The Spatial and Temporal Hazard of Debris Flows in Glacier National Park, Montana
Wilkerson, Forrest Dunlap
This dissertation concerns the mass wasting process of debris flows that occur within the eastern portion of Glacier National Park. There are two major foci of this research. The first concerns the magnitude and frequency of debris flow landforms observed in the field. The second focuses attention on the distribution of the landforms within the park and both foci are intertwined with the hazard that debris flows pose to park visitors, personnel, and infrastructure, primarily roads and trails. Debris flows are one of the many active slope-forming processes within Glacier National Park, Montana. Most debris flow landforms exhibit classic morphology with a distinct failure scarp, incised channel, channel levees, and toe slope deposits that often develop a lobate form. The Precambrian metasediments that dominate Glacier National Park's geology, weather into angular clasts ranging in size from platy gravels to boulders. Classic debris flows occur in areas where the topographic expression provides a debris source from cliff faces and an accumulation of regolith, often in the form of talus slopes. Many of these debris flows have long runout zones and can travel many hundreds of meters. Often they cross hiking trails or roads, including the main east-west highway, the Going-to-the-Sun Road. Debris flows impacting the road have resulted in several near fatalities, and hikers have been forced to cross active debris flows to reach safe ground. The magnitude of debris flows varies between high magnitude channel incising events and low magnitude channel filling and/or reworking events. The frequency of debris flow events is irregular and appears to be controlled by the hydrology of triggering storms and antecedent moisture conditions, not by the debris supply. As a result, debris flow magnitude is not a function of frequency, but is more closely related to the characteristics of antecedent conditions and individual storms. Debris flows occur in geomorphically distinct areas. Particularly in places with moderately steep slopes that allow debris and slope material to accumulate at near the angle of repose. Debris flows also require a large input of water to trigger an event. This water is most often delivered by heavy rain, particularly on top of a snow pack, by sudden snowmelt, or often a combination of rainfall, melting snow, and saturated ground conditions. Debris flows in Glacier National Park have proven to be highly variable in space and time as well as magnitude and frequency. However, they are somewhat predictable depending upon local site conditions, soil moisture, and above all, inputs of water from storms and snowmelt. Debris flows remain a hazard to the visitors and staff of Glacier National Park and are deserving of much more attention toward the mitigation of this often neglected hazard.
debris avalanches, landslide hazard analysis, landforms, Montana, Glacier National Park
Wilkerson, F. (2004). The spatial and temporal hazard of debris flows in Glacier National Park, Montana (Unpublished dissertation). Texas State University-San Marcos, San Marcos, Texas.