Winters in Glacier National Park are known for their massive accumulations of snow. The average snowfall in the high country is over 10 feet and snow depth at the 'Big Drift' during spring plowing of the Going-to-the-Sun Road often exceeds 60 feet! Spring weather usually determines whether or not hydrologic activity will be in the headlines. In 1964 a heavy winter snowpack combined with warm spring rains to produce catastrophic flooding over large areas of the Park as well as portions of the Flathead Valley. Floods have also damaged the Polebridge area (on the North Fork) and flooding periodically occurs in the St.Mary River system, especially along Divide Creek near the village of St. Mary. In November 2006, the park experienced severe flooding, requiring extensive re-construction on the Going-to-the-Sun Road as well as many park bridges and trails. Despite the potentially destructive impacts of flooding in developed areas, the sculpting action of water runoff is a natural force that profoundly influences Park landscapes. Without the periodic scouring and cleansing effect of floods and high water, stream channels would fill with sediment and silt and become clogged with woody debris. Substrate gravels on stream bottoms, essential habitat for fish spawning, would be covered over with silt rending them unsuitable for egg incubation. Moreover, they could not sustain aquatic invertebrates which comprise an important food item for fish.