A USGS count puts 78,049 residents inside the zone a Mount Rainier lahar would sweep

Inside the area that a large Mount Rainier mudflow would inundate, 78,049 people live, according to a federal tally of the lahar-hazard zone south and east of Tacoma. The same zone holds tens of thousands of jobs, thousands of businesses and some of the busiest freight corridors in western Washington. Unlike most volcanic threats, the flow does not need an eruption to start.

That combination of density and surprise is what makes the count matter.

The Electron Mudflow scenario behind the count

The figure comes from USGS Scientific Investigations Report 2009-5211, written by Nathan J. Wood and Christopher E. Soulard and prepared with the Washington Military Department’s Emergency Management Division. The authors drew the hazard zone around the Electron Mudflow, a lahar that ran down the Puyallup River about 500 years ago, and then counted who and what sits inside it across King, Lewis, Pierce and Thurston Counties. Their summary is blunt: “The zone contains 78,049 residents,” and lahars “are significant threats to downstream development.”

According to the USGS account of lahar monitoring at Mount Rainier, “scientists have found no evidence of eruptive activity associated with the most recent large lahar.” A landslide off the west flank triggered it instead, and the same page says that flank remains potentially vulnerable to a future large collapse. The volcano’s own overview calls a lahar the greatest potential volcanic hazard at Mount Rainier, with past flows that reached as far as the Puget Sound lowlands.

Jobs, sales and property inside the zone

People are only part of the exposure. The report’s full publication record puts 59,678 employees, about 4% of the four-county labor force, at 3,890 businesses inside the zone. Those businesses generate $16 billion in annual sales, about 7% of the four-county total, with manufacturing, retail trade, transportation and warehousing, wholesale trade and construction the main sectors. Tax parcels in the zone are valued at $8.8 billion combined, roughly 2% of the study area, a smaller share than the sales figure.

Exposure is uneven across the 27 communities the authors assessed. The City of Puyallup has the highest number of people and assets in the zone, while Carbonado, Fife, Orting and Sumner have the highest percentages of their own populations and assets exposed. On a composite index combining those measures, Puyallup, Sumner and Fife rank highest overall. Road and rail corridors that serve the Port of Tacoma cross the zone, so the authors warn that losses could ripple into indirect economic damage well beyond the valley floor.

The residents themselves are not a uniform group. About 11% are over 65, 21% live outside cities and unincorporated towns, and 39% of households are renter-occupied. The zone also takes in schools, child day-care centers, hotels, religious organizations, police and fire stations, Mount Rainier National Park and the Puyallup Fairgrounds. Schools and day-care centers in the zone are the reason the evacuation drills described below put children on foot.

Acoustic flow monitors and the minutes they buy

Warning in this valley depends on hardware and on geography. USGS says the system “identifies lahars with a network of tripwires and small sensors called acoustic flow monitors,” buried devices that pick up ground vibration from a passing flow. The original sites, installed in 1998, reported every two minutes. Between 2017 and spring 2021, 14 new real-time sites went in, with up to 20 more planned for 2021 through 2023, and the finished network is expected to exceed 40 real-time stations. Modeled arrival times are short: about 5 minutes for residential areas inside the park and 15 to 60 minutes outside it.

For Orting, the closest large town, a USGS-developed model indicates that a large lahar “could reach the community of Orting in as little as 50 minutes,” according to the agency’s account of the world’s largest lahar evacuation drill. That drill ran on March 21, 2024, coordinated by the East Pierce Interlocal Coalition’s emergency management team. More than 45,000 students took part from Puyallup, Sumner-Bonney Lake, Orting, White River and Carbonado: about 13,000 walked up to 2 miles to ground outside the mapped zone, and the other 32,000 sheltered in place at schools already outside it.

What the 2009 number does and does not say

The count describes one modeled scenario, not the population of any county, and it is more than a decade old. USGS has since described a more recent study that estimates over 90,000 people live in Mount Rainier lahar hazard zones, so 78,049 is best read as a snapshot of the Electron-sized scenario rather than a ceiling. The same agency account notes geologists have found evidence of at least 11 large lahars in the last 6,000 years, and that a landslide-triggered “no notice lahar” could leave the nearest communities only tens of minutes to act.

The practical lesson is the one the siren network and the school drills already encode. Where alerts cannot reach people, USGS advises watching for ground rumbling with a roar like a jet or locomotive, and moving to high ground immediately instead of waiting for confirmation.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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