Skagit River watershed is the largest draining into Puget Sound and has the most extensive glacial cover of any basin > 5,000 km2 in the US outside of Alaska. We examined the importance of these glaciers to the basin's summer water balance using an empirical approach. In 1959 approximately 396 glaciers covered 170.23 ± 8.50 km2 of the basin. Since then, combined glacier area has decreased by 32.02 ± 1.60 km2 (- 19%), with most of the loss between elevations of 1600 m and 2100 m. Fifty years ago surface melting of snow, firn, and ice from Skagit glaciers provided from 0.440 ± 0.055 to 0.742 ± 0.093 km3 of water in summer (May through September) to the Skagit River at Concrete. Today, the surface melt component has decreased (- 24% ± 9%) and now ranges from 0.333 ± 0.042 km3 of water in cool-wet years to 0.559 ± 0.070 km3 in warm-dry years. Surface melt from the remaining glaciers continues to provide 6–12% of the river's total summer runoff, and roughly twice that fraction during August and September. Cold glacial meltwater is concentrated in tributaries Thunder Creek, White Chuck River, Suiattle River, Baker River, and Cascade River. Between 1959 and 2009 average cumulative annual mass balance of five monitored glaciers was -20.35 ± 3.63 m water equivalent. This has resulted in glacial water volume loss of 3.01 ± 0.69 km3 basin-wide, representing the elimination of ∼ 100 years of fresh water supply for Skagit County at the current rate of consumption.
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Vol. 90 • No. 1