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The retreat of glaciers (see PDF at end of page) in Glacier National Park, Montana, has received widespread attention by the media, the public, and scientists because it is a clear and poignant indicator of change in the northern Rocky Mountains of the USA. In 2017, the USGS and Portland State University released a dataset which describes the areas of the 37 named glaciers in Glacier National Park and two glaciers on the U.S. Forest Service’s Flathead National Forest land. Glacier areas are described for 1966, 1998, 2005 and 2015/2016, marking 49 years of change for most of the glaciers and 50 years of change for a few. The difference in record length is due to adequate satellite data not being available for a few glaciers in 2015.
Glacier Margin Time Series Results for the Named Glaciers of GNP (1966, 1998, 2005, 2015/2016):
· All glacier areas reduced between 1966 and 2015
· Average area reduction between 1966 and 2015 = 39%
· Largest area reduction (Boulder Glacier) = 85%
· Smallest area reduction (Pumpelly Glacier) = 10%
· In 2015, 26 of the named glaciers remain large enough to be considered glaciers (see "Glacier Area Information Table" PDF at end of page)
Glacier areas were determined by digitally mapping the perimeters of the glaciers in late summer when seasonal snow melted sufficiently enough to reveal the extent of the glacial ice. Digital aerial photography and satellite imagery were used with a Geographic Information System (GIS) to conduct the mapping. Terrestrial photographs taken from nearby ridges and summits were also used as references. Site visits were made to a number of the glaciers over several years to investigate portions of glaciers that were covered by rock debris which made delineation from aerial photographs and satellite images difficult.
New Insights From Satellite Imagery
Using satellite imagery for analysis in recent years provides much higher resolution and greater clarification of glacier margins when compared to aerial imagery. In several cases, the high-resolution 2015 satellite imagery was used to help map glacier margins from previous aerial analyses where rock debris covering ice had been excluded from the ice perimeter or where heavy shading had made margin determination difficult. One example of this is the identification of a large portion of Siyeh Glacier that was previously unidentified. For this glacier, the aerial photos of 1966, 1998 and 2005 were heavily shaded and the only identifiable ice was mapped as a small horseshoe-shaped glacier along the headwall of Mount Siyeh. However, 2015 satellite imagery allowed scientists to discover that rock debris obscured more than half of the actual glacier area, accounting for a much larger glacier footprint than previously mapped. A site visit confirmed these findings and the area increase was updated for the previous years, which meant that Siyeh ice was large enough to be considered a glacier for all years in the time series. The previous number of named glaciers in Glacier National Park had been documented at 25, but satellite imagery has improved the accuracy of analysis and revealed that 26 of the named glaciers remain large enough to be considered glaciers in 2015. The availability of satellite imagery for the purpose of tracking landscape change, such as glacier area, will allow the USGS to update margin data more frequently and with greater accuracy.
Appropriate use of these data include comparing Glacier National Park's glacier areas between the years included in the USGS data release, and from different parts of the world. The change in glacier area can be used to test models of glacier-climate interactions, and to estimate glacier contributions to streamflow.
Area Change Does Not Reflect Volume Change: Interpretation of the glacier area data requires careful consideration of characteristics inherent of small alpine glaciers, such as those in Glacier National Park. The first is that the glacier margins, and calculated area, reflect only the footprint of the glaciers. Ice depth data does not exist for the majority of the glaciers so ice volume cannot be directly calculated. Glacier thickness can change in ways that may not be captured by area data alone. For example, a thin glacier can melt away quickly, resulting in substantial ice loss, while a neighboring glacier that is twice as thick will take longer to melt despite the same climate influences. Although in general, larger area glaciers will have more volume, interpreting absolute size between individual glaciers based on area data alone must be done with caution. Similarly, glacier area change does not necessarily reveal immediate or absolute glacier change.
Local topographic features influence retreat rates: Each glacier is influenced by local variables associated with its unique location. Debris-cover, elevation, aspect, ice flow, and the presence of a lake or other water body at the glacier’s edge, may impact glacier retreat to varying degrees. Another consideration is that, for some glaciers, continued warming begins to have less influence on glacier size once the (now smaller) glaciers retreat to the upper confines of a cirque basin. These glaciers are positioned under steep slopes and headwalls where shading slows melting, and snow avalanches and wind-drifted snow add mass to the glacier. As the overall glacier area shrinks, these factors impact a much larger proportion of the glacier area. The net result is that these compensating factors can cause ice retreat and glacier area contraction to slow down, even as regional air temperatures continue to increase.
Glacier Area Information Table - named glaciers of GNP and Flathead National Forest (see PDF directly below)
Back to Status of Glaciers in Glacier National Park
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Repeat photography and aerial / satellite photo analysis provide evidence of glacier loss in terms of shape and area. The USGS Benchmark Glacier project has collected mass balance data on a network of glaciers in Alaska, Washington, and Montana for decades, quantifying trends of mass loss at all sites. Extensive field data collection at these sites includes twice yearly visits to measure seasonal...