More than 90,000 people live in the zones that a Mount Rainier lahar, a fast-moving flow of water, mud and rock, could inundate. The count comes from a 2015 study by the U.S. Geological Survey, which separately counted over 50,000 employees working at about 3,800 businesses inside the same hazard areas. Residents and workers are tallied apart, so the totals should not be added together.
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Lahars flow nearly as fluidly as water yet carry the density of wet concrete, which is why USGS says a large one can bulldoze or bury nearly everything in its path. The agency’s modeling concentrates on the mountain’s west flank, which earlier USGS research identified as the most landslide-prone part of the volcano and which drains into densely developed valleys. Those two facts together explain why emergency managers there treat the volcano as a standing planning problem, not a rare curiosity.
Two river valleys carry the exposure
The USGS feature on its lahar-modeling tool names the Puyallup and Nisqually river valleys as the two drainages on Rainier’s western flank that are densely developed. Those valleys channel any flow that leaves the volcano toward communities, which makes them the practical boundary of the 90,000 figure.
The population count itself is documented in the 2015 hazard study published in the Journal of Applied Volcanology, available as the lahar-hazard-zone study the USGS cites. It is a snapshot of a single study year, so the number describes how many people live inside mapped hazard zones, not a forecast of casualties from any particular event, and it does not say how many would be reached by a flow of any given size. The separate figure of over 50,000 employees reflects how many people spend working hours there, which USGS counts separately from the resident figure.
What a lahar is and how it can start
Lahars begin on volcanoes and can arise two ways. An eruption can trigger them, as at Mount St. Helens on May 18, 1980, but a landslide on a steep, weakened slope can also set one off with no eruption at all, sometimes without warning. The west flank of Rainier was identified by earlier USGS research as the most landslide-prone part of the volcano, and the baseline description of the mountain’s hazards appears in the agency’s volcano hazards assessment.
That matters for planning because a flow with no eruption to announce it leaves little room for a warning. The report that the USGS tool supports does not predict when the next large lahar will occur; it estimates what such a flow would do if one began, which is the information residents and planners need to judge exposure rather than timing. The larger scenario, at about 260 million cubic meters, is five times the volume of the smaller 52 million cubic meter case, and running both lets planners see how the reach of a flow can change with the size of the slope failure that starts it, a comparison the agency built into the design of the simulations.
D-Claw and the simulations behind the maps
The tool is a computational model called D-Claw, built by USGS scientists with university researchers to simulate how a flow of debris and water moves. It is applied in USGS Open-File Report 2021-1118, Modeling the Dynamics of Lahars that Originate as Landslides on the West Side of Mount Rainier, Washington, authored by David George, Richard Iverson and Charles Cannon of the agency’s Cascades Volcano Observatory and Oregon Water Science Center.
The simulations start from the Sunset Amphitheater and the Tahoma Glacier headwall and model two landslide volumes, about 260 million cubic meters and 52 million cubic meters. Each volume is run twice, once as dense rock with low mobility and once as weakened, clay-rich rock with high mobility. The outputs predict flow depth, speed and range of movement, which emergency managers can set against the places where people live and work.
D-Claw is not limited to hypotheticals. The USGS notes it was previously used to simulate the 2014 landslide at Oso, Washington, which the agency examined in a study of the Oso disaster, and the model can reconstruct past flows as well as test hypothetical ones. That dual use lets the researchers compare its output with events that actually happened.
The modeling feeds real preparations. USGS says it is working with emergency managers at Mount Rainier National Park and in local communities and counties, and that an automated lahar detection system installed in 1998 is being upgraded and expanded. Detection matters most for the landslide-triggered flows described above, because a sensor network can shorten the gap between a flow starting and a warning reaching the valleys, even when no eruption has signaled that anything is wrong.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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