Bridge for Kids and Mount Rainier

What is a lahar?

A lahar is a mudflow originating on a volcano. The mud and debris in a lahar typically follows down a river path, or drainage, and is the consistency of wet concrete. This natural hazard can be triggered in a number of ways. Some lahar events give advanced warnings, such as detectable subsurface magma activity, and others occur without any advanced warning what so ever.

There is also a range of severity associated with lahars, from flows that only move a short distance from their origin, to those that travel over many miles.

Lahar flows can reach speeds up to 50 miles per hour. Orting is only 30 miles away from Mount Rainier, presenting an estimated arrival time of 40 minutes.

Lahar Classifications

Like hurricanes, tornadoes, floods and other disasters, lahars have a "ratings" system:

Case M events are the largest, occurring on a 10,000 year timeline. The most recent flow of this magnitude was the Osceola Mudflow, which covered 212 square miles.

Case 1 events are more common, occurring on a 500-1000 year timeline. Approximately 500 years ago, the Electron Mudflow buried an old growth forest in the Orting Valley. The mud from this could reach 18 feet deep.

Case 2 events are moderate in severity, recurring every 100-500 years. These are typically caused by an eruption swiftly melting ice or snow. There have been more than 12 such events in the last 6000 years.

Case 3 events are the most common of all and relatively small -- occurring anywhere from annually to every 100 years. They can be triggered by sudden release of water stored in glaciers, such as the 2001 outburst in Nisqually. Debris from events like this may not travel out of the National Park, but can cause flooding in lower areas as glacier water displaces river water.

Lahar Detection for the NW Corner of Mt Rainier

Acoustic Flow Monitor (AFM) systems were placed on the Puyallup and Carbon River drainages of Mt Rainier in 1995.  An array of five AFM units are in place on both river systems outside the boundaries of the National Park.  Each array is placed to detect and verify a major lahar flow.  They are placed at one point on each river ranging from close to river level, uphill to a level above any project flow volume.  Be

In a real flow situation, were a flow to be heading down one drainage only, all five AFM’s on it will send out an alarm to human monitors at two locations that are staffed 24 hours a day, 365 days a year.  The lower AFM units on each array were placed so as to seize alarm when they become inundated with mud.  At that time, the flowing mass will be determined to large enough to Sumner and Puyallup.  And it will be at the point that the public alarm system will be activated.

Debris flows rustle down the flanks of Mt Rainier most every day.  That is why the AFM systems were placed well outside the Park boundaries to assure that only a “real” event is taking place. 

Therefore while a lahar may get initiated on the side of the mountain at one point in time, it will have to travel almost an hour before gaining mass volume enough to be detected, confirmed and alerts get issued.

For more information, check out the links below!


Rock Slide Mislabeled as a Lahar

In Depth Information

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