This picture shows part of the Osceola Mudflow on the flank of Mt. Rainier in the upper White River drainage, 50 miles upstream from Enumclaw. The Osceola Mudflow can be traced from higher on the side of Mt. Rainier all the way down the river valley to Puget Sound. Details of the geological record of the deposits around Mt. Rainier indicate that it was erupting at the time of the Osceola Mudflow. A sector of the east side of the volcano then collapsed and turned into a major debris avalanche, which mixed with water from melting snow and glacial ice to turn into the largest lahar to have come from Mt. Rainier.

Base of a Lahar

This picture shows boulders eroding out of the base of the Osceola Mudflow, along the White River upstream from the town of Enumclaw. The lahar known as the Osceola Mudflow occurred 5,600 years ago according to carbon-14 dating of trees caught in the lahar. It was able to carry boulders of this size far into the Puget Sound lowland. Select the image to see a larger view. Use your browser's back button to return to this page.

Two Lahar Types

It has taken a lot of digging and careful scientific work to put together the geological record of Mt. Rainer's lahars. This picture shows the bank of the Puyallup River a few miles upstream from the town of Orting. There are three layers of sediment exposed in the bank of the river, from bottom to top (older to younger):
1. an older soil that had been oxidized to a rust-red color
2. on top of the soil, partly hidden in the grass, is an unnamed lahar deposit of approximately 2,300 years age, which contains lots of gray clay and a few angular pebbles
3. the upper, boulder-rich layer is the Electron Mudflow, a lahar that came down the Puyallup River valley about 600 years ago.
Select the image to see a larger view. Use your browser's back button to return to this page.

Stump in Lahar

The stump in the photo was carried by the Electron Mudflow, a lahar that came down the Puyallup River from Mt. Rainier 600 years ago. The stump, which was excavated at a site where a new school was being built, was cut open with a chain saw and the tree ring sequence studied in order to pin down the exact year of the Electron Mudflow. Select the image to see a larger view. Use your browser's back button to return to this page.

The Electron Mudflow is unusual among the large lahars that flowed all or most of the way toward Puget Sound from Mt. Rainier. It does not contain large, angular volcanic rocks, which are found in the other lahars. The angular volcanic rocks in other lahars indicate that those lahars were probably set off by explosive volcanic eruptions of Mt. Rainier. The Electron Mudflow may have begun as a flood surge from melting snow and glaciers, which picked up rocks and sediments on its way down, thus becoming a lahar. Small lahars that begin as sudden releases of ice and snow meltwater have been occurring on streams of Mt. Rainier since it was declared a National Park, most recently in the summer of 2001. The Electron Mudflow may have been a much larger version of a non-eruptive, flood-surge lahar.

Although the small lahars of the last century have not destroyed buildings or caused human casualties, improved knowledge of the major Mt. Rainier lahars of the last 10,000 years has led to greater concern about the threat of lahars to people in the surrounding creek and river valleys. This concern has led to the development of a lahar warning system and evacuation signs for the major rivers, in particular the Puyallup River.

Glacial and Lahar Stratigraphy

This picture is taken from a viewpoint near Mud Mountain Dam, which is on the White River near Enumclaw, Washington. The White River starts at Emmons Glacier on Mt. Rainier and joins the Puyallup River near Tacoma. This picture suggests how exposures of geology are sometimes hard to find in western Washington, due to the dense tree cover. Select the image to see a larger view. Use your browser's back button to return to this page. Follow this link for a labeled version of the photo.

This picture also shows that, where geological exposures can be found in the White River valley, they reveal evidence of the advance of the Vashon glacier from Canada followed by post-glacial lahar activity from Mt. Rainier. Lahars (large volcanic mudflows) are the major threat that Mt. Rainier poses to people who visit or make their home in the river valleys around Mt. Rainier.

In the labeled version of this picture, the sedimentary formations are in the following sequence, from older to younger:

1. Esperance Sand, advance outwash from the Vashon glacier. These are beds sand, silt and clay that were deposited by stream and lake water in front of the advancing glacier, 20,000-15,000 BP.
2. Vashon Till, a mixture of clay, sand, and larger rock debris deposited directly from the ice of the Vashon Glacier, 15,000-13,000 BP.
3. Osceola Mudflow, the largest lahar known to have come off of Mt. Rainier, simultaneously with an explosive, ash-laden eruption of the volcano 5,600 BP. The Osceola Mudflow can be traced from the upper slopes of Mt. Rainier all the way to Puget Sound. Many towns and houses have been built on it.
4. A younger lahar, roughly 2,000 BP, that filled in part of the inner valley of the White River, after the river had eroded down into the older deposits.

Glossary terms that appear on this page: lahar; outwash; till; debris avalanche