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Debris Flow At Mount Rainier National Park Causes Minor Damage

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This photo spotlights the source of the debris flow from the South Tahoma Glacier at Mount Rainier National Park/NPS

This photo spotlights the source of the debris flow from the South Tahoma Glacier at Mount Rainier National Park/NPS, Scott Beason

Hawai'i Volcanoes National Park's Kīlauea and Mauna Loa volcanoes are the two National Park System volcanoes that have garnered most news during the past year, but the volcano that is Mount Rainier National Park isn't without activity. Just the other day it released a "debris flow" that, while not uncommon, is not an everyday occurrence at the park.

"Debris flows" are basically an outpouring of water, rocks, mud, sand, whatever is the makeup of the ground through which the water flows. In this case, park staff say the event occurred between approximately 6:48 p.m. and 7:58 p.m. Monday, originating from a sudden and significant change in the primary outlet stream from the terminus of the South Tahoma Glacier. This change caused a surge of water within the glacier and turned into a debris flow.

The park’s Westside Road and Tahoma Creek Trail sustained some damage, so in consultation with the USGS Cascades Volcano Observatory, the park temporarily closed the Westside Road. No damage is expected outside park from this event. This debris flow may indicate that subsequent events may occur in the coming weeks or months.

“Debris flows are not uncommon during periods of hot weather in the park’s dynamic landscape,” said Rainier's acting superintendent, Tracy Swartout. “Visitor and staff safety are our priority, so limited closures are appropriate at this time.”

Park staff detected the event on Tuesday morning when the Tahoma Creek appeared extremely sediment-rich. Park geologists conducted aerial reconnaissance of the area Tuesday and identified the source and runout areas. They also checked the South Tahoma Glacier for additional outburst geologic hazards. All visitors and staff in the area were accounted for on Tuesday.

Back in September 2015, a half-acre portion of the South Tahoma Glacier broke off and spawned a glacial outburst that flooded Tahoma Creek. About seven waves of debris flow occurred in that event. As the outburst flood moved down valley, it carried sediment, rocks, and uprooted trees and deposited the debris within the Tahoma Creek valley near Mount Wow in the park, according to staff. A stream gage on the Nisqually River at National registered the 0.5 foot river rise on Thursday afternoon. That debris flow, and the most recent one, had no impact to properties outside of Mount Rainier National Park.

Glacial outburst floods and debris flows occur with some regularity in the rivers that drain glaciers within the park. A glacial outburst flood is a large, abrupt release of water from a glacier. Although the mechanism remains unknown, geologists report that stagnant and slow moving ice on the lower part of a glacier combined with faster moving ice on the upper glacier has been associated with such events. The Tahoma Creek valley has had at least 32 outburst floods and debris flows since 1967, according to park staff.

The park reminds all visitors to remain alert for changes in water levels and unusual sounds or shaking ground. In the event of rapidly rising water or a loud roaring sound coming from up valley, move quickly to higher ground at least 200 feet above the valley bottom. Visitors who may have witnessed the event on Monday are encouraged to report observations to Scott Beason, park geologist, at [email protected]. 

Comments

The local media all ran stories about this relatively minor event.  The comments in the Seattle Times quickly devolved into the typical climate-change squabble, but I was amused by the suggestion that if people would just elect Republicans, "the glaciers would be too scared to retreat."

In addition to the Westside Road, the Chinook Pass entrance to the park (SR 410) is closed by a recent rockslide.


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