Geological Survey Professional Paper 1547 Sedimentology, Behavior, and Hazards of Debris Flows at Mount Rainier Washington

DEBRIS AVALANCHES AND THE TAHOMA LAHAR

A small debris avalanche derived from a shallow slope failure is a second type of flow that is best included in the final general category, that of the smallest but most frequent flows and avalanches. Although less frequent than glacial-outburst debris flows, several historical debris avalanches have occurred. The two largest examples traveled (1) from the Sunset Amphitheater onto the Tahoma Glacier in the early 20th century (fig. 21) and (2) from Little Tahoma Peak onto and beyond the Emmons Glacier in 1963 (fig. 4). Smaller debris avalanches fell onto the Winthrop Glacier in 1974 (Frank, 1985, p. 138), onto the Cowlitz Glacier in 1975 (Frank, 1985, p. 138—139), and onto the Winthrop Glacier in 1989.

Figure 21. Debris avalanche on the surface of the Tahoma Glacier, at the head of the South Puyallup River. Note the lighter, hydrothermally altered debris originating in the Sunset Amphitheater, contrasting with the darker morainal sediment, foreground, on and lateral to the Tahoma Glacier. The origin of this flow was similar to that of the Case III flow, the Tahoma Lahar.

The most recently documented debris avalanche originated August 16, 1989, from upper Curtis Ridge, as did the 1974 flow, and descended from 3,600 to 3,700 m (11,800 to 12,100 ft) to 1,950 m (6,400 ft) in altitude. Runout occurred over a horizontal distance of 4.1 km. The flow deposits were noncohesive in texture; deposit thickness is surprisingly thin, as are the thicknesses of the deposits of the debris avalanches on the Tahoma and Emmons Glaciers. The total volume of the flow is probably in the range of 0.1 to 0.5 million m³, based on an average thickness of about 20 cm. The main seismic record of the avalanche consisted of complex, high-amplitude signals at 1706, 1714, 1715, and 1721 hours UTC on August 16, with the 1721 event lasting for 9 minutes (Norris, in press). The length of the signal is too great to reflect the velocity of the flow and probably reflects either continuing failure or the continued rolling of house-size boulders.

Multiple rock avalanches originated in pre-Rainier rocks that form the ridge known as Mount Wow and inundated the bottom of the Tahoma Creek valley with several lobes of debris, and another extended from the east end of the ridge of pre-Rainier terrane known as Mother Mountain almost to the Carbon River. The Mount Wow avalanches might have been triggered by the April 13, 1949, Olympia earthquake (M 7.1), as suggested by the decomposition stage of killed trees (decay sequence in Franklin and others, 1981).

These avalanches warrant serious attention because of their extremely rapid, catastrophic emplacement and their known frequency and hazard at Mount Rainier and many other stratovolcanocs. Therefore, we focus on one large avalanche-derived flow that is typical of several young flows known at Mount Rainier. The Tahoma Lahar is the case history most suitable for planning within Park boundaries. It is distinct from the large sector-collapse debris avalanches and landslides but like those flows, also mobilized to a downstream lahar. Based on the record of all known flows, the smaller avalanches will not pose a large hazard outside the Park.

The Tahoma Lahar is interpreted as a variably disaggregated debris avalanche mainly transformed to a lahar (tables 2, 3). Its deposits form a distinctive unit in the Tahoma Creek watershed; they are mainly cohesive but are locally noncohesive in some lateral exposures. Like the Paradise Lahar, the unit is characterized by a yellow color and hydrothermally stained clasts. It is post-set W in age and thus much younger than the Paradise Lahar. The Tahoma Lahar, named here, is 0.5 to 2.0 m thick on valley-side slopes; more than 20 m thick in cross section near the base of Neoglacial deposits about 0.5 km upstream from the Wonderland Trail bridge across Tahoma Creek; and at least 4.3 m thick in the valley bottom as seen in exposures only 4.8 km upstream from the Highway 706 bridge. Most deposition probably occurred in this area, filling the expanding lower valley of Tahoma Creek, where deposits are now covered by those of glacial-outburst floods. Clay content is variable but is characteristic of cohesive debris flow in most exposures. Because of further disaggregation of megaclasts in the flow and a more clayey recession phase (as we also propose for the Osceola Mudflow and other cohesive lahars), clay content is highest downstream where deposits were seen locally near the center of the valley.

The flow deposits overlie set W and compose the uppermost unit of most stratigraphic sections downstream from the Neoglacial terminus (about 100 m upstream from the trail bridge), as reported by Crandell (1971, p. 58) and verified in new exposures. The older Round Pass Mudflow supports trees as much as 700 to 800 years old, some of which were killed by the Tahoma Lahar and others which were killed at least 100 years ago by flows from Mount Wow. Significant attenuation of the Tahoma Lahar began as the flow left the confined channel upstream from the former picnic area. Although deposition was pronounced between the ex-picnic area and a point about 3 km downstream, the distal configuration is estimated (pl. 1) based on levels revealed by peak-stage deposits on valley-side slopes where they are not covered by younger deposits.

The stratigraphic relation of the Tahoma Lahar to Neoglacial morainal deposits and the estimate of tree ages on the lahar surface by Crandell (1971, p. 58) establish the time of the flow as shortly following the A.D. 1480 deposition of layer Wn, or about 400—500 years ago. Radiocarbon dates are variable, however. The outermost 25 rings of a tree that grew on the Round Pass Mudflow and that possibly was killed by the Tahoma Lahar provided an age of 560±75 radiocarbon years. A radiocarbon date from the outermost wood from an apparently similar tree near the site of the picnic ground was 200±50 years. That date is in a time interval for which the correlation between radiocarbon and calendar years is poor, and it could correlate with a calendar age of A.D. 1665 to 1955 (Stuiver and Becker, 1986). No volcanic activity is recorded from Mount Rainier near the probable time of the lahar.

The coloring of the surficial unit, which might be taken to indicate soil formation, instead reflects an origin as a mobilized debris avalanche of hydrothermally altered rock from the Sunset Amphitheater. The source is probably a different sector of the Sunset Amphitheater than that yielding the modern clay-rich debris avalanche on the Tahoma Glacier (fig. 21; see Crandell, 1971, p. 17). The trend of the distinctively colored Tahoma deposits within the Neoglacial moraine (incised by the branch of Tahoma Creek draining South Tahoma Glacier) suggests an origin above the Tahoma Glacier rather than from the South Tahoma Glacier. A debris avalanche above the Tahoma Glacier, however, should have created a correlative lahar in the South Puyallup River downstream from the Tahoma Glacier, and no such unit has yet been indisputably identified. A highly likely correlative, however, is unit 4 of Crandell's measured section 8 (1971, p. 57), which is younger than the Electron Mudflow, as is the Tahoma Lahar, and is texturally similar to the Tahoma Lahar.

The Tahoma Lahar locally has a hummocky surface. Megaclasts form mounds in forested backwater areas of the fanhead downstream from the former picnic ground. The megaclasts are similar in composition (but with less clay) and color of alteration products to those in the modern debris avalanche on the surface of Tahoma Glacier. Many mound-forming megaclasts were eroded or buried by the glacial-outburst flood and debris flow of October 15, 1988. The strength of the Tahoma Lahar is indicated by a lake dammed by the lateral levee of the peak flow about 0.5 km upstream from the trail head. The lake had a maximum depth of about 2 m, a width of 30 m, and a length of approximately 100 m in 1989.

The peak flow of the Tahoma Lahar probably was too cohesive for a runout phase to have formed. A lahar-runout deposit of similar age occurs in the Nisqually River near National (fig. 9, tables 3, 4). That deposit contains wood with an age of 410±75 radiocarbon years, corresponding to a true age of about 540 years (before 1994). It is more likely that the debris avalanche did not transform beyond a lahar and that the runout flow is a separate event.