Colorado does host a small, finite collection of ice that meets the scientific criteria of a glacier, though these masses are far removed from the massive valley glaciers that carved the Rocky Mountains. These remnants persist primarily in the highest peaks of the Front Range, shielded from direct sunlight and insulated by deep snow drifts. Understanding their presence requires a precise definition, as many named features are actually permanent snowfields or other forms of perennial ice.
Defining a True Glacier
A true glacier is not merely a patch of ice that survives the summer melt season. Scientific classification requires a persistent body of dense ice that exhibits internal movement under the pressure of its own weight. This deformation and flow, known as internal plastic deformation and basal slip, differentiates an active glacier from a static ice mass. The ice must be thick enough to compress accumulated snow into dense glacial ice, allowing it to move downslope due to gravity.
The existence of a glacier depends on a positive mass balance, meaning the accumulation of new snow and ice must exceed the loss from melting and evaporation (ablation). The vast majority of Colorado’s glaciers are classified as cirque glaciers, which are relatively small, bowl-shaped ice masses. These glaciers form in sheltered, amphitheater-like basins high on mountain slopes, where topography enhances snow collection and steep walls provide shade. These unique microclimates allow these small ice bodies to flow, even minimally, in a typically arid, temperate climate.
Identifying Colorado’s Active Glaciers
While the United States Geological Survey (USGS) officially lists over a dozen named glaciers in Colorado, glaciologists agree that only a handful are still active. These features are concentrated almost entirely in the high peaks of the Front Range, specifically within Rocky Mountain National Park and the neighboring Indian Peaks Wilderness.
Among the most consistently recognized as truly active are Arapaho Glacier and Andrews Glacier. Arapaho Glacier, situated southeast of North Arapaho Peak, is historically considered the state’s largest and most robust, with researchers observing evidence of internal movement. Andrews Glacier, located below Otis Peak, retains the characteristics of an active, flowing ice body, exhibiting signs like crevasses and a cloudy, glacially-fed tarn.
Conversely, popular destinations like St. Mary’s Glacier, which retains the historic name, are generally classified as semi-permanent snowfields or ice patches that no longer flow under their own weight. The total number of true, actively deforming glaciers in the state is likely fewer than 10. The two most prominent examples serve as the last verifiable remnants of Colorado’s glacial past.
The Status of Colorado’s Glacial Ice
The small, remaining glaciers in Colorado are highly sensitive to regional warming and are experiencing recession. Scientific monitoring by the USGS and university researchers shows a clear trend of shrinking area and volume loss linked to rising average temperatures and reduced winter snowpack. Arapaho Glacier, for instance, lost 52% of its surface area during the 20th century. Projections suggest it could disappear entirely within a few decades if the current rate of loss continues.
Another monitored example, Arikaree Glacier near Niwot Ridge, was observed to be thinning by approximately three feet per year over a 15-year period. This rapid loss is compounded by regional climate data showing that April 1 snow water equivalent—a measure of the seasonal snowpack—has been consistently lower across the state’s major river basins compared to the mid-20th-century average. The loss of these ice masses impacts local ecosystems, as glacial melt provides a sustained source of cold water to alpine streams throughout the late summer.
Colorado is home to an estimated 3,500 rock glaciers, which are a mixture of ice and rock debris that also move downslope. These features are far more numerous and often contain a greater volume of ice than the exposed ice glaciers. The thick layer of rock debris on the surface acts as an insulating blanket, protecting the underlying ice from solar radiation. This makes rock glaciers significantly more resilient to climate change and a stable source of cold water for the headwaters of many streams.