Skip to main content
U.S. flag

An official website of the United States government

Monitoring Lahars at Mount Rainier

The greatest potential volcanic hazard at Mount Rainier is a lahar, also known as a volcanic mudflow. 

Why monitor lahars at Mount Rainier?

Scientist standing next to solar panels on monitoring station with snowy mountain in the background.
Lahar detection systems, such as this monitoring site at Mount Rainier, Washington, will become activated when a lahar is moving down a nearby drainage. The system will quickly notify emergency officials who issue warnings to downstream residents. (Credit: Rebecca Kramer. Public domain.)

Geologists have found evidence for at least nine large lahars (or volcanic mudflows) from Mount Rainier in the last 5,600 years that reached into the Puget Lowlands.  Several areas around the volcano inundated by these large lahars are now densely populated and contain critical infrastructure such as highways, bridges, ports, and pipelines.  Geologic evidence tells us that most large lahars occur during Mount Rainier eruptions. However, scientists have found no evidence of eruptive activity associated with the most recent large lahar (the Electron Mudflow of ~1500 A.D.), which instead was caused by a large landslide off the west flank of Mount Rainier.  Scientific studies show that the west flank of Mount Rainier is potentially vulnerable to a future large-scale collapse, which could produce a large lahar down the Puyallup, Mowich, and/or Tahoma Creek and Nisqually River drainages. Mathematical models indicate that such a lahar would reach residential areas inside the park in about 5 minutes and residential areas outside the park in 15 to 60 minutes.

Mount Rainier Lahar Detection System

Rainier Lahar Detection System, 1998-2021

In response to lahar-hazard concerns, the Pierce County Department of Emergency Management (PCDEM), the U.S. Geological Survey (USGS), the Washington State Emergency Management Division (WA EMD), and other local and county emergency-management agencies initiated a multi-agency effort in 1995 to develop a lahar warning system for the Carbon and Puyallup River valleys in the southeastern Puget Sound region hazard. After a multi-year development effort, the system become operational in 1998 and is still in operation today. 

The lahar warning system includes a detection component and a warning-dissemination component. The detection component is automated and identifies lahars with a network of tripwires and small sensors called acoustic flow monitors (AFMs). AFMs are embedded underground and measure ground vibrations made by passing lahars. Signals from the field stations are transmitted to computers in the Washington State Emergency Operations Center and Pierce County Department of Emergency Management (PCDEM), where they are continuously analyzed. When a lahar is detected, the computer systems transmit alerts to 24–hour emergency monitoring and notification centers at WA EMD and South Sound 911 who initiate the warning component of the system. The warning system transmits alerts in many ways, including the WA EMD Emergency Alert System  and networks of sirens operated by PCDEM and WA EMD. Because lahars from Mount Rainier can reach populated areas in a matter of minutes, warning messages are intended to trigger immediate, preplanned emergency-response actions.

Rainier Lahar Detection System, 2021-onwards

4 images showing boxes of equipment with wires being put into the ground in a forest setting.
Lahar monitoring equipment is housed in a secure box with multiple types of instruments to detect approaching lahars. (Credit: Rebecca Kramer. Public domain.)

In 2015 the PCDEM and USGS Cascades Volcano Observatory (CVO) initiated a new planning effort to modernize the lahar detection system to provide earlier warning on more drainages. In 2017 the USGS received appropriated funding to start the work, with the first station upgrades occurring later that summer. Between 2017 and Spring 2021, 14 new sites were installed, including several that replace the 1998-vintage sites on the Puyallup River. 

Up to 20 additional sites are planned for installation in 2021-2023. This includes 12 proposed sites inside Mount Rainier National Park, which are currently being evaluated by the National Park Service through an Environmental Assessment process. All new sites will feature broadband seismometers, and many will also include at least one other sensor type including tripwire arrays, infrasound sensors, web cameras, and GPS receivers. Along with CVO and Pacific Northwest Seismic Network (PNSN) monitoring stations already in operation at Mount Rainier, the combined Mount Rainier lahar and volcano monitoring network will include over 40 real-time monitoring stations once installations are complete. 

The new lahar detection system will function much like the original system, with several important improvements:

  1. Signals from all sites will be transmitted in real-time (the 1998-vintage sites transmitted signals every two minutes). This improves detection and warning times by several minutes.
  2. Drainages covered by the detection system will expand to include the Tahoma Creek/Nisqually drainage, in addition to the Puyallup River.
  3. The USGS is developing more sophisticated lahar detection software that will detect lahars earlier and determine the location, speed, and/or size of a lahar as it moves downstream.
  4. The new system will also improve detection of seasonal debris flows inside Mount Rainier National Park—in particular along the Tahoma Creek drainage where at least 33 debris flow events are known to have occurred since 1967.

The new lahar detection system will run alongside the old system to ensure everything works well before stations from the old system are removed.

Know how to respond to a lahar warning.

Volcano Evacuation Route Sign
Volcano Evacuation Route Sign (Public domain.)

Residents, employees, and visitors in lahar–hazard areas will be notified of an approaching lahar through multiple channels of communication. Many schools and other public and commercial facilities will receive notice directly from the Emergency Alert System. Notifications will be publicized on television, radio stations, NOAA Weather Radio, emergency management social media accounts, and via other broadcast platforms. A system of All Hazard Alert Broadcast sirens in cities and towns from Orting to the Port of Tacoma will provide evacuation alerts and protective measures related to an approaching lahar. Check with local officials to find out what is available in your community, school, or workplace.

Once people in the Puyallup and Nisqually River valleys receive a lahar warning, they need to respond instantly. Pierce County and other Washington State agencies have developed an evacuation plan with marked evacuation routes to aid residents and visitors. Parts of some communities rely on evacuation to high ground by foot, especially in areas where highways may become clogged with traffic.

In at-risk areas that are too remote to receive notification by one of the above methods, it is necessary to be aware of the natural warning signs of an approaching lahar—ground rumbling accompanied by a roaring sound like a jet or locomotive. Moving to high ground immediately is the recommended course of action.


The USGS works in partnership with Pierce County Department of Emergency Management (PCDEM), the Pacific Northwest Seismic Network (PNSN), Washington Military Department’s Emergency Management DivisionSouth Sound 911, and Mount Rainier National Park to develop, maintain, and operate the lahar detection and warning systems. The USGS maintains and operates the lahar detection component with technical assistance from PCDEM. PCDEM, South Sound 911, and WA EMD maintain and operate the alarming notification system. PCDEM, WA EMD, and CVO house base stations for the detection system. Twenty-four-hour emergency monitoring and notification centers at WA EMD and South Sound 911 are responsible for initiating a lahar notification.

Further Information