Patterns of glide avalanche release: Insights from 23 years of observations along the Going-to-the-Sun Road, Montana, USA

Erika Hart Birkeland; Maddey Jude Frey; Sarah Elizabeth Williams; Erich Peitzsch

doi:10.15788/1790098947

Home/Archive/USGS

USGS · 70283052

Patterns of glide avalanche release: Insights from 23 years of observations along the Going-to-the-Sun Road, Montana, USA

Erika Hart Birkeland·Maddey Jude Frey·Sarah Elizabeth Williams·Erich Peitzsch

View original source ↗Export metadata ↓

Abstract

Glide avalanches pose a notable hazard during annual spring snow removal operations along the Going-to-the-Sun Road (GTSR) in Glacier National Park (GNP), USA. Glide cracks consistently form on specific slopes near and above the GTSR. Some glide crack locations produce glide avalanches most years (known as repeater paths), whereas others are less predictable and either release or melt in place. Previous studies of glide avalanches have focused on different meteorological, snow-pack, and terrain parameters that influence glide release. The timing of glide avalanche release remains difficult to predict, which can be a challenge. To the authors' knowledge, no previous work has approached the issue of timing by comparing the chronological release order of glide paths within an operation. To address this, we examine the release order of repeater paths along the GTSR and investigate different topographical factors that may influence release timing. To investigate the timing of glide avalanche release, we analyzed a dataset of annually recorded glide avalanches from 2003 to 2025 along the GTSR from mid-March to June. We identified 359 glide avalanches with starting zones covering all aspects between 1,600 and 2,700 meters, and performed analysis on topographic factors including elevation, aspect, slope angle, and bed surface. Additionally, we assessed the likelihood and sequence of the release of seven repeater paths. Seven repeater paths along the GTSR generate glide avalanches nearly every year (65–100% of the study period), with two paths (Peak 7774 and Peak 7850) releasing first in over half of the years observed (p-value < 0.01). These paths have been informally recognized as "indicator paths," though this distinction has not been quantified until now. Identifying factors that drive the timing of glide avalanches using a days-scale operational signal can support the safeguarding of infrastructure and personnel. By examining release order and related variables, this study provides practical insights to inform operations managing glide avalanche hazards. Our methods may help forecasters pinpoint indicator paths and potentially recognize the onset of glide avalanche cycles, offering a framework for other operations.

Explore related subjects

Conference Paper

Source-reported bounding extent: 48.61159695691762° to 48.824724984597566° latitude; -114.00684065662138° to -113.5122958000857° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

Erika Hart Birkeland, Maddey Jude Frey, Sarah Elizabeth Williams, Erich Peitzsch. 2026-09-28. Patterns of glide avalanche release: Insights from 23 years of observations along the Going-to-the-Sun Road, Montana, USA. https://doi.org/10.15788/1790098947

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Stability changes after weak layer collapse: Insights from an unexpected dataset

While conducting avalanche mitigation or traveling through the backcountry, avalanche professionals sometimes experience a "whumpf": a collapse-mode crack accompanied by surface cracking that does not produce an avalanche. These whumpfs are a clear warning sign of instability in the surrounding terrain. But assessing the collapsed slope itself, if it is steep enough to avalanche, carries much more uncertainty. Can slopes that whumpf and are steep enough to slide still avalanche? The answer to this question supports avalanche mitigators and ski guides in assessing whether to open such slopes. Previous research on collapsed slopes is limited to one study conducted before tests that estimate crack propagation propensity, such as the Propagation Saw Test (PST), were developed. This work showed that weak layer shear strength decreased immediately after collapsing and then increased over time. However, no research has examined crack propagation propensity after a whumpf. Studying propagation propensity is key for assessing how whumpfs immediately affect slope stability. To improve our understanding of changes in propagation propensity following weak layer collapse, we conducted PSTs on a test slope in the hours and days after an unexpected collapse occurred on a layer of surface hoar. Two PSTs at our study site three days prior to this collapse provided an estimate of weak layer propagation propensity. PST cut lengths decreased immediately following the collapse, suggesting that the collapse increased the propagation propensity. However, over the next 4-5 hours, PST cut lengths steadily increased at our site, likely due to ongoing sintering and strengthening in the collapsed weak layer. By the end of day two, we were no longer able to initiate propagation, suggesting substantial stabilization of the weak layer. Our results support prior findings and observations suggesting that slopes may remain capable of producing avalanches immediately after a collapse but likely stabilize over the next day or two. Although this research does not provide answers about precisely when a slope becomes stable after a collapse, it does highlight uncertainties in our understanding and interpretation of the stability of slopes that have whumped.

Montana↗

USGS2026

Revisiting avalanche destructive scale and associated mass estimates

Avalanche destructive (D) size classifications are essential for forecasting, communication, and risk management, yet size estimation remains one of the more subjective elements of avalanche assessment. Practitioners typically rely on visual impressions of destructive potential, path length, and estimated mass and volume. Recent work indicates that size is often underestimated even by experienced observers, especially when based on photographs or distant observations. Although mass is central to defining destructive size, the empirical foundations of mass–size relationships have rarely been revisited, and expert variability in size assignment could be decreased if more consistent operational tools were available. We evaluated whether a simple, practitioner-focused calculation tool using slab dimensions, snow density, and snow water equivalent can provide reliable mass estimates that correspond meaningfully with existing destructive size classes. Using this tool, we estimated avalanche mass and compared the results to observer-assigned D sizes. We also simulated mass outputs using a dense flow avalanche simulation module and compared observer estimates and simple-tool outputs to recent volume-based methods for D-size classification. Across 143 well-documented dry snow avalanches from multiple avalanche climates, the size estimation tool tends to classify avalanches larger than field-based observers. We found a mean bias of 0.47 D-size units and a typical (mean absolute) disagreement of 0.56. Our results show that 95% of events landed within one whole D-class of each other and 71% within a half-class. Agreement between the simple tool mass estimates and dense flow avalanche simulations was generally strong, though discrepancies emerged in cases with substantial entrainment or complex terrain. We compared the size tool to the simulation module with entrainment (n=4) and without entrainment (n=8). With entrainment, the two methods estimate the same D-size class in 4 of 4 comparable events and 7 of 8 events without entrainment. Also, the simulation module estimates larger sizes than the observer in 7 of 8 cases without entrainment and all 4 cases with entrainment. These findings support that development of practical, mass-based tools could improve D-size consistency and emphasize the value of revisiting historical mass–size relationships. The accompanying application offers practitioners an accessible and operationally useful resource for improving D-size estimation from field observations.

Conference Paper↗

USGS2026

Start-zone characteristics and timing of wet slab avalanches In Colorado

Wet slab avalanches (WSAs) release when liquid water reduces the strength of a buried weak layer or interface, and their timing is governed primarily by the snowpack energy balance and the movement of meltwater. We classified 717 WSAs from the Colorado Avalanche Information Center database (2011–2025) into five start-zone terrain categories and analysed the timing of naturally triggered, near- and above-treeline events using rank-based and Bayesian hierarchical methods. Start-zone terrain strongly governs the natural occurrence of WSAs: approximately 64% released in Rocky start zones (Cliffs/Rocks), while only ≈4% occurred on Open Slopes. After controlling for latitude and aspect, Rocky start zones released approximately five days before Open Slopes (90% CrI: 0.6–9.9 days). Latitude was the dominant timing predictor, shifting release by roughly 3–4 weeks from southern to northern Colorado. Cornice start zones concentrated WSA activity in early to mid-spring, with 40 of 41 releasing before mid-May. These findings support start-zone terrain classification as a low-cost addition to observation protocols for improving the spatial targeting of wet slab outlooks.

Colorado↗