8 August 2019
The South Tahoma Glacier/Tahoma Creek Outburst Flood & Debris Flow on 5th August 2019
With thanks to Scott Beason, Park Geologist, Mount Rainier National Park.
On Monday evening, 5th August 2019, an outburst flood and debris flow occurred at Mount Rainier National Park. Between approximately 6:48 – 7:58 PM PDT (8/6/2019 01:48 – 2:58 UTC), the park’s seismographs picked up four separate debris flow sequences in the Tahoma Creek area. Scott notes that:
“On arrival in the park on Tuesday morning, I observed extreme sediment transport/hyperconcentrated flows at the road bridge, approximately 13 km downstream of the glacier. Once I got in the office, I checked the seismic records and saw a clear debris flow signature (see below). After checking with the USGS/Cascades Volcano Observatory, who concurred on the debris flow signature, we closed the park’s West Side Road (which parallels Tahoma Creek and is downstream of the South Tahoma Glacier) until we could undertake a ground and aerial reconnaissance to determine the source and additional hazards”
Seismic data shows that the debris flows had four discrete pulses (see the earthquake spectra image and seismic records below) and Scott has found numerous debris flow levees and boulder fields in the field.
Scott and his colleagues were able to collect images of the aftermath of the event via a helicopter flight and fieldwork. These are available via a Flickr page – they are fabulous:-
Initial satellite image analysis by Scott suggests that the outlet stream from the South Tahoma changed locations:-
Scott’s initial hypothesis that this was a catastrophic change that then rapidly incised into debris-covered stagnant ice and ground-cored moraine just downstream and between bedrock “steps” below the glacier. Once it had enough momentum, it then rapidly bulked up into little ice age ground moraine several km’s below the terminus. The run out ended about 6.5 – 7 km’s downstream where the boulder fields were deposited.
Interestingly, this event is remarkably similar to the 2015 debris flow sequence.
There is much more to write about this event, and I will return to it in the next few days. In the meantime, many thanks to Scott for highlighting it to me, and for the amazing data.