Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 174

FIGURE 4.21 An aerial view of avalanche paths immediately north of Hebgen Lake in southwest
Montana. The complex starting zone, well-defined track, and a runout zone that crosses the high-
way are shown for one of the paths. (Photo by K. Birkeland.)
TERRAIN
Avalanches usually occur repeatedly in the same places (referred to as avalanche paths )
because of weather and terrain relationships. The area where an avalanche initiates is
the starting zone, the area through which the avalanche runs is the track, and the area
where deceleration and deposition take place is the runout zone (Fig. 4.21). In timbered
areas, the path and its component parts are usually easy to identify because of the dam-
age to, and clearing of, vegetation (Tremper 2008). The most important terrain factor
for avalanches is the slope angle of the starting zone, with favored angles between 30°
and 45°; the majority of avalanches originate on slopes from 36° to 39° (Fig. 4.22). In
snow climates with moist snowfall, slope angles for avalanche starting zones may be
much steeper than 45°. Avalanches on slopes steeper than 55° occur in some moun-
tain ranges but, typically, continuous sluffing cleans the steeper slopes. Slopes less than
about 30° are generally not steep enough for avalanche initiation, because snow that
fractures cannot overcome the residual friction and slide down the slope, although ava-
lanches in motion can flow down slopes shallower than 10° for quite a distance. The
momentum of large avalanches can even carry snow up the opposite canyon wall. Over-
all, the probability of avalanches increases with steeper slopes up to a certain point and
then decreases.