HISTORIC GLACIER CHANGES IN THE AMERICAN WEST

Our GIS analysis of topographic maps indicate that more than 8400 alpine glaciers and permanent snow fields, cover an area greater than 650 km2 in seven western states of the American West, exclusive of Alaska. Evaluation of glacier change in several regions, based on historic maps and photos, indicate steady retreat after an enhanced retreat during the early 1900s. In some regions, the glaciers have not significantly changed due to local topographic settings that enhance snow accumulation through avalanching and wind redistribution. The dominant topographic control on glacier shrinkage is size. Smaller glaciers exhibit greater fractional area loss than larger glaciers. Using area-volume scaling relations, the total volume of melt water lost from the shrinkage of small glaciers (<1 km²) is equivalent to that from larger glaciers. Thus small glaciers and snowfields play an important hydrologic role. Glacier shrinkage reduces their buffering capacity to minimize effects of droughts in alpine basins and reduces their area of influence. In addition to these "clean" glaciers, extensive areas are populated with debris-covered glaciers are important to alpine hydrology. Our preliminary estimates indicate that the area of debris-covered glaciers exceed “clean” ice areas by perhaps 3:1.