DESCRIPTION:
Electron 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. ... At least 6 smaller debris avalanches have spawned lahars in the past 5,600 years. One of these, the Electron Mudflow, which was derived from a slope failure on the west flank of Mount Rainier about 600 years ago, has not been correlated with an eruption. The Electron Mudflow was more than 30 meters (yards) deep where it entered the Puget Sound lowland at the community of Electron. Its deposits at Orting are as much as 6 meters (yards) thick and contain remnants of an old-growth forest.

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. ...

Another clay-rich lahar, the Electron Mudflow, has been dated at 530 Carbon-14 years B.P. This lahar, which evidently began as a failure of part of the western edifice, has not been correlated with any eruptive activity at Mount Rainier and may have occurred without precursory eruptive phenomena. The Electron Mudflow apparently was very fluid and underwent minimal downstream attenuation of discharge. This is demonstrated approximately 36 kilometers west of Mount Rainier where the Electron was still about 30 meters deep as it exited the Cascade mountain front and flowed onto the Puget Lowland. ...

NOTE: Woods and Kienle article (contribution by Patrick Pringle) states 60 meters deep, corrected to 30 meters deep via request of Patrick Pringle, May 1998

... the Electron Mudflow which occurred about 550 radiocarbon years ago in both forks of the Puyallup River. There is no evidence of association of the flow with eruptive activity. Although less than one-tenth of the volume of the Osceola, the Electron Mudflow inundated at least 36 square kilometers of Puget Sound Lowland. Because of post-settlement deforestation and the consequent changes in the hydraulic roughness of flood plains, a modern lahar of similar volume and rheology would flow faster and inundate a larger area.

The mean peak velocity of the Electron Mudflow was about 20 meters per second at the boundary of the lowland, and an average velocity of about 22 meters per second is estimated between Mount Rainier and that point. An equivalent maximum celerity if 79 kilometers per hour (49 miles per hour). ...

... Several large flows in the Puyallup River drainage originated from the Sunset Amphitheater. The most typical example is the Electron Mudflow. Deposits of that flow, which occurred about 500 years ago, form the valley surface around Orting.

Puyallup Glacier originates in Sunset Amphitheatre near the summit (of Mount Rainier) on the mountain's northwest side. One theory suggests that the Electron Mudflow resulted from a cliff collapse to form this grandiose cirque. ...

The Electron Mudflow was named from Electron, a small community beside the Puyallup River at the west margin of the Cascade Range (Crandell, 1963b, p.A50). Remnants of the mudflow underlie much of the valley floor from Electron northward to the outskirts of Sumner. The mudflow is an unsorted mixture of subangular to subrounded rock fragments in a purplish-gray matrix of sand, silt, and clay. it generally exhibits a size gradation upward from coarse material at the base to fine material at the top. ...

A wood fragment obtained from the mudflow near Electron had a radiocarbon age of 530+/-200 years (Crandell, 1963b, P.A51). When corrected, this age is about 600 years. ...

The Electron Mudflow underlies about 14 square miles of the Puyallup River valley in the Puget Sound lowland. In this area its thickness ranges from a few inches to more than 26 feet; it is generally about 15 feet thick from McMillin southward to a point 2 miles south of Orting. If an average thickness of 15 feet is assumed for the area beyond the mountain front, the deposit has a volume of a little more than 200 million cubic yards. An additional amount of unknown area and volume lies between the lowland and its source, 22 miles away, on the west flank of Mount Rainier.

The clay-mineral content of the Electron Mudflow suggests an origin like that proposed for the other clayey mudflows from Mount Rainier; thus, the Electron probably was caused by massive slides of moist, hydrothermally altered rock. These slides may have been triggered by an earthquake or by a volcanic explosion.

The Electron Mudflow may have occurred at a time when the volcano was dormant, for we have found no deposits that indicate an eruption about 600 years ago.

The Electron Mudflow, as interpreted by Crandell (1963, p.69), dammed the drainage of Kapowsin Creek to form Lake Kapowsin. The lake has a maximum depth of 9 meters (Crandell, 1963), which is deep for a lahar-margin lake (Scott, 1986) and suggests that the strength of the flow was significant (Johnson, 1984, equation 8.6c). The original flow margin may or may not have had that much relief, however, according to local residents, the lake level has risen substantially in the last century due to outlet blockage, either natural or constructed.

Electron Mudflow - Crandell, 1971

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