"At Mount Rainier, the risk from lahars is greater than from lava flows, volcanic ash fall, or other volcanic phenomena because some pathways for future lahars are densely populated and contain important infrastructure such as highways, bridges, ports, and pipelines. Lahars look and behave like flowing concrete, and they destroy or bury most manmade structures in their paths. Past lahars probably traveled 45 to 50 miles per hour and were as much as 100 feet or more thick where confined in valleys near the volcano. They thinned and spread out in the wide valleys downstream, slowing to 15 to 25 miles per hour. Deposits of past lahars are found in all of the valleys that start on Mount Rainier's flanks." (https://geology.com/usgs/rainier/)

Lahars and Time of Travel

Mount Rainier can potentially be the most dangerous volcano in the United States. If Mount Rainier erupts, there is a limited amount of time for people to escape the lahars especially communities that are nearby the mountain.

The amount of time that is anticipated for a debris flow, lahar, flood, or avalanche (estimated at 1000,000,000 cubic feet of volume) to travel geographically from either the source of the event or the point where the AFM alarm is sounded is classified into the following time of travel zones:

Time Zone A: On the Nisqually and White River systems: which is the area within an estimated one-hour travel distance from the source of the event. On the Puyallup and Carbon River systems: which is the area within an estimated one-half hour travel distance from the point where the AFM alarm is sounded.

Time Zone B: On the Nisqually and White River systems: which is the area within an estimated one and one-half hour travel distance from the source of the event. On the Puyallup and Carbon River systems: which is the area within an estimated one hour travel distance from the point where the AFM alarm is sounded.

Time Zone C: On the Nisqually and White River systems: which is the area within an estimated two hours travel distance from the source of the event. On the Puyallup and Carbon River systems: which is the area within an estimated one and one-half hour travel distance from the point where the AFM alarm is sounded.

Time Zone D: On the Nisqually and White River systems: that area more than an estimated two hours travel distance from the source of the event. On the Puyallup and Carbon River systems: that area more than an estimated one and one-half hour travel distance from the point where the AFM alarm is sounded.

Lahars and Evacuation Routes

If Mount Rainier erupts then the most important place to evacuate to is high ground to avoid the lahars. These routes shown in blue are the evacuation routes that will be put in effect when there is an emergency to avoid the lahars.

Data is from Pierce County Open GeoSpatial Data Portal.
Other information about Mount Rainier is from National Geographic (https://www.nationalgeographic.com/science/2018/07/news-most-dangerous-volcano-mount-rainier-supervolcanoes-yellowstone/) and geology.com (https://geology.com/usgs/rainier/)