Annal s 0/ Cla ciology 13 1989 @ International Glaciological Society GLACIER OUTBURST FLOODS AT MOUNT RAINIER, WASHINGTON STATE, U.S.A. by Carolyn L. Driedger (U.S. Geological Survey, Pacific Northwest District, Suite 600, 1201 Pa c ific Avenue , Tacoma, WA 98402, U .S.A .) and Andrew G. Fountain (U.S. Geological Survey, P.O. Box 25046, MS-412 Denver, CO 80225 , U.s.A. ) ABSTRACT During the twentieth century, glacial outburst flood s have been the most destructive natural events on Mount Rainier, a stratovolcano in the Cascade Range in Washington State, U.S.A . In the period between 1930 and 1980 numerous floods were reported from five glaciers on the mountain, most of which flowed from Nisqually, Kautz, or South Tahoma Glaciers on its southern flank. Such floods threaten lives and property because they occur without warning and quickly mobilize the loose volcanic debris into debris flows. A monitoring program was begun in 1987 which was designed to measure the dimensions and timing of outburst floods, but this has been unsuccessful because no floods have yet occurred on the monitored streams. Four floods did burst from South Tahoma Glacier that was unmonitored , but in spite of this they have been useful in providing evidence of flood storage and relea se mechanisms. All flood volumes were found to be of approximately similar orders of magnitude, of 1 x 10 5 m 3 of water, indicating that all floods probably had similar mechanisms for storage and release of water. Hydraulic pressure considerations indicate that such a large volume of flood water would be stored at the bed of the glacier rather than in isolated englacial cavities. The stepped bedrock terrain provides an ideal setting for the formation of subglacial cavities capable of storing the volumes of flood water noted. INTRODUCTiON Glacier outburst floods, j6kulhlaups, are sudden discharges from water bodies dammed within or at the margins of glaciers. At Mount Rainier, the largest dormant volcano in the central Cascade Range of Washington State , U.S.A., one or more glacier outburst floods may occur in any given year, although in some years none occur. Glacial streams on Mount Rainier erode through loose volcanic debris and sudden increases in discharge such as those that occur in outburst floods commonly mobilize this eroded material as debris flows . In the geological record for Mount Rainier, and similarly for other Cascade volcanoes such as Mount Shasta, California, U.S.A ., stratigraphic evidence of debris flows (Osterkamp and others, 1985) indicates that these flows are seldom associated with eruptive activity (Crandell, 197 I). The outburst floods at Mount Rainier apparently originate from below the glacier surface and no ice-dammed lakes have been observed on the mountain . This paper presents the results of an analysis of the historical flood records for the Mount Rainier National Park, and theorizes about the source, storage, and release of outburst water. THE SETTING Mount Rainier rises to 4400 m a.s.1. and is a strato- volcano composed of overlapping layers of lava and tephra deposits capped by glaciers (Fig. I). It last erupted in the mid-nineteenth century (Crandell , 1971), and some low-level geothermal activity persists. Twenty-five glaciers exist on Mount Rainier, with a total area of 92 km 2 of snow and ice cover (Dri edg er and Kenn a rd , 1986). The morphol ogical features of five of Mount Raini er's flood-producing glaciers, Nisqually, Kautz, South Tahoma, Carbon, and Winthrop, are s hown in Table I. Five major river s have their origins in glaciers on Mount Raini er, Kautz Creek, Tahoma Creek , and the Nisqually Riv er drain the major outburst-producing glaciers , and each has a s ummer discharge of approximately NORTH longmire • UNITED STATES OF AMERICA :2 3 MILES r---,-....... --r'----,---" KIlOMETERS Fig. I. Glaciers and permanent snowfields cover about 92 km 2 of Mount Rainier. The three glaciers producing most floods , South Tahoma, Kautz , and Nisqually , are located on the southern flank of the mountain. Some floods have emanated from Winthrop Glacier, and outburst flood activit y is also sus pected on South and North Mowich Glaciers, and Carbon Glacier. 51 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0260305500007631 Downloaded from https://www.cambridge.org/core. IP address: 5.1.65.236, on 24 Sep 2017 at 06:43:15, subject to the Cambridge Core terms of use, available at