- Debris flow initiation is controlled by a complex interaction of geology, geomorphology, climate, and weather. In the Cascade Range of Pacific Northwest and mountainous areas globally, patterns of temperature and precipitation are being altered by climate change, which may in turn impact debris flow initiation. Temperature has increased and patterns of precipitation have changed, potentially impacting the timing, geography, and triggering mechanisms of debris flows. Glacier retreat since the end of the Little Ice Age has exposed volumes of unstable sediment on steep slopes prone to debris flow initiation. Earlier spring snowmelt extends the snow-free window when rainstorms may mobilize sediment, resulting in debris flows.
To ascertain the presence of a climate change signal we examined the timing, geography, and initiation mechanisms of recent (2001 to 2006) non-volcanic debris flows from Mount Rainier, Washington, the highest volcano in the Cascade Range with the largest ice-volume in the conterminous United States. Debris flows damage infrastructure, requiring costly repairs. Debris flows also deposit volumes of sediment in streams, potentially exacerbating future flood hazards. To characterize recent debris flows, field reconnaissance was conducted summer 2008 along suspected debris flow paths from initiation to deposition. Results from summer fieldwork were used in conjunction with analysis of aerial photography, Light Detection and Ranging (LiDAR), and other data to determine characteristics of debris flow initiation sites, such as elevation, slope, orientation, upslope contributing area, and proximity to glaciers. Recent debris flow initiation sites were also examined in reference to glacier characteristics, such as elevation of glacier termini, glacier retreat, orientation, area, and volume, for the years 1913, 1971, and 1994 from past work by Nylen (2004). Characterization of debris flow initiation sites and definition of the locations of longitudinal transitions in debris flow behavior allows estimation of future debris flow hazards also allows inferences to be drawn regarding initiation mechanisms to be inferred and suggests a trajectory for changing debris flow hazards due to climate change.
Debris flows at Mt. Rainier occur in late summer through fall and recent events were no exception, occurring from August through November. A total of twelve debris flows occurred in six stream channels during the period of 2001 to 2006. Three channels not previously known to have experienced debris flows, two south-facing and one north-facing, were impacted. Debris flows tracks led up to glacier meltwater fed, steep-walled channels or gullies in unvegetated, unconsolidated Quaternary-age material immediately downslope of glacier margins. Debris flows initiated at an average elevation of 2181 m and an average channel gradient of 39°. While glaciers appear to play a key role in debris flow initiation, simple glacier metrics could not be used to distinguish glaciers near debris flow heads from those without proximal debris flows heads.
All but one of the twelve debris flows initiated during rainfall. The single debris flow that occurred during dry-weather is described by Vallance et al. (2002). Rainfall induced debris flows in 2003, 2005, and 2006 were not associated with landslide scarps, rockfalls, or other indications of large slope failures. Rather, debris flows initiated in steep-walled gullies fed by glacier meltwater that were visible on aerial photography prior to the first known debris flow initiation in a particular channel. The steep flanks of Mt. Rainier contain many similar gullies that have not previously been associated with debris flows, but debris flow producing gullies are at higher elevations than gullies not associated with debris flows.
The small population of recent debris flows and incomplete historic record of debris flows for the periods 1926 to 1985 and 1993 to 2001 limits analysis of changes in debris flow timing, geography, or triggering mechanism. The magnitude of recent events may have initially appeared greater than historic events as the 2005 and 2006 storms are the only ones known to have produced multiple debris flows in the recorded history of Mt. Rainier National Park. Yet much of the damage initially attributed to debris flows was due to widespread, severe flooding. Ongoing, detailed record keeping and possibly reconstruction of past events through paired geomorphic reconnaissance and dendrochronology is needed before conclusions regarding the impacts of climate change on debris flow initiation can be reached.