Glacier-derived August runoff in northwest Montana

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This article is part of the Fall 2015 issue of the Earth Science Matters Newsletter.

The second largest concentration of glaciers in the U.S. Rocky Mountains is located inside and immediately west of Glacier National Park (GNP), Montana. Here, mountain glaciers are water reservoirs that store winter precipitation as snow and ice and then release meltwater during the summer months. Late summer is typically hot and dry in this region. It is also a time when much of the previous winter’s snowmelt has melted and run off.  By August, glacier meltwater can help maintain base flows in streams and keep water temperatures cool during seasonal droughts. 

The total glacier-covered area in GNP has decreased by ~35% over the past 50 years, a trend that epitomizes the impacts of a warming climate on the landscape.  Since cold water sourced from melting glaciers is considered a vital natural resource, the shrinkage and potential loss of glaciers has raised substantial concerns about declining stream flows, warming water temperatures, and the associated ecological impacts.

outlet streams below Old Sun Glacier

Outlet streams below Old Sun Glacier, Glacier National Park. Continued glacier loss in the park will likely lead to significantly reduced stream flow in the future.

(Credit: Adam Clark, USGS. Public domain.)

The loss of glaciers in GNP has been well documented, and there is plentiful previous work describing the possible consequences of diminishing glaciers here and elsewhere.  However, until now no study had quantified glacier melt characteristics explicitly across GNP, leaving important scientific uncertainties associated with the hydrological and ecological processes of these glaciers. In a recent study, researchers from both the University of Montana’s Geosciences Department and U.S. Geological Survey worked in a partnership to address these uncertainties for GNP.  Together they developed a regional glacier melt model that was calibrated with weather and glacier melt measurements on five remote glaciers in the park.  Because the glaciers can only be reached on foot, the scientists used an innovative weather station design, which allowed the stations to be broken down and carried in backpacks across rugged mountainous terrain. 

Results from the model, which was run for the months July, August, and September during years 2009 and 2010, produced robust estimates of the summer meltwater production by glaciers.  The researchers found that during the month of August, glaciers in the region produce approximately 25x106 m3 of potential runoff.  They then estimated the glacier runoff component in five gaged streams sourced from GNP basins containing glaciers.  Glacier-melt contributions ranged from 5% of runoff in a basin where glaciers only covered 0.12% of the watershed catchment to more than 90% in a basin where glaciers covered 28.5% of the catchment.  The model results indicated that glacier loss will likely lead to lower discharges and warmer temperatures in streams draining basins greater than 20% glacier-covered.  Lower flows could be expected in streams draining basins with as little as 1.4% glacier cover if glaciers continue to shrink.

The paper, published in Arctic, Antarctic, and Alpine Research is available at: http://aaarjournal.org/doi/pdf/10.1657/AAAR0014-033.

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