May 15, 2011

Ptarmigan Ridge Glacier Retreat and Separation, North Cascades

Posted by Mauri Pelto

Ptarmigan Ridge Glacier is nestled on the north side of Ptarmigan Ridge in the North Cascades of Washington. This is a small glacier that has been retreating and has separated into several smaller glaciers over the last 20 years. From 1984-2010 we have visited this glacier every year assessing the terminus in half of those years, in others it has still been buried by snowpack. The main terminus has retreated 120 meters. Given the total maximum glacier length is 360 m in 2010, this is a substantial loss. The sequence of images below indicate the retreat from the top a map based extent 1979, to a 1993 USGS aerial photograph middle, and the 2009 satellite image from Google Earth. The glacier boundary from the map is in magenta. . The glacier retreat has been associated with an earlier melt out of this alpine region in general. This has been beneficial to the local mountain goats with a sharp population rise after 2002. We do a mountain goat census here every year at the request of the United States Forest Service. Despite the population increase along the ridge there is no sign of overgrazing. A closeup look at the terminus region illustrates not just retreat, but thinning as well when comparing the 1993 and 2009 imagery. The locations of thinning are shown by M thumbtacks in the bottom image. The terminus remains thin even on the steep slope as seen in the 2005 image of the terminus below. Above the terminus the steep slopes are typically well snowcovered into August as in 2007 second image. In 2011 a La Nina winter was predicted to lead to substantial snowfall and a cool winter inn the Pacific Northwest. The first half of the winter did not live up to the forecast, but the second half right through April did. The result is extensive snowpack. A comparison of the snowpack at Lyman Lake the best high altitude site that the USDA Snotel system has in the North Cascades indicates that as of May 10 the snowpack is quite high in 2011 (bottom image). It is not unusual at this site to have snowpack not decline significantly before May 20. This suggests that summer will not feature a substantial melt out of the Ptarmigan Ridge Glacier. This is my daughter Jill’s favorite field area and this year maybe we will not have to paint on a positive mass balance.