Avalanche, the fall of a mass of ice or snow from a mountain slope. Ordinary avalanches are of frequent, even momentary, occurrence in mountain regions, but they are ordinarily so small or so far above the line of habitation that they do no harm. Occasionally, however, immense accumulations slip loose and go crashing down through the forests into the valley below. The Union Pacific Railway protects long stretches of its tracks with strongly framed snow sheds calculated to uphold the weight of snow that comes sliding down all winter. The region most noted for avalanches, or at least the region in which they have been studied most closely, is that of the Alps. Mountain climbers regard the danger from avalanches as the greatest risk they take.