September 22, 2010
Rainbow Glacier Mass Balance
Posted by Mauri Pelto
In 2010 at the end of a four day period of cool rainy weather we hiked into to our base camp on Ptarmigan Ridge to measure the mass balance of the Rainbow Glacier on Mount Baker in the North Cascades of Washington for the 27th consecutive year. Below is a view of Rainbow Glacier as we approach it. This is a valley glacier that begins on the slopes of Mount Baker at 2200 m and descends to a terminus that is often avalanche covered at 1350 m. The year proved to be the most variable in terms of glacier mass balance of any of our 27 years. Assessing the mass balance requires melting the extent and depth of snowpack on the glacier. We had a chance to measure the snow depth in 121 locations using crevasse stratigraphy and probing. The below image has all of the measurement locations, blue dots, and the rough contours of mass balance marking the snowline in green-blue, the 1 meter of snowpack water equivalent (swe) in purple and the 2 m of swe in blue. Glacier margin is in orange-brown.. The initial field assessment of mass balance for the Rainbow Glacier in 2010 was +0.81 m. At this time the significant melt season is at an end, new snow is projected for tomorrow 9/23. The average over the previous 26 years has been -0.40 m/year. Of the ten glaciers we monitor there was a split with six having negative balances and four positive, the variation is unusual. The probe is a half inch diameter steel rod that is easily driven through the snowpack until the hard icy layer marking last years summer surface is reached. This can either be bare glacier ice or the firn from the previous year. In either case it cannot be penetrated. The second means is to lower a tape measure down the wall of a vertically sided crevasse. This provides a two dimensional measure and view of snow depth versus the point measurement of probing. By late summer the density of the snowpack is uniform in the North Cascades. We survey the blue ice regions using a GPS to map the boundaries. Melting is assessed by observing the progressive ablation of snow and ice. On Rainbow Glacier snowpack was normal below 1800 m, where probing is dominantly used. The snowline was at 1450 m in early August and had risen to 1600 m by late September. Above 1600 m the snowpack increased very rapidly this year from 1.5 m at 1800 m to 5.5 m at 2100 m. This reflects the unusually warm winter that led to a dearth of snow below 1800 m by winters end. Above this elevation several winter events that were rain below were snow. Than melting was well below normal in the summer of 2010. Again spring snow storms retarded melt above 1800 m, while those were rain events below this elevation.
Crevasse stratigraphy was the dominant tool of measuring snowpack on the Rainbow up to is divide with Mazama Glacier. Navigating these crevasses takes considerable care using the snow probe as a crevasse probe. . The area of bare glacier ice is riven by some large streams, which are also the focus of annual observation. T The terminus this summer was buried in snow from an avalanche, as was the case last year. In 2007 the terminus was fully exposed and we could measure the retreat at 450 m in the last 25 years. This glacier’s mass balance history follows that of the other northwestern North American glaciers which also is right in line with global mass balance. All data is from the WGMS. One of the best parts of this location is the gorgeous campsite we use for a base camp. It is above the glacier so we have to hike uphill at the end of the day. . his area is noted for its mountain goats as well which we count annually. We will complete a final analysis in the next month and report this data to the World Glacier Monitoring Service.