Using tree-rings to unravel avalanche frequency and associated climate drivers in the northern Rocky Mountains
In the western United States, avalanches are the most frequently occurring lethal form of mass movement and, on an annual basis, cause more fatalities than earthquakes and all other forms of slope failure combined. In addition to their effects on human safety and commerce, avalanches are a major driver of ecological disturbance by modifying habitat for plants and animals. There are numerous geophysical causes of avalanches with often complex interactions, including weather, climate, and snowpack structure, that make avalanche prediction difficult. To improve public safety and protect resources, a better understanding of the spatio-temporal relationships of avalanches is needed. Understanding avalanche frequency and magnitude of the past helps contextualize current and future avalanche behavior, including impacts of climate shifts from cold and dry conditions to warm and wet on avalanche type and timing. Under a warming climate, destructive wet snow avalanches pose an increasingly frequent threat to humans, yet they are poorly understood.
Avalanches tend to recur at the same locations on mountain slopes, and the amount of time between avalanche events at a location is called the avalanche return interval. Long-term, reliable, and consistent avalanche observation records are necessary for calculating avalanche return intervals, which are needed for infrastructure planning, avalanche forecasting operations, and natural resource protection in mountainous regions. However, such records are often sparse or non-existent. Therefore, to infer avalanche frequency, scientists are using tree-ring records from trees within avalanche paths to document the frequency of past events and extrapolate results from many trees in numerous avalanche paths to determine avalanche frequency over regional scales. Recently, USGS researchers and collaborators used tree-rings to:
- reconstruct a long-term avalanche chronology in the northern Rocky Mountains,
- examine the frequency of large magnitude avalanches at the regional and individual avalanche path scales, and
- identify specific seasonal climate or atmospheric circulation variables that contributed to years with large magnitude avalanche events across the region.
Results from the study show that approximately every five years there is widespread large magnitude avalanche activity across the region, and that a decreasing trend in the probability of large magnitude avalanches across the region from 1950-2017 exists. Historically, large magnitude avalanche years in the region were characterized by stormy winters with above average snowpack, but over recent decades avalanche years were increasingly influenced by warmer temperatures and a shallow snowpack, lowering the chances of a large avalanche.
As continued climate warming drives further regional snowpack reductions in the northern Rocky Mountains, our results suggest that large avalanches will be less frequent during winters with large snowpacks and that there’s a potential for more large avalanches when temperatures warm and spring precipitation increases. These results highlight how tree-ring records can provide data on the potential scale and spatial extent of avalanches as well as the influence of climate on avalanche occurrence. Information from this study is used by the National Park Service, USDA Forest Service, and state agencies to guide decision making on infrastructure planning, resource management, and public safety.
Two papers from this work, “A regional spatiotemporal analysis of large magnitude snow avalanches using tree rings” and “Climate drivers of large magnitude snow avalanche years in the U.S. northern Rocky Mountains” were published in Natural Hazards and Earth Systems Science and Nature Scientific Reports, respectively.