ACOUSTIC SOUNDSCAPE MONITORING AS MEANS TO DETECT DEBRIS FLOWS AT MOUNT RAINIER NATIONAL PARK, WASHINGTON

Debris flows (DFs) occur with some regularity on the glaciated Mount Rainier (MORA) volcano (4,392 m). These events are related to hydrologic anomalies: forming either as outburst floods from glaciers in the summer or spawned during high intensity precipitation events. These events "bulk up" in steep debris-rich proglacial margins. While all major glaciated basins at MORA have generated DFs, Tahoma Creek and South Tahoma Glacier have a rich history of DF generation, with at least 37 discrete events since 1967. The destructive nature of these events have caused a change in management of the park's West Side Road and affected infrastructure. After a relative lull in activity between 2007 and 2014, two separate DF sequences occurred in the Tahoma Creek basin on August 13 and September 12, 2015. These DFs appear to have been sourced at the terminus of the South Tahoma Glacier. The exact initiation mechanism is not certain, but prevailing weather, the underlying bedrock and detached debris-rich stagnant ice appear to be major drivers of the 2015 events. The August 13 DF was especially well documented by park visitors, staff, seismic instruments and stream gauges. One visitor videoed a DF with his cell phone as it passed within meters of his location. Of particular interest to the monitoring effort was a National Park Service soundscapes array that was located near the end of the DF runout zone. This array recorded an anomalous decrease in "river noise" two hours prior to DF onset and the passage of four discrete DF sequences, all of which were correlated to field and instrument observations. Soundscape instruments may be a new and novel approach to monitoring stream flow and passage of DFs at MORA. We are investigating the use of this technology to detect DFs and warn park visitors and staff. Documentation of the decrease in "river noise" may be evidence for either intra- or supra-glacial collapse which dammed glacial outflows and enhanced the generation of DFs.