DESCRIPTION:
Osceola Mudflow, Mount Rainier, Washington

[Map,20K,InlineGIF]
Extent of the Osceola and Electron Mudflows
-- Modified from: Crandell, et.al., 1979

The largest lahar originating at Mount Rainier in the last 10,000 years is known as the Osceola Mudflow. This cohesive lahar, which occurred about 5600 years ago, was at least 10 times larger than any other known lahar from Mount Rainier. It was the product of a large debris avalanche composed mostly of hydrothermally-altered material, and may have been triggered as magma forced its way into the volcano. Osceola deposits cover an area of about 550 square kilometers (212 square miles) in the Puget Sound lowland, extending at least as far as the Seattle suburb of Kent, and to Commencement Bay, now the site of the Port of Tacoma. The communities of Orting, Buckley, Sumner, Puyallup, Enumclaw, and Auburn are also wholly or partly located on top of deposits of the Osceola Mudflow and, in some cases, of more recent debris flows as well.

Mount Rainier, the highest and third most voluminous volcano in the Cascade Range, is potentially the most dangerous volcano in the range because of the large population living around its lowland drainages. These areas are at risk because of the mountain's great relief and the huge area and volume of ice and snow on the cone (92x10^6 square meters, and 4.4x10^9 cubic meters, respectively) that could generate lahars during eruptions. In addition, large (>2x10^8 cubic meters) sector collapses of clay-rich, hydrothermally altered debris from the cone have occurred at least 3 times in the last 6,000 years (Osceola, Round Pass, and Electron mudflows. ...

Post-glacial deposits at Mount Rainier are dominated by lahars; over 60 have been identified. Although relations between Holocene tephra and flowage deposits remain speculative, at least some lahars were probably eruption induced, most notably the Paradise lahar and the Osceola Mudflow, which has been dated at 5,040 Carbon-14 years B.P., had a volume >10^9 cubic meters, and a profound geomorphic effect on the Puget Sound shoreline, over 100 kilometers from the mountain. As interpreted from well logs (neglecting minor relative sea-level changes), syn- and post-Osceola sedimentation has pushed the shoreline seaward 25 and 50 kilometers, respectively, in two Puget Sound embayments and added approximately 460 square kilometers of new land surface. ...

The Osceola Mudflow was a cohesive debris flow with a volume (3 cubic kilometers) at least 10 times that of the next largest flow in a distinct subpopulation of large, cohesive lahars. Its deposits are also the most clay-rich (average of 7 percent) in this group of lahars, indicating that the sector collapse that produced the Osceola Mudflow penetrated the hydrothermally altered core of the volcano more deeply than most such events.

The mean peak velocity of the Osceola Mudflow at the boundary of the Cascade Range and the Puget Sound Lowland was at least 20 meters per second. The relation between that velocity and the actual velocity of the flow wave (celerity) can only be estimated because of (1) uncertainties in the velocity determinations, (2) the probable similarity of a debris flow path to that of a caterpillar-tractor tread, in which material may be repeatedly recycled from, into, and back out of the high-velocity center of the flow, and (3) other factors such as characteristics of the measurement site.

The travel time of the maximum lahar to the Puget Sound Lowland is estimated to range between a value based on the estimated flow velocity, on the high side, and that based on the ratio of flow velocity and celerity of a large cohesive debris flow at Mount St. Helens, probably on the low side. The assumption of an average flow velocity of about 25 meters per second over the course from volcano to the lowland yields an equivalent but unlikely celerity of 90 kilometers per hour (56 miles per hour). Based on behavior of a similar cohesive flow at Mount St. Helens, the actual flow wave may have moved only at a rate of approximately 6 meters per second, or 22 kilometers per hour (14 miles per hour). Distances from Mount Rainier to the lowland or the nearest downstream reservoir range from 38 to 77 kilometers. Corresponding travel times are in the range of 0.4 to 3.6 hours. ...

Debris flows are distributed over time like floods. The largest ones are also the rarest. ...

By far the largest flow in the history of Mount Rainier is the Osceola Mudflow. About 5,000 years ago, a huge landslide removed 3 cubic kilometers (0.7 cubic miles) from the summit of Mount Rainier. (The landslide removed the top 600 meters (2,000 feet), leaving a summit crater. Subsequent volcanic eruptions created the modern summit cone with the crater.) The landslide penetrated highly altered rock in the core of the volcano, and the huge clay-rich mass mobilized almost immediately to a debris flow. Large blocks of the landslide form numerous mounds in lateral deposits along the White River valley before spreading of the flow over a wide area of the Puget Sound Lowland. Osceola deposits occur along trails near the White River Campground.

[Image,161K,GIF]
View of east side of Mount Rainier, showing area of failure of the Osceola Mudflow. The crater left by the flow was later filled by volcanic formation of the snow-clad summit cone. The Osceola Mudflow overran Steamboat Prow and split into branches down the White River and West Fork White River (right of photo). Landslide from Little Tahoma Peak in 1963 formed dark material on lower Emmons Glacier.

From Sunrise:   Walk to Emmons Nature Trail Overlook above White River for view of the source area of the Osceola debris flow (2 cubic kilometers) that poured down the White River and West Fork White River and spread across more than 260 square kilometers, mostly in the Puget Sound lowland, 4-70 kilometers from the volcano (Crandell and Waldron, 1956; Crandell, 1971). Logs in the Osceola give a bristlecone-corrected Carbon-14 age of about 5,700 years B.P. (Crandell, 1969, 1971).

The Osceola everywhere has a high content of clay (6-12% [Crandell, 1971]), presumably derived from hydrothermally altered rocks high on the volcano. The source area is now hidden beneath the broad upper part of the Emmons and Winthrop Glaciers and possibly includes material removed from a former summit region above 4,250 meters (Crandell, 1963b; Crandell, 1971). As Crandell and Waldron (1056) pointed out, "the very existence of this broad expanse of ice suggests the possibility that the ice occupies a depression that resulted from the explosive destruction of part of the volcano, or from eruptive action followed by partial collapse of this area." The Osceola occurs on the ridgetops above Glacier Basin and on top of Steamboat Prow (2,957 meters), so clearly it originated even higher on the volcano (Crandell and Waldron, 1956; Crandell, 1971). A remnant of the debris flow is visible from here as a terrace along Inter Creek just upstream form the White River. Conceivable the debris flow could have been generated by collapse of the altered flank of the cone without accompanying eruptive activity. Whether an eruption occurred is uncertain. Tephra layer F (Mullineaux, 1974) from Mount Rainier is clay-rich like the Osceola and ha a corrected Carbon-14 age of 5,700-5,800 years; it has been found neither above nor below the Osceola, however, perhaps owing to unfavorable wind directions. A reasonable supposition is that layer F records phreatic activity preceding or accompanying generation of the avalanche and resulting debris flow. The Osceola is 2-7 meters thick along its margins but presumably much thicker in its center. Remnants of the debris flow on the sides of the White River and West Fork White River valleys show that the thickness aw locally at least as much as 150 meters while the debris flow was moving. A three-dimensional model in the visitor center helps to visualize the geometry and enormity of the Osceola. A measure of its size is the profound effect it had on Puget Sound more than 100 kilometers from the volcano; the shoreline was displace seaward 27-50 kilometers as more than 460 square kilometers of new land was created!

The Osceola Mudflow is by far the largest mudflow of postglacial age from Mount Rainier and is one of the largest known volcanic mudflows in the world. ...

The Osceola Mudflow apparently originated in avalanches of hydrothermally altered rock form the summit of Mount Rainier. The presence of the mudflow high on the flank of the volcano at Steamboat Prow indicates that the flow came from above that point, and its distribution in Glacier Basin and the cirque north of the basin suggests that debris that descended the volcano surmounted Steamboat Prow and cascaded into Glacier Basin in a sheet or sheets many hundreds of feet thick. The clay mineralogy of the mudflow is most readily explained if the avalanches originated in an extensive mass of rock that had previously been hydrothermally altered in large part to clay. ...

The volume of the Osceola Mudflow can be estimated on the basis of the known area of distribution of the flow, if several assumptions are made concerning its total inferred area and thickness: The mudflow originally covered a total land area of at least 100 square miles, in addition to a submerged area in the Puyallup River valley where the mudflow extended into a former arm of Puget Sound. The mudflow probably extended as far south in the Puyallup River valley as the confluence of the Puyallup and Carbon Rivers, as far west as the town of Puyallup, and northward in the Green River valley to the outskirts of Kent and covered a submerged area of at least 27 square miles. The mudflow has subsequently been removed from a considerable part of its former total area by stream erosion or has been buried by younger alluvium. The mudflow ranges in thickness from a few feet to at least 200 feet. It is a reasonable assumption that the Osceola had an original average thickness of 20 feet over the land areas it covered; if it did, the mudflow would have had a volume of about 2 billion cubic yards. If the Osceola had had a comparable average thickness in the submerged areas, there would have been an additional volume of about 660 million cubic yards, making a probable total volume of a little more than half a cubic mile.

Investigation of the Osceola (Crandell and Waldron, 1956; Crandell, 1963b, Mullineaux, 1961, 1965a, b) has shown that the mudflow overlies the Vashon Drift and that the upper part of the drift contains a soil profile. This soil was formed after the Puget glacier lobe withdrew from the southeastern part of the Puget Sound lowland, about 14,000 years ago, and before the mudflow occurred. Four radiocarbon dates obtained from wood incorporated in the mudflow range from 4,700+/-250 (W-564) to 5,040+/-150 (University of Washington radiocarbon date NO.62). When corrected for variations in atmospheric Carbon-14 and a a Carbon-14 half life of 5,730 years, these ages range from 5,550 to 5,800 years; a "true" age of about 5,700 years is here arbitrarily assumed for the Osceola. ...

Debris flows are one of the most common and devastating geologic phenomena in the postglacial history of the volcano. The largest debris flows from Mount Rainier probably originated in volcanic explosions that caused large-scale avalanching of rock debris. Other debris flows were caused by such factors as heavy rainfall and rapid snowmelt, which are unrelated to volcanism; their occurrence at Mount Rainier results from the availability there of large quantities of loose rock debris on steep slopes. ...

The largest postglacial mudflow, which is estimated to have had a volume of nearly half a cubic miles, is the 5,000-year-old Osceola Mudflow (Crandell and Waldron, 1956). This clay-rich mudflow probably originated from the avalanching of rock, previously altered partly to clay by steam, from the summit and upper slopes of the volcano (Crandell, 1963a,b). Others (Fiske and others, 1963) have proposed, instead, that the mudflow was formed by "the collapse and flowing out of a thick fill of water-saturated sediments in the upper part of the White River valley." However, the source of the mudflow was above rather than within the valley of the White River, as is clearly shown by remnants of the Osceola deposit higher on the flanks of the volcano at Steamboat Prow, to the west of Winthrop Glacier, and on the ridge crests at the head of Inter Fork valley (Crandell and Waldron, 1956; Crandell, 1963a). Thus, the distribution of the mudflow on the northeast side of Mount Rainier, inferred from detailed mapping of its remnants, indicates an origin at or near the former summit. The avalanches that caused the mudflow may have resulted from one or more steam explosions.

During its movement, the Osceola Mudflow submerged the White River valley at the site of White River campground beneath at least 400 feet of mud and rock debris. It traveled 40 miles downvalley to the mountain front, then spread out in a lobate mass that covered 65 square miles of Puget Sound lowland. There it buried the sites of the present communities of Enumclaw and Buckley under as much as 70 feet of mud and probably extended at least as far northwest as the present town of Auburn. ...

It should be noted that the larger debris flows from Mount Rainier apparently did not form permanent fills in valleys to the maximum height of their remnants on the valley walls. Instead, these remnants probably mark transient flow crests, analogous to those of stream floods.

Osceola Mudflow - Crandell, 1971

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