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Mendenhall Glacier is a temperate (non-polar) valley glacier located on the maritime side of the northern Coast Mountains of southeastern Alaska (see figure below).

It is one of forty large (and numerous smaller) outlet glaciers draining the Juneau Icefield, the fifth largest icefield in North America.

From an ice divide at 1600 m above sea level (msl), the glacier flows more then 20 km to a calving terminus in Mendenhall Lake, 23 msl.
Figure showing the terminus of mendenhall glacier (study area) at different times
Location of Mendenhall Glacier in southeastern Alaska. Post-LIA terminus positions are overlain on a 1996 aerial photograph. Datum: UTM8N, NAD27. (Motyka et al, 2002)

Mendenhall Glacier has been retreating continuously since reaching its maximum extent during the Little Ice Age (LIA), around 1760 (Motyka et al., 2002). Information on the post-LIA retreat of the glacier comes from moraines, twentieth century topographic maps and air photos, and survey data acquired over the past decade (Motyka et al., 2002).

Mendenhall Lake did not exist before 1930 (Motyka et al., 2002); it formed in conjunction with the glacier's retreat into its own over-deepened channel. Currently 1/3 of the terminus is lacustrine, while the remaining part has receded out of Mendenhall Lake and is once again terrestrial.

The onset of lacustrine calving is significant in evaluating the retreat history of Mendenhall Glacier. Iceberg calving is a very efficient ablation (ice loss) mechanism, and a much larger rate of mass loss is achieved by calving than by surface melting alone (van der Veen, 1996). According to a commonly accepted model, calving rate is related to water depth at the terminus, with retreat of the glacier snout into deeper water, leading to larger calving rates and accelerated retreat (Brown et al., 1982).

Thus, a glacier's bed topography is an important factor in controlling changes in the terminus position (Meier and Post, 1987). This causal mechanism has been the subject of much debate, and the empirically derived relationships between water depth and calving rate are not applicable to all calving glaciers.

It is apparent that the long-term behaviour of Mendenhall Glacier is consistent with current climatic trends. However, dynamic and topographic factors may allow the terminus to become decoupled from short-term climatic trends. In studying such a glacier, it is important to distinguish changes that may be attributable to climatic factors, from changes caused by other variables affecting the terminus. This requires an accurate record of changes in terminus positions, which is where remote-sensing techniques may be applied.

Over the past five years, Mendenhall Glacier has experienced accelerated rates of thinning and retreat. This culminated in a dramatic breakup event in the summer of 2004, during which the glacier retreated more than 200 m.

 
logos Geophysical Institute University of Alaska Fairbanks Alaska Space Grant Program Earth System Science Education for the 21st Century