About 500 years ago, a wall of mud and rock tore down the west side of Mount Rainier, rolled through the Puyallup River valley, and left a deposit that geologists still map today. That event, the Electron Mudflow, is the most recent large lahar named in the latest modeling work from the United States Geological Survey. Nothing of its size has happened since, which sounds reassuring and is actually misleading, because the geologic record shows that the mountain keeps no schedule.
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A lahar is a fast-moving slurry of water, mud, rock and woody debris. At Rainier, the dangerous ones are not always the ones that follow an eruption.
The Electron Mudflow in the Puyallup valley
The USGS volcano science pages describe the Electron Mudflow as a flow of about 0.26 cubic kilometers, roughly 0.06 cubic miles, set off by an avalanche of hydrothermally altered rock from the volcano’s west side. Rock that has been cooked and softened by hot, acidic fluids inside the mountain is weak and clay-rich, which is why it can fail and then liquefy as it moves. The flow raced down the Puyallup drainage at least as far as the present-day town of Sumner, far from the summit and well into country that is now lived in.
Scale matters here. The Osceola Mudflow, about 5,600 years ago, carried around 4 cubic kilometers of debris after an eruption-driven collapse of the northeastern flank and reached the sites of present-day Tacoma and Kent. By comparison the Electron event was a middleweight, yet it is the one USGS used as its yardstick for the west side, because it came from the same slopes that threaten the Puyallup and Nisqually valleys today.
The deeper record is what makes the 500-year figure hard to read. Over roughly 11,000 years, the same USGS account shows lahars in most of Rainier’s eruptive periods, and they cluster: three or more during the Cowlitz Park period between 7,400 and 6,700 years ago, multiple flows in the Summerland period from 2,700 to 2,000 years ago, far-traveled flows about 1,500 years ago, and one or two more around 1,100 to 1,000 years ago, before the Electron Mudflow. The page gives no average recurrence rate, only intervals running from hundreds to thousands of years, so a gap of five centuries cannot honestly be called overdue or safe.
Landslide-triggered lahars and the warning gap
The warning problem comes from how some lahars begin. In its description of a new modeling tool, USGS explains that lahars can develop from landslides on steep slopes without any eruption, and that such flows “may occur without warning.” A flow that begins as a slope failure does not depend on an eruption to start, so an eruption alert cannot be counted on to precede it. The Electron event began as an avalanche of altered rock on the west flank, a failure of that general kind.
USGS is careful about what its work does not claim. The accompanying report, Open-File Report 2021-1118, was written by David George, Richard Iverson and Charles Cannon of the USGS Cascades Volcano Observatory and the USGS Oregon Water Science Center, and it “does not predict when the next large lahar will occur.” A figure caption in the agency’s story adds that real flows may be larger or smaller than the modeled ones and that arrival times will vary in an actual event.
Distance buys time, but not much of it. A USGS account of the world’s largest lahar evacuation drill, held in March 2024, says the D-Claw model indicates a large lahar could reach Orting in as little as 50 minutes, and that the closest communities would have only tens of minutes to leave if no notice came at all. A lahar warning system for the Carbon and Puyallup valleys has operated since 1998, but a system can only warn about a flow that sensors have already detected.
D-Claw and the planning it supports
D-Claw is a computational model developed by USGS scientists with university researchers. Its equations draw on decades of large-scale experiments at the USGS debris-flow flume in Oregon, described in a 2007 USGS report on those experiments, and it has been used to reconstruct the 2014 Oso, Washington, landslide. Unlike a simple map of past deposits, it can project how a flow of a given volume and wetness would move across real terrain.
For Rainier, the report modeled landslides from the two most landslide-prone parts of the volcano, the Sunset Amphitheater and the Tahoma Glacier Headwall. It ran two volumes, 260 million cubic meters, which approximates the Electron Mudflow, and 52 million cubic meters, comparable to a 2010 landslide at Mount Meager in British Columbia. It also ran a low-mobility case, with dense, hard rock, and a high-mobility case, with weakened, clay-rich rock. The outputs are inundation patterns, depths, speeds and arrival times for the Puyallup and Nisqually river valleys.
Those outputs feed evacuation route planning, emergency response plans and the school lahar drills that Washington State requires in the hazard zones, and USGS says the automated detection system installed in 1998 is being upgraded and expanded. The practical lesson of the Electron Mudflow is not a countdown to the next one. It is that a response built only around eruption warnings would miss the lahar that starts as a slope failure, and the drill and detection work described by USGS reflects a response measured in minutes.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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