What Is a Lahar, and Why They Matter in Oregon

May 18, 2026, marks 46-years since the eruption of Mount St. Helens—an event that reshaped landscapes, communities and the Pacific Northwest’s understanding of volcanic hazards. This anniversary is a moment to reflect on the past and recognize the hazards Oregon still faces today.

One of the most destructive events during the 1980 eruption was the generation of lahars — fast-moving flows of mud, water and volcanic debris that surged down river valleys, destroying bridges, homes and infrastructure. And while Mount St. Helens sits just across the Washington border, similar hazards exist much closer to home. Oregon has active volcanoes, and understanding these risks helps us stay ready.

That is why Oregon’s 2026 state-level preparedness exercise — Getting Through Lahar’d Times, scheduled for October 13–16, 2026 — focuses on volcano preparedness. Exercises like this strengthen our ability to respond to natural hazards, protect communities and support recovery.

What is a lahar?

A lahar is a rapidly flowing slurry of volcanic ash, rock, soil and water that moves down the slopes of a volcano and into surrounding river valleys. It behaves like a flash flood made of concrete, and can travel miles from a volcano, sometimes at highway speeds.

Lahars can start in several ways:

• Rapid melting of snow and ice during an eruption
• Heavy rainfall on loose volcanic ash
• Sudden release of water from a crater lake or melted ice
• Landslides on volcanic slopes

Because they follow valleys and rivers, lahars can affect communities far from the volcano itself. We saw many lahars form during the 1980 eruption.

How lahars could affect Oregon

The Cascade Range of Oregon and Washington is home to several volcanic peaks capable of generating lahars. These include Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams in Washington State and Mount Hood, Three Sisters, Newberry, and Crater Lake in Oregon. Each has produced lahars in the past and will do so again in the future. Today, the Cascade Range of Oregon and Washington are at normal background activity levels. But we know the mountains can wake up in the future.

The United States Geological Society (USGS) monitors all active volcanoes in the Cascades through the Cascades Volcano Observatory.

Potential impacts include:

Fast-moving flows affecting communities

Many Oregon towns, recreational areas and highways sit in river valleys that start on volcanic slopes. In an eruption or landslide-triggered event, lahars could reach populated areas within minutes to hours.

Damage to transportation corridors

Highways, bridges and rail lines often follow river valleys, making them vulnerable to burial, washout or blockage by debris flows.

Impacts on water supplies

• Lahars can fill reservoirs, damage water systems and affect water quality.
• Long-term effects on landscapes and ecosystems
• Lahars can reshape river channels, alter flood patterns and deposit thick layers of sediment that take years to stabilize.

Why volcano preparedness matters

Getting Through Lahar’d Times, Oregon’s statewide exercise in October 2026 will focus on how agencies, tribes, local governments, utilities and community partners coordinate weeks after a volcanic emergency as communities move into the recovery phase. These exercises happen every two years and strengthen communication and build the capacity to support communities before, during and after a disaster. But preparedness is not just for responders. Every household plays a key role.

Oregonians are encouraged to Be2Weeks Ready, meaning:

• Have enough food, water, medications and essentials to last 14 days
• Know evacuation routes and communication plans
• Stay informed about hazards in their area

Whether the hazard is a volcanic eruption, earthquake, winter storm, wildfire or flood, the steps you take now help you stay safe in any emergency.

Staying ready for the unexpected

Mount St. Helens taught the region lasting lessons about volcanic power and the importance of preparedness. As we mark the 46 years since that historic event, we also look toward the future, working to keep Oregon informed, ready and resilient.

Lahars are a natural part of our volcanic landscape, but with awareness, planning and practice, communities can reduce risks and stay safer.

For more information about emergency preparedness planning in Oregon, visit the Oregon Department of Emergency Management’s website.