Examining Snow Avalanche Frequency and Magnitude

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Snow avalanches pose substantial risks to human safety, commerce, and infrastructure in mountainous regions across the globe.  Avalanches also act as drivers of important ecological change by creating and modifying habitat for flora and fauna. To better understand the dynamic processes of avalanches at multiple scales, the USGS Snow and Avalanche project uses a variety of methods to study avalanche magnitude and frequency. By advancing the understanding and predictive capabilities of avalanche disturbance, scientists aim to reduce the hazard to humans and more fully understand the ecological role of avalanches. Results from this project provide land-use planners, natural resource managers, and avalanche forecasters a more thorough understanding of how avalanches act as both a hazard and a driver of landscape change.

Avalanche debris deposit

A researcher stands in front of a large wet slab avalanche debris deposit in Glacier National Park, Montana. The start zone and fracture line can be seen near the top of the ridge in the upper middle part of the image. (Credit: USGS Northern Rocky Mountain Science Center. Public domain.)

What is an avalanche? 

An avalanche is a mass of snow sliding, tumbling, or flowing down an inclined surface. Slab avalanches can be particularly powerful and destructive due to the speed and force of the mass of snow and the rush of air that sometimes precedes the avalanche.  For a slab avalanches to occur, these four conditions are necessary:

  • A sufficiently steep slope (most avalanches occur on slopes between 30 and 45 degrees)
  • A weak layer in the snowpack
  • A slab of snow above that weak layer
  • A trigger initiates the fracture and collapse of the weak layer.  Triggers can be the accumulation of snow, rain, wind loading, or human/animal disturbance

How do avalanches impact society?

Hazard - When avalanches intersect with humans, there can be substantial cost associated with impacts to commerce, damage to infrastructure, and loss of human life. 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 (Voight et al, 1990). They can also impact roads and railways causing substantial damage and disruption to commerce.

Disturbance - Ecologically, avalanches are instrumental in modifying the landscape and adding ecological complexity to mountain ecosystems. By creating and maintaining paths free of large trees, avalanches disturbance creates a mosaic of vegetation types which support a diverse range of plant and animal species. 

Avalanche debris

Snow removal operators work carefully to remove a large accumulation of avalanche debris on Going-to-the-Sun Road in Glacier National Park, Montana. (Credit: USGS Northern Rocky Mountain Science Center. Public domain.)

Challenge – Avalanches are a complex phenomenon due to the complicated interactions of weather, climate, and snowpack structure, and USGS research assessing current and future trends in avalanche activity aims to fill a knowledge gap needed to improve forecasting and public safety, to protect resources, and to lend insight into the ecological role of avalanches.  

Snow and Avalanche Project Research Goals

The ability to improve avalanche forecasting in the context of a changing climate depends on a solid understanding of the interplay between the drivers that contribute to avalanche frequency, magnitude, and character. USGS scientists and their collaborators study avalanches across multiple climatic zones throughout the U.S. Rocky Mountains.  Dendrochronology (the study of tree rings) and remote sensing provide scientists opportunities to assess avalanche frequency and behavior in the context of changing climate.  Research goals seek to address:

1) how avalanche frequency and character vary across space and time 

2) what are the primary climate and atmospheric drivers of avalanche variability

Map of tree-ring data sites from three distinct climate zones

Tree-ring data sites from three distinct climate zones, Maritime (southeast Alaska) Intermountain (northern Rockies) and Continental (central/southern Rockies), allow ​USGS scientists​ ​and their collaborators to understand how regional climate patterns influence avalanches. (Credit: USGS Northern Rocky Mountain Science Center. Public domain.)

Avalanche science – the past informs the future

To understand the relationships between avalanche cycles and climate, scientists can look back in time using dendrochronology, the study of tree rings, and create a chronology of past avalanche events.  Annual growth rings of trees that survive avalanches often hold records of large magnitude avalanche events due to tree damage. This mechanical damage can be visible in many forms including scars, reaction wood, or traumatic resin ducts. USGS scientists collect many tree samples across avalanche path study areas and carefully cross-date the growth rings to build a historic record of large magnitude avalanches for each site.

By coupling these avalanche chronologies with historic climate data, USGS scientists tease apart the topographic and climate factors that contribute to avalanche occurrence at local and regional scales. These techniques assist with the understanding of avalanche cycles in the broader context of atmospheric circulation patterns, such as the Pacific Decadal Oscillation (PDO) or El Niño Southern Oscillation (El Niño and La Niña). Through careful dissection of the connections between past climate and avalanche cycles, USGS scientists aim to provide improved avalanche forecasts to reduce the loss of property and life.  

Results from the northern Rocky Mountains


Additional Resources:

Tree-ring sample from an avalanche path

This tree-ring sample from an avalanche path captures 256 years of data between the years 1777 and 2013. The scars in 1818 and 1974 are examples of mechanical damage caused by an avalanche. The pith is the center of the tree. (Credit: USGS Northern Rocky Mountain Science Center. Public domain.)