The USGS puts the odds of a Mount Hood mudflow reaching Oregon valleys at 1-in-15 to 1-in-30 over 30 years

The U.S. Geological Survey puts the odds of a future Mount Hood eruption sending lahars down the Sandy and White River valleys at 1-in-15 to 1-in-30 over the next 30 years, according to a hazard assessment from the agency’s David A. Johnston Cascades Volcano Observatory. A lahar is a fast-moving slurry of volcanic rock, ash, mud and water that behaves less like a lava flow and more like wet concrete accelerating downhill, and USGS scientists say it is the hazard most likely to reach communities living far from the volcano’s slopes.

Mount Hood last erupted in the 1790s, not long before Lewis and Clark’s expedition passed through the region, and minor explosive activity was reported by local residents in the mid-1800s. The mountain has been quiet since, but USGS researchers Cynthia Gardner, William Scott, Jon Major and Thomas Pierson, who authored the agency’s assessment, write that the volcano’s roughly 500,000-year history shows quiet stretches lasting centuries to more than 10,000 years between active periods — meaning silence today is not evidence the volcano is finished.

Lahars, not lava, are Mount Hood’s defining hazard

Mount Hood does not have a history of violent explosive eruptions the way Mount St. Helens does. Instead, lava flows rarely travel more than 6 to 8 miles from their source, and molten rock more often piles up over the vent as a lava dome hundreds of feet high. On Mount Hood’s steep upper slopes, those domes have repeatedly collapsed into fast-moving pyroclastic flows — and when those hot flows melt snow and ice, the meltwater mixes with rock debris to form a lahar that can travel far beyond where the pyroclastic flow itself stops. Over the past 30,000 years, USGS scientists say dome growth, collapse and lahar generation have dominated the volcano’s activity more than any other process.

That is why the hazard map for a future eruption centers on river valleys rather than the mountain’s flanks. Scientists expect the next eruption to bring small explosions, more dome collapses, ash clouds and lahars, and they say lahars pose the greatest risk simply because more people live in the valleys downstream than live on the volcano itself.

The Sandy and White River valleys carry the highest odds

The 1-in-15 to 1-in-30 figure applies specifically to the Sandy and White River valleys, the two drainages a future eruption is most likely to fill with fast-moving debris. By comparison, the probability of the Hood River valley on the volcano’s north side being affected by a similarly sized lahar runs about ten times lower. The USGS divides the surrounding terrain into proximal zones, close enough that a lahar could arrive in under 30 minutes, and distal zones farther out, where a lahar might take more than 30 minutes to reach — enough warning, in that case, for people to move to higher ground if alerted in time.

Water infrastructure sits inside those distal zones along the Sandy River, including pipes carrying the Bull Run Watershed supply that serves Portland, which is why the hazard assessment treats transportation and water-system disruption as a near-certain consequence even in a moderate eruption, separate from any direct threat to life in the immediate path. A more recent USGS hazard summary adds that Interstate 84 and the Union Pacific Railroad line, which both cross the Sandy and Hood Rivers near the Columbia River, sit in the same exposed corridor, along with the Columbia River shipping channel itself, which sediment-choked floodwater could also disrupt.

A Christmas Day storm proved lahars don’t need an eruption

Mount Hood’s most destructive recent lahar had nothing to do with volcanic activity at all. On Christmas Day 1980, an intense rainstorm rapidly melted snow and triggered a landslide in loose debris above Polallie Creek. The resulting flow moved downvalley at 25 to 35 miles per hour, killed a camper, and temporarily dammed the East Fork Hood River. When that makeshift dam failed, the flooding that followed destroyed five miles of highway, three bridges and a state park, at a cost the USGS put at a minimum of 13 million dollars. Small lahars on that scale happen every few years at Mount Hood, the agency says, though few have been nearly as destructive.

Two ancient lahars set the outer bounds of what’s possible

The USGS record includes two much larger events that show how bad a future lahar could get. Around 100,000 years ago, a large section of the volcano’s north flank and summit collapsed into a debris avalanche that transformed into a lahar roughly 400 feet deep where it reached the site of the modern town of Hood River, before crossing the Columbia River and surging up the White Salmon River valley in Washington. A second, smaller event about 1,500 years ago sent a lahar the length of the Sandy River valley, depositing boulders as large as 8 feet across some 30 feet above the present riverbank near where Wemme and Wildwood now stand — an event the USGS says was still only about one-tenth the size of the 100,000-year-old collapse.

The 1,500-year-old flow kept going well past Wemme and Wildwood, too. A separate USGS summary of the same debris avalanche traces it roughly 55 miles downstream to Troutdale, where the Sandy River meets the Columbia, and credits the same collapse with carving the breached summit crater that still channels most of the volcano’s eruptive material toward the Sandy River basin today.

Scientists say Mount Hood’s next eruption is far more likely to resemble the smaller, 1,500-year-old event than the catastrophic one, but the agency has continuously monitored the volcano for earthquake swarms since 1977 for exactly that reason: it cannot say in advance which scenario is coming, or when.

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