Climate Influences on, and Interannual Variability of, Natural Avalanches in Three
Avalanche Paths in Glacier National Park, Montana
1,2 and C. J. Caruso2
B. A. Reardon1, D. B. Fagre1, G. T. Pederson1,2
1U.
Background
ABSTRACT: We compiled chronologies of natural avalanche activity in three avalanche paths in John F. Stevens Canyon, at the southwestern corner of Glacier
National Park, for a 97-year period (1910-2005). Natural avalanche activity in these paths is concentrated in January and February, so we compared avalanche
occurrence in each path with mean January through February Pacific Decadal Oscillation (PDO) and ENSO Southern Oscillation (ENSO) indices and March 1st snow
water equivalent (SWE) and snow depth anomalies at a nearby Natural Resources Conservation Service (NRCS) snowcourse. We ranked the climate indices for the
97-year period, then grouped them into terciles. We then tallied the frequency with which an avalanche winter occurred in each tercile for the individual paths and for
the paths as a group. The results show that avalanche occurrence in the individual paths has few consistent relationships with the climate indices and snowpack
anomalies. Considered as a group, however, avalanches occurred more frequently in winters with negative PDO indices, neutral ENSO indices and positive SWE and
depth anomalies. Avalanches occurred in 18 of 32 negative PDO winters, 21 of 30 ENSO neutral winters, and 16 of 25 winters with positive snowpack anomalies.
During the 97-year period, there were 17 winters in which avalanches occurred in more than one of the three avalanche paths; eight of these coincided with PDO
negative years or years with positive snow depth anomalies and seven with neutral ENSO conditions. The coincidence of positive SWE anomalies, negative PDO and
neutral ENSO indices with winters with avalanche years common to two to three avalanche paths suggests that above-average snowfall driven by climate oscillations
may result in more widespread avalanche activity throughout a winter, with correspondingly greater economic and ecological effects. Thus, natural avalanches may be
a mechanism through which climate has large scale and long-term ecological effects on montane forests.
S. Geological Survey Northern Rocky Mountain Science Center
2Big Sky Institute - Montana State University
Are avalanches more common in some climate
patterns?
To investigate this question, we compared
avalanche occurrence in Stevens Canyon with
seasonal scores for two climate patterns.
•We developed avalanche chronologies for
three avalanche paths for the 1910-2005
period by combining tree-ring chronologies
with historical records. We also compiled a
historic record of all avalanches that disrupted
transportation in the canyon during the same
period.
Do climate patterns influence the variability of natural snow avalanches?
•Generally considered random natural hazards.
•Known to have important ecological effects.
•For each path, the paths as a group, and the
canyon as a whole, we designated each year
as either an avalanche year or a nonavalanche year.
•May be mechanism by which climate shapes montane forests.
Answer requires long-term record of natural avalanches.
•Most records of natural avalanches from sites where avalanches controlled through explosives or
skier compaction
•Because natural avalanche activity in
Stevens Canyon is concentrated in January
and February, we calculated mean January
through February PDO and Nino 3.4 scores
for each winter, as well as a combined PDONino 3.4 score for each winter.
•Transportation infrastructure in Stevens Canyon, at southwestern corner of Glacier National Park,
provides nearly century-long record.
What climate patterns might influence natural avalanche variability?
•We ranked the seasonal scores for the 97year period, then grouped them into terciles.
The highest terciles correspond with warm
phase PDO or El Nino conditions and the
lowest with cool phase PDO or La Nina
conditions.
•Pacific Decadal Oscillation (PDO) is the primary source of interannual snowpack variability.
•El-Nino-Southern Oscillation (ENSO), particularly the Nino 3.4 region, is a secondary influence on
snowpack variability (McCabe and Dettinger, 200?; Selkowitz et al, 2002).
Study Site and Data Sources
Historic Record
•97 winters (1910-2006)
•Compiled from variety of primary sources
•Great Northern Railway records
•Glacier NP ranger logs
•Montana Dept. Of Transportation logs
•Local newspapers
•No consistent period of record or data
recording
Shaded Relief Map of Stevens Canyon showing outlines
of avalanche paths (blue), snowsheds (red), and
transportation infrastructure.
•Bias towards large-magnitude avalanches
that damaged infrastructure, injured or killed
workers, or disrupted traffic in canyon.
Climate Records
•PDO: http://jisao.washington.edu/pdo/PDO.latest
•ENSO: http://www.cgd.ucar.edu/cas/catalog/climind/TNI_N34/index.html#Sec5
Tree-Ring Record
•Chronologies for three avalanche
paths developed in previous studies
Photos of the three avalanche paths that
were the focus of this study: Shed 7 (a, top),
Shed 10.7 (b, lower left), and Goat Lick (c,
lower right). Photos by Darwon Stoneman.
• We then tallied the frequency with which an
avalanche winter occurred in each tercile for
the individual paths, for the paths as a group,
and for the entire canyon.
Avalanche
Path
Shed 10.7
Goat Lick
Shed 7
Any of 3
paths
2 or more
paths
Any path in
canyon
Highest
Tercile
7
2
10
12
PDO Score
Middle
Tercile
11
7
7
16
Lowest
Tercile
9
6
14
18
Highest
Tercile
8
0
5
10
Nino 3.4 Score
Middle
Tercile
11
8
13
21
•Prior to conducting the T-Test, we examined each group for equal variance and normal
distribution.
Comparisons for PDO years involved 97 years; comparisons for Nino 3.4 and PDO + Nino 3.4
involved 92 years.
Avalanche Path(s)
Shed 10.7
Goat Lick
Shed 7
Any of 3
paths
2 or more of
three paths
Any path in
Canyon
27 avalanche
years
15 avalanche
years
32 avalanche
years
49 avalanche
years
18 avalanche
years
54 avalanche
years
Est. of Diff:
p-value:
Est. of Diff:
p-value:
Est. of Diff:
p-value:
Est. of Diff:
p-value:
Est. of Diff:
p-value:
Est. of Diff:
p-value:
PDO Index
Nino 3.4
Index
PDO + Nino
3.4 index
-0.0719
-0.0303
-0.149453
0.757
0.922
0.748
-0.608000
-0.509777
-1.32951
0.091
0.015
0.006
-0.363579
-0.294411
-0.787603
0.115
0.237
0.044
-0.392949
-0.131404
-0.593413
0.057
0.597
0.128
-0.388657
-0.476785
-0.994729
0.206
0.142
0.061
-0.420579
-0.353579
-0.804076
0.039
0.17
0.044
•Results again showed little consistent relationship between individual avalanche paths and
climate scores.
•Avalanche years showed greatest difference from non-avalanche years for the combined
PDO + Nino 3.4 score.
•Avalanche years tended to show more statistically significant difference from non-avalanche
years as the number of avalanche paths increased, though test results were generally not
significant or borderline significant at the 95% confidence level.
Lowest
Tercile
6
5
9
12
4
5
8
3
7
5
15
18
21
14
19
18
Individual value plots and boxplots for combined PDO and Nino 3.4 scores for Shed 7 (left),
2 or more of the 3 avalanche paths (middle), and the entire canyon (right). Though scores
were generally lower for all three groups, the difference between avalanche and nonavalanche years was only significant for Shed 7 and the canyon as a whole.
•Different methodologies and
periods of records
Conclusions
1990
•The precision of the data was not consistent at all levels of the analysis. Data for some
avalanche paths, particularly Goat Lick, was biased towards large magnitude avalanches due to
inconsistent recording and lack of tree-ring data.
1985
1993
Figure 7: Dated cross section taken from
avalanche path Shed 10.7. Note the
scarring, reaction wood and years that
correspond to injuries and growth
anomalies.
To determine whether there was significant difference between avalanche years and nonavalanche years, we conducted 2-sample T-Tests comparing avalanche years for individual
avalanche paths, the avalanche paths as a group, and all avalanche paths within the canyon.
The results show that avalanche occurrence in the individual paths has few consistent
relationships with the climate indices anomalies. Considered as a group, however, avalanches
occurred more frequently in winters with negative PDO indices and neutral ENSO indices.
(Butler and Malanson, 1985; Reardon et al, submitted).
1993
Are avalanche years significantly different than nonavalanche years?
Examples of sources used to create
the historical record: clipping from
the Hungry Horse News, 1950 (top)
and 1911 photo (bottom; courtesy
of Stumptown Historical Society).
•It appears that relationships with PDO and Nino 3.4 grow clearer and more significant when
larger numbers of avalanche paths are included in the comparisons. This suggests that these
climate patterns influence natural avalanche frequency by creating more widespread avalanche
activity across a landscape.
Graph showing sample depth by year and years
avalanches recorded by trees in Shed 10.7 avalanche
path.
•An accurate assessment of how climate patterns such as PDO and ENSO influence interannual
variability of natural avalanches would require tree-ring based avalanche chronologies from
multiple avalanche paths in a drainage.