Snow Avalanche Path Terrain and Vegetation, Glacier National Park, Montana

Butler, David

Snow avalanche paths are major geomorphic features in the Rocky Mountains of Glacier National Park, Montana. They are the most common and geographically widespread form of mass-wasting in the central portion of the park. Field work and aerialphoto studies identified over 800 such paths in the study area. Other mass-wasting phenomena are also common. Snow avalanche path directions in the study area are concentrated in the southeastern, southern, and northwestern compass octants. The most common type of avalanche path has a bowl-shaped catchment area; a narrow track, often associated with a stream channel; and a tongue-shaped runout zone. Geographically, the avalanche paths are concentrated in the high-relief, high-precipitation, western portions of the study area. Statistical analysis of the variables which affect avalanche path runout zones revealed that track width accounted for 36% of the variance in runout widths. Runout zone length variations were not adequately explained by any of the terrain variables measured, including those of the starting zone. Twelve avalanche paths were studied in the field in some detail; these are described. Longitudinal sampling of forest vegetation on these avalanche paths revealed that Abies lasiocarpa, Alnus spp., and Acer glabrum are the most common trees on such sites. Progressing upslope, the number of conifers diminishes, relative to deciduous trees. Ground-cover vegetation is often related to moisture conditions on the avalanche paths. General vegetative conditions on avalanche paths in the study area indicated frequent avalanching, often at least once per season. Conifers on five avalanche paths were subjected to dendrochronologic analysis in order to accurately determine avalanche frequency. One such path experienced avalanches in 1945, 1950, 1954, 1963, 1965?, 1966, 1972, and 1974. This analysis revealed a lack of slope-to-slope synchroneity relative to periods of avalanche activity. Avalanche impact pressures and velocities may be determined from damage to trees on and alongside avalanche paths. Impact pressures and velocities for average (impact pressure, 0.21 to 0.28 t m-1; velocity, 3.7 to 4.3 m sec-1) and maximum (impact pressure, 9.6 to 11.9 t m-2; velocity, 25.3 to 28.2 m sec-1) avalanche events were calculated for an avalanche path in the Snyder Lake valley, and are considered typical for the study area.

Snow Avalanche Path Terrain and Vegetation, Glacier National Park, Montana

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