The geography of rock glaciers in the American West

Rock glaciers are relatively common in conterminous United States west of the 100° meridian, yet their distribution and number are not well known. Using aerial and satellite imagery, we inventoried the population of rock glaciers, yielding 2,257 rock glaciers totaling 234.28 km2 with an area of uncertainty of 7 percent. Of these, 1,561 (155.44 km2) are considered intact (moving) and 696 (76.84 km2) relict. Based on published estimates of ice content, total water-equivalent volume is roughly 2.52 km3. Rock glacier elevations increase from the northwest (Washington) to the southeast (Colorado), following a spatial trend of decreasing winter precipitation and increasing summer air temperature. Compared to alpine glaciers, rock glaciers are found in warmer, drier environments where winter precipitation < 1,200 mm, and relict rock glaciers inhabit warmer and drier conditions than intact rock glaciers. Locally, alpine glaciers exist at higher, colder, and wetter elevations than rock glaciers, and intact rock glaciers are found at higher elevations that relict ones. We also noted 7,850 “features of interest” that appeared to be nonflowing ice-rich landforms, many of which are similar in form to protalus ramparts and found in a climate common to relict rock glaciers.