beyond the road closure, bouldery deposits have accumulated at the extremely rapid rate of nearly 0.5 meters per year (1.5 feet per year) since 1988. About 1 kilometer (0.6 miles) of the Tahoma Creek hiking trail, which began at the picnic area, has been obliterated.
South Tahoma Glacier and the uppermost reach of Tahoma Creek, including the gorge eroded by recent debris flows, are visible from Mirror Lakes, near Indian Henry's Hunting Ground. The lowermost part of the gorge is crossed by the Wonderland Trail suspension bridge at a point about 3 kilometers (2 miles) upstream of the destroyed picnic area. Erosion by passing debris flows has deepened the gorge beneath the bridge from 10 meters (30 feet) to 25 meters (80 feet) since 1986.
FURTHER READING
Crandell, D.R., 1969, The geologic story of Mount Rainier: U.S. Geological Survey Bulletin 1292, 43 p. -----1971, Postglacial lahars from Mount Rainier Volcano, Washington: U.S. Geological Survey Professional Paper 677, 75 p. Driedger, C.L., 1986, A visitor's guide to Mount Rainier glaciers: Pacific Northwest National Parks and Forests Association, 80 p. Driedger, C.L., and Fountain, A.G., 1989, Glacier outburst floods at Mount Rainier, Washington State, U.S.A.: Annals of Glaciology, v. 13, p. 51-55. Scott, K.M., Pringle, P.T., and Vallance, J.W., 1992, Sedimentology, behavior and hazards of debris flows at Mount Rainier, Washington: U.S. Geological Survey Open-file Report 90- 385, 106 p.
FIGURE CAPTIONS
Figure 1: Schematic diagram of major glaciers on Mount Rainier.
Figure 2: Debris flow at Tahoma Creek, July 26, 1988. Photograph by G.G. Parker.