December 26, 2011

Lewis Glacier Disappears, North Cascades, Washington

Posted by Mauri Pelto

In 1985 during my second visit to the Lewis Glacier, North Cascades, Washington was the first time I confronted the idea of a glacier disappearing. We were able to peer down several crevasses and see the bottom of the Lewis Glacier, measurements indicated a maximum depth of 12 meters over an area the size of a football field.

This glacier had been selected for the North Cascade Glacier Climate Projects’s mass balance program(Pelto and Riedel, 2001) assessing mass balance on 10 glaciers across the glacier clad mountain range. It was a small glacier in the drier part of the range, near Rainy Pass. This size made it attractive to observe in terms of response to climate change. The USGS map indicates a significant glacier with an area of 0.12 square kilometers in the 1950’s.  By 1985 (top image) the glacier had lost half of its mapped area, there were still some significant blue ice areas, and areas of firn, snow several years old that is not yet glacier ice. Return visits each summer over the next few years chronicled the demise of the glacier. By 1988 (middle image) the glacier had shrunk dramatically even since 1985, with no area of blue ice even the size of a basketball court, the thickest ice measured was 5 meters. By 1990 the glacier was gone (bottom image), no blue ice left in the basin, the blue arrows indicate the lateral moraine above the now empty glacier basin. At the time I had not developed the model for forecasting glacier survival (Pelto, 2010).














Lewis Glacier 1985


Lewis Glacier 1988


Lewis Glacier 1990

Google Earth imagery from 1998 and 2006 indicate the basin does retain snowcover late into the summer during most summers. However, the areal extent is much smaller than the glacier had been. The blue arrows in the 1998 image indicate the moraine marking the extent of the glacier during the Little Ice Age.  This moraine is quite large-high for such a small glacier, typical vertical extent from cirque basin to moraine crest is 15 m. This moraine was not pushed into place by glacier movement as most are instead it is a combination moraine and pro-talus rampart with rocks travelling down the surface of the glacier via rockfall or avalanching and ending up in the moraine from Corteo Peak above. The red arrows in the 2006 image indicate the moraine crest.  in the summer when all other snow in a basin has typically been gone.  The next glacier to the west has the same type of moraine/rampart below the cliffs of Corteo Peak. Runoff observation conducted below the glacier indicate a 70& drop in August streamflow after glacier loss. This is the principal impact of glacier retreat, a reduction in summer runoff, particularly acute for rivers late (Pelto, 2008)

1998 Google Earth image

lewis glacier 2006

2006 Google Earth image  note moraine

Climbers heading for the north route

Moraine crest below glacier just above Wing Lake and Lewis Glacier.