A comprehensive mid-20th century inventory of glaciers and perennial snowfields (G&PS) was compiled for the American West, west of the 100° meridian. The inventory was derived from U.S. Geological Survey 1:24,000 topographic maps based on aerial photographs acquired during 35 years, 1955-1990, of which the first 20 years or more was a cool period with little glacier change. The mapped features were filtered for those greater than 0.01 km 2. Results show that 5036 G&PS (672 km 2 , 14 km 3) populate eight states, of which about 1276 (554 km 2 , 12 km 3) are glaciers. Uncertainty is estimated at ±9% for area and ±20% for volume. Two populations of G&PS were identified based on air temperature and precipitation. The larger is found in a maritime climate of the Pacific Northwest, characterized by warm winter air temperatures and high winter precipitation (~2100 mm). The other population is continental in climate, characterized by cold winter air temperatures, relatively low winter precipitation (~880 mm), and located at higher elevations elsewhere. The G&PS in the Pacific Northwest, especially in the Olympic Mountains, are particularly vulnerable to warming winter air temperatures that will change the phase of winter precipitation from snow to rain, further accelerating glacier shrinkage in the future. Comparison with a recent inventory suggests that the total G&PS area in the American West may have decreased by as much as 39% since the mid-20th century.

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Abstract. This report summarizes the methods used to construct an inventory of glaciers and perennial snowfields of the contiguous United States. The inventory is based on interpretation of mostly aerial imagery provided by the National Agricultural Imagery Program, US Department of Agriculture with some satellite imagery in places where aerial imagery was not suitable. The inventory includes all perennial snow and ice features greater than 0.01 km2. Due to aerial survey schedules and seasonal snow cover, multiple imagery dates were required. The earliest date is 2013 and the latest is 2020, but more than 73 % of the outlines were acquired from 2015 imagery. The inventory is compiled as shapefiles within a geographic information system that includes feature area and location. The inventory identified 1331 (366.52 km2) glaciers, 1776 (31.00 km2) perennial snowfields, and 35 (3.57 km2) buried-ice features. The data including both the shape files and tabulated results are publicly available at https://doi.org/10.15760/geology-data.03 (Fountain and Glenn, 2022).

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Glaciers and perennial snowfields are important to alpine ecosystems and regional hydrology. Quantifying volume change of a population of glaciers widely distributed over a region is difficult and expensive. We employed NASA’s novel Airborne Glacier and Ice Surface Topography Interferometer (GLISTIN) to rapidly map surface topography of alpine glaciers across the western USA. In five flight days 3289 glaciers and perennial snowfields were surveyed. Comparison with lidar over control sites showed a mean difference of +0.17 ±1.78 m at a spatial scale of 3 m. Data coverage increased and elevation uncertainty decreased with the mosaicking of multiple passes due to the complex terrain. Elevation change since the National Elevation Dataset shows a thinning (and volume loss) over the last ~56 years, averaging -0.3 ± 0.2 m and accelerating since 1980. GLISTIN can be a valuable tool for rapidly mapping ice surfaces in the alpine environment.

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