Skykomish River, Washington: Impact of ongoing glacier retreat on streamflow



Glacier retreat and changes in summer runoff have been pronounced in the Skykomish River Basin, North Cascades, Washington from 1950 to 2009. An analysis comparing USGS streamflow records for the 1950–1985 to the 1985–2009 period indicates that during the recent period the Skykomish River summer streamflow (July–September) has declined 26% in the watershed, spring runoff (April–June) has declined 6%, while winter runoff (November–March) has increased 10%. The minimum mean monthly August discharge from 1928 to 2010 occurred in 2003 and 2005 when streamflow was 15·1 and 15·2 m3s−1, respectively. From 1929 to 1985, streamflow was less than 14 m3s−1 during the glacier melt season on a single day in 1951. From 1986 to 2007 there were 217 days with discharge below 14 m3s−1 with 9 periods lasting for 10 consecutive days. In the Skykomish River watershed from 1958 to 2009, glacier area declined from 3·8 to 2·1 km2. Columbia, Foss, Hinman and Lynch Glacier, the primary glaciers in the basin, declined in area by 10, 60, 90 and 35%, respectively, since 1958. Annual mass balance measurements completed from 1984 to 2009 on Columbia, Foss and Lynch Glacier indicate a mass loss of 13·1 m w.e. Despite 15% higher ablation rates during the 1985–2009 period, the 45% reduction in glacier area led to a 38% reduction in glacier runoff between 1958 and 2009. The 38% reduction in glacier runoff did not lead to a significant decline in the percentage summer runoff contributed by glaciers under average conditions; the contribution has remained in the range of 1–3% from July to September. The glacier runoff decline impacted river discharge only during low flow periods in August and September. In August 2003 and 2005, glacier ablation contributed 1·5–1·6 m3s−1 to total discharge, or 10–11% of August discharge. While declining glacier area in the region has and will lead to reduced glacier runoff and reduced late summer streamflow, it has limited impact on the Skykomish River except during periods of critically low flow, below 14 m3s−1 when glaciers currently contribute more than 10% of the streamflow. Copyright © 2011 John Wiley & Sons, Ltd.