March 16, 2011
Saywer Glacier, Alaska Retreat
Posted by Mauri Pelto
Sawyer Glacier is an Alaskan glacier that many people have observed the retreat of. It is one of three glaciers that calves into Tracy Fjord, that is often visited by cruise ships. The 2.3 km retreat over the last 50 years has made approach to the actual terminus difficult for larger cruise ships. The terminus of the glacier was almost to the main arm of the fjord in 1961, USGS map, blue arrow. The 2003 Google Earth images indicates a 2.3 km retreat, green arrow.
The green line down fjord marks the 1961 terminus, the upfjord line is at the 1100 foot contour of the USGS map and is parallel with the current terminus location. The snowline of the glacier was noted to be at 1125 meters by Pelto (1987), using Landsat images. At the time usable images were rare. Today images are acquired daily by MODIS and if the weather is clear, which is not common, the snow line is evident. Recent satellite images from 2006 and 2009 indicate the snow line still in the 1100 to 1200 meter elevation region, however, the 300+ meter thinning at the terminus has led to thinning upglacier and the location of the 1100 meter contour has shifted 1-1.5 kilometers upglacier. Landsat images from 1990 to 2013 indicate extensive retreat. The retreat from the 1990 terminus, red arrow, to the 2013 terminus, yellow arrow is 2.8 km, 120 m per year. The western feeder just above the terminus has narrowed at the pink arrow from 1 km wide in 1990 to 500 m wide in 2013. This suggests that retreat will soon lead to separation of Sawyer Glacier with the western arm and eastern arm both continuing a retreat. The 300 meter thinning in the last 50 years has left a sharp trimline beyond the current terminus marking the former ice height, that is well above the current fjord. The glacier is losing mass and retreating as is the nearby North Dawes Glacier.
The thinning combined with the rising snowline has resulted in a smaller accumulation zone. At the same time the retreat has reduced the calving rate as the terminus has narrowed and entered shallower water. The calving rate is related to water depth on these glaciers, Pelto and Warren(1991) . The reduced calving leads to a more positive mass balance, the increased snow line and reduce height of the glacier surface leads to reduced mass balance. This is equivalent to your bank account when you finish paying off your mortgage, but at the same time your income is declining. It will be interesting to observe the balance point of these two opposite forces. The University of Alaska Fairbanks began periodic altimeter surveys of the glaciers in the Sawyer Glacier region in 2000 and have noted a sharp decrease in surface elevation from the terminus to 1000 meters for the 2000-2009 period (Larsen et al, 2009).