Nature Notes

Vol. II January-February, 1987 No. 11


"There's boulders as big as the Entrance Booth bouncing down the creekbed in a river of mud!"

That is what a visitor reported to Ranger Jenni Whitney at the Nisqually Entrance Station late Sunday afternoon October 26, 1986. After one of the driest Octobers on record, almost five inches of rain had fallen in two days contributing to a "jokulhlaup" (jo ka loip) on the South Tahoma Glacier.

A jokulhlaup is an Icelandic term for a glacier outburst flood. Outburst floods occur when there is a sudden change in the pluming or internal conduit system of a glacier. The interior of the glacier contains pockets which normally store water. If there is a sudden influx of water from melt or rain, a large amount of water will burst out from the snout or other weak point in the glacier. Often part of the glacier will collapse and chunks of ice are released with the water. These waters usually pick up large amounts of alluvial and glacier deposits which quickly turn them into mudflows or "lahars" (lahars are mudflows that occur on the slopes of a volcano).

The October 26 lahar covered half of the Tahoma Creek trailhead parking area with six inches to over one foot of mud and debris. Up to 10% of the material deposited from the outburst flood was glacial ice or frozen morainal material. The large volume of material that flowed through the area is evident from mud lines 12 feet high on trees and boulders up to 10 feet in diameter randomly deposited far from the creekbed. The creek itself was diverted into a new channel by a log jam which forced the water to cut through a section of old growth forest before joining its former channel 1.5 miles downstream.

But did those boulders really bounce? That is possible because the lahars have a consistency of wet cement and are referred to as viscus flows. The specific gravity cf the boulders was approximately equal to the specific gravity of the viscus flow allowing the giant boulders to roll and tumble on top of the slurry.

Was this an isolated event? Not at all on a glacially active mountain like Mt. Rainier. There used to be a campground at the Tahoma Creek Trailhead until a jokulhlaup from the South Tahoma Glacier closed it on August 31, 1967. In the park's 87-year history, there have been over a dozen witnessed outburst floods resulting in both large and small mudflows. Who knows how many unrecorded ones have occurred on a mountain with 26 glaciers and about 1 cubic mile of ice? With a rate of one debris flow every 3-10 years, we can be sure that jokulhlaups and lahars will continue their surprise appearances on the slopes of Mt. Rainier.

Lynn Arthur

From Origin of Geographic Names of Tacoma/Pierce County Washington By Gary Fuller Reese

CARTER FALLS -- One of the beautiful features of the lower Paradise River; first falls above Nisqually River. Named for Harry Carter who did much of the work to build the first Paradise Trail. Carter homesteaded on Bear Prairie in 1893 and received exchange land along Puget Sound from the forest service for his holdings at Bear Prairie.

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