Quantifying Glacier-Derived Summer Runoff in Northwest Montana

Adam Michael Clark, The University of Montana

Abstract

Glacier National Park, located in northwest Montana contains the second largest concentration of glaciers in the U.S. Rocky Mountains. However, total glacier-covered area has shrunk by almost 30% over the past 50 years. This has important implications for local glacier-fed streams, especially during the typically hot and dry summer month of August. This study is the first attempt to quantify the volume of glacier meltwater entering local streams. We simulate glacier-melt generation during the summers of 2009 and 2010 using a modified temperature-index model that includes solar radiation as input. The model was calibrated with data from weather stations installed on five different glaciers and in situ measurements of surface ablation. Simulated glacier-melt hereafter "glacier runoff" from within gauged basins was then compared to measured stream discharge for the month of August to assess the relative contribution of glacier runoff to stream flows. We find the fraction of August stream discharge potentially derived from glaciers is not linearly proportionate to glacier cover. In watersheds only 0.12% glacierized, glacier runoff equals 5%-6% of the August total discharge; glacier runoff is 23%-28% of the discharge in a stream draining a basin 1.4% glacierized; and, in the small alpine catchments where glacier cover exceeds 25% of the basin area, glacier runoff likely accounts for almost all of the total runoff. Our work suggests that during dry summer months, glacier runoff is a primary control on both water availability and water temperature in small alpine basins with a catchment area on the order of tens of km2 and smaller and that are located within 20 km of the continental divide. However, the significance of meltwater diminishes quickly when travelling downstream and that in larger rivers situated 50 km or more from glaciers, glacier runoff is minimally important.